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Oldham ML, Zuhaib Qayyum M, Kalathur RC, Rock CO, Radka CD. Cryo-EM reconstruction of oleate hydratase bound to a phospholipid membrane bilayer. J Struct Biol 2024; 216:108116. [PMID: 39151742 PMCID: PMC11385989 DOI: 10.1016/j.jsb.2024.108116] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2024] [Revised: 08/08/2024] [Accepted: 08/13/2024] [Indexed: 08/19/2024]
Abstract
Oleate hydratase (OhyA) is a bacterial peripheral membrane protein that catalyzes FAD-dependent water addition to membrane bilayer-embedded unsaturated fatty acids. The opportunistic pathogen Staphylococcus aureus uses OhyA to counteract the innate immune system and support colonization. Many Gram-positive and Gram-negative bacteria in the microbiome also encode OhyA. OhyA is a dimeric flavoenzyme whose carboxy terminus is identified as the membrane binding domain; however, understanding how OhyA binds to cellular membranes is not complete until the membrane-bound structure has been elucidated. All available OhyA structures depict the solution state of the protein outside its functional environment. Here, we employ liposomes to solve the cryo-electron microscopy structure of the functional unit: the OhyA•membrane complex. The protein maintains its structure upon membrane binding and slightly alters the curvature of the liposome surface. OhyA preferentially associates with 20-30 nm liposomes with multiple copies of OhyA dimers assembling on the liposome surface resulting in the formation of higher-order oligomers. Dimer assembly is cooperative and extends along a formed ridge of the liposome. We also solved an OhyA dimer of dimers structure that recapitulates the intermolecular interactions that stabilize the dimer assembly on the membrane bilayer as well as the crystal contacts in the lattice of the OhyA crystal structure. Our work enables visualization of the molecular trajectory of membrane binding for this important interfacial enzyme.
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Affiliation(s)
- Michael L Oldham
- Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, TN, 38105, USA
| | - M Zuhaib Qayyum
- Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, TN, 38105, USA
| | - Ravi C Kalathur
- Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, TN, 38105, USA
| | - Charles O Rock
- Department of Host Microbe Interactions, St. Jude Children's Research Hospital, Memphis, TN, 38105, USA
| | - Christopher D Radka
- Department of Host Microbe Interactions, St. Jude Children's Research Hospital, Memphis, TN, 38105, USA; Department of Microbiology, Immunology, and Molecular Genetics, University of Kentucky, Lexington, KY, 40536, USA.
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2
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Radka CD, Grace CR, Hasdemir HS, Li Y, Rodriguez CC, Rodrigues P, Oldham ML, Qayyum MZ, Pitre A, MacCain WJ, Kalathur RC, Tajkhorshid E, Rock CO. The carboxy terminus causes interfacial assembly of oleate hydratase on a membrane bilayer. J Biol Chem 2024; 300:105627. [PMID: 38211817 PMCID: PMC10847778 DOI: 10.1016/j.jbc.2024.105627] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2023] [Revised: 12/19/2023] [Accepted: 12/22/2023] [Indexed: 01/13/2024] Open
Abstract
The soluble flavoprotein oleate hydratase (OhyA) hydrates the 9-cis double bond of unsaturated fatty acids. OhyA substrates are embedded in membrane bilayers; OhyA must remove the fatty acid from the bilayer and enclose it in the active site. Here, we show that the positively charged helix-turn-helix motif in the carboxy terminus (CTD) is responsible for interacting with the negatively charged phosphatidylglycerol (PG) bilayer. Super-resolution microscopy of Staphylococcus aureus cells expressing green fluorescent protein fused to OhyA or the CTD sequence shows subcellular localization along the cellular boundary, indicating OhyA is membrane-associated and the CTD sequence is sufficient for membrane recruitment. Using cryo-electron microscopy, we solved the OhyA dimer structure and conducted 3D variability analysis of the reconstructions to assess CTD flexibility. Our surface plasmon resonance experiments corroborated that OhyA binds the PG bilayer with nanomolar affinity and we found the CTD sequence has intrinsic PG binding properties. We determined that the nuclear magnetic resonance structure of a peptide containing the CTD sequence resembles the OhyA crystal structure. We observed intermolecular NOE from PG liposome protons next to the phosphate group to the CTD peptide. The addition of paramagnetic MnCl2 indicated the CTD peptide binds the PG surface but does not insert into the bilayer. Molecular dynamics simulations, supported by site-directed mutagenesis experiments, identify key residues in the helix-turn-helix that drive membrane association. The data show that the OhyA CTD binds the phosphate layer of the PG surface to obtain bilayer-embedded unsaturated fatty acids.
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Affiliation(s)
- Christopher D Radka
- Department of Microbiology, Immunology, and Molecular Genetics, University of Kentucky, Lexington, Kentucky, USA; Department of Host Microbe Interactions, St Jude Children's Research Hospital, Memphis, Tennessee, USA.
| | - Christy R Grace
- Department of Structural Biology, St Jude Children's Research Hospital, Memphis, Tennessee, USA
| | - Hale S Hasdemir
- Theoretical and Computational Biophysics Group, Department of Biochemistry, and Center for Biophysics and Quantitative Biology, NIH Resource for Macromolecular Modeling and Visualization, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
| | - Yupeng Li
- Theoretical and Computational Biophysics Group, Department of Biochemistry, and Center for Biophysics and Quantitative Biology, NIH Resource for Macromolecular Modeling and Visualization, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
| | - Carlos C Rodriguez
- Theoretical and Computational Biophysics Group, Department of Biochemistry, and Center for Biophysics and Quantitative Biology, NIH Resource for Macromolecular Modeling and Visualization, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
| | - Patrick Rodrigues
- Hartwell Center of Biotechnology, St Jude Children's Research Hospital, Memphis, Tennessee, USA
| | - Michael L Oldham
- Department of Structural Biology, St Jude Children's Research Hospital, Memphis, Tennessee, USA
| | - M Zuhaib Qayyum
- Department of Structural Biology, St Jude Children's Research Hospital, Memphis, Tennessee, USA
| | - Aaron Pitre
- Cell and Tissue Imaging Center, St Jude Children's Research Hospital, Memphis, Tennessee, USA
| | - William J MacCain
- Department of Host Microbe Interactions, St Jude Children's Research Hospital, Memphis, Tennessee, USA
| | - Ravi C Kalathur
- Department of Structural Biology, St Jude Children's Research Hospital, Memphis, Tennessee, USA
| | - Emad Tajkhorshid
- Theoretical and Computational Biophysics Group, Department of Biochemistry, and Center for Biophysics and Quantitative Biology, NIH Resource for Macromolecular Modeling and Visualization, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
| | - Charles O Rock
- Department of Host Microbe Interactions, St Jude Children's Research Hospital, Memphis, Tennessee, USA
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3
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Tavasoli M, Lahire S, Reid T, Brodovsky M, McMaster CR. Genetic diseases of the Kennedy pathways for membrane synthesis. J Biol Chem 2020; 295:17877-17886. [PMID: 33454021 PMCID: PMC7762932 DOI: 10.1074/jbc.rev120.013529] [Citation(s) in RCA: 30] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2020] [Revised: 10/20/2020] [Indexed: 11/06/2022] Open
Abstract
The two branches of the Kennedy pathways (CDP-choline and CDP-ethanolamine) are the predominant pathways responsible for the synthesis of the most abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine, respectively, in mammalian membranes. Recently, hereditary diseases associated with single gene mutations in the Kennedy pathways have been identified. Interestingly, genetic diseases within the same pathway vary greatly, ranging from muscular dystrophy to spastic paraplegia to a childhood blinding disorder to bone deformations. Indeed, different point mutations in the same gene (PCYT1; CCTα) result in at least three distinct diseases. In this review, we will summarize and review the genetic diseases associated with mutations in genes of the Kennedy pathway for phospholipid synthesis. These single-gene disorders provide insight, indeed direct genotype-phenotype relationships, into the biological functions of specific enzymes of the Kennedy pathway. We discuss potential mechanisms of how mutations within the same pathway can cause disparate disease.
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Affiliation(s)
- Mahtab Tavasoli
- Department of Pharmacology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Sarah Lahire
- University of Reims Champagne-Ardenne, Reims, France
| | - Taryn Reid
- Department of Pharmacology, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Maren Brodovsky
- Department of Pharmacology, Dalhousie University, Halifax, Nova Scotia, Canada
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4
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Izrael R, Marton L, Nagy GN, Pálinkás HL, Kucsma N, Vértessy BG. Identification of a nuclear localization signal in the Plasmodium falciparum CTP: phosphocholine cytidylyltransferase enzyme. Sci Rep 2020; 10:19739. [PMID: 33184408 PMCID: PMC7665022 DOI: 10.1038/s41598-020-76829-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2020] [Accepted: 11/02/2020] [Indexed: 12/30/2022] Open
Abstract
The phospholipid biosynthesis of the malaria parasite, Plasmodium falciparum is a key process for its survival and its inhibition is a validated antimalarial therapeutic approach. The second and rate-limiting step of the de novo phosphatidylcholine biosynthesis is catalysed by CTP: phosphocholine cytidylyltransferase (PfCCT), which has a key regulatory function within the pathway. Here, we investigate the functional impact of the key structural differences and their respective role in the structurally unique pseudo-heterodimer PfCCT protein in a heterologous cellular context using the thermosensitive CCT-mutant CHO-MT58 cell line. We found that a Plasmodium-specific lysine-rich insertion within the catalytic domain of PfCCT acts as a nuclear localization signal and its deletion decreases the nuclear propensity of the protein in the model cell line. We further showed that the putative membrane-binding domain also affected the nuclear localization of the protein. Moreover, activation of phosphatidylcholine biosynthesis by phospholipase C treatment induces the partial nuclear-to-cytoplasmic translocation of PfCCT. We additionally investigated the cellular function of several PfCCT truncated constructs in a CHO-MT58 based rescue assay. In absence of the endogenous CCT activity we observed that truncated constructs lacking the lysine-rich insertion, or the membrane-binding domain provided similar cell survival ratio as the full length PfCCT protein.
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Affiliation(s)
- Richard Izrael
- Institute of Enzymology, Research Centre for Natural Sciences, 1117, Budapest, Hungary.
- Doctoral School of Multidisciplinary Medical Sciences, University of Szeged, 6720, Szeged, Hungary.
- Department of Applied Biotechnology, Budapest University of Technology and Economics, 1111, Budapest, Hungary.
| | - Lívia Marton
- Institute of Enzymology, Research Centre for Natural Sciences, 1117, Budapest, Hungary
| | - Gergely N Nagy
- Institute of Enzymology, Research Centre for Natural Sciences, 1117, Budapest, Hungary
- Department of Applied Biotechnology, Budapest University of Technology and Economics, 1111, Budapest, Hungary
- Division of Structural Biology, Wellcome Centre for Human Genetics, University of Oxford, Oxford, OX3 7BN, UK
| | - Hajnalka L Pálinkás
- Institute of Enzymology, Research Centre for Natural Sciences, 1117, Budapest, Hungary
- Doctoral School of Multidisciplinary Medical Sciences, University of Szeged, 6720, Szeged, Hungary
- Department of Applied Biotechnology, Budapest University of Technology and Economics, 1111, Budapest, Hungary
| | - Nóra Kucsma
- Institute of Enzymology, Research Centre for Natural Sciences, 1117, Budapest, Hungary
| | - Beáta G Vértessy
- Institute of Enzymology, Research Centre for Natural Sciences, 1117, Budapest, Hungary.
- Department of Applied Biotechnology, Budapest University of Technology and Economics, 1111, Budapest, Hungary.
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5
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Taneva SG, Lee J, Knowles DG, Tishyadhigama C, Chen H, Cornell RB. Interdomain communication in the phosphatidylcholine regulatory enzyme, CCTα, relies on a modular αE helix. J Biol Chem 2019; 294:15517-15530. [PMID: 31488547 DOI: 10.1074/jbc.ra119.009849] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2019] [Revised: 08/18/2019] [Indexed: 12/14/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT), the rate-limiting enzyme in phosphatidylcholine (PC) synthesis, is an amphitropic enzyme that regulates PC homeostasis. Recent work has suggested that CCTα activation by binding to a PC-deficient membrane involves conformational transitions in a helix pair (αE) that, along with a short linker of unknown structure (J segment), bridges the catalytic domains of the CCTα dimer to the membrane-binding (M) domains. In the soluble, inactive form, the αE helices are constrained into unbroken helices by contacts with two auto-inhibitory (AI) helices from domain M. In the active, membrane-bound form, the AI helices are displaced and engage the membrane. Molecular dynamics simulations have suggested that AI displacement is associated with hinge-like bending in the middle of the αE, positioning its C terminus closer to the active site. Here, we show that CCTα activation by membrane binding is sensitive to mutations in the αE and J segments, especially within or proximal to the αE hinge. Substituting Tyr-213 within this hinge with smaller uncharged amino acids that could destabilize interactions between the αE helices increased both constitutive and lipid-dependent activities, supporting a link between αE helix bending and stimulation of CCT activity. The solvent accessibilities of Tyr-213 and Tyr-216 suggested that these tyrosines move to new partially buried environments upon membrane binding of CCT, consistent with a folded αE/J structure. These data suggest that signal transduction through the modular αE helix pair relies on shifts in its conformational ensemble that are controlled by the AI helices and their displacement upon membrane binding.
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Affiliation(s)
- Svetla G Taneva
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
| | - Jaeyong Lee
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
| | - Daniel G Knowles
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
| | - Chanajai Tishyadhigama
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
| | - Hongwen Chen
- Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
| | - Rosemary B Cornell
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada .,Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
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6
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Cornell RB, Taneva SG, Dennis MK, Tse R, Dhillon RK, Lee J. Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability. J Biol Chem 2018; 294:1490-1501. [PMID: 30559292 DOI: 10.1074/jbc.ra118.006457] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2018] [Revised: 12/05/2018] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) is the key regulatory enzyme in phosphatidylcholine (PC) synthesis and is activated by binding to PC-deficient membranes. Mutations in the gene encoding CCTα (PCYT1A) cause three distinct pathologies in humans: lipodystrophy, spondylometaphyseal dysplasia with cone-rod dystrophy (SMD-CRD), and isolated retinal dystrophy. Previous analyses showed that for some disease-linked PCYT1A variants steady state levels of CCTα and PC synthesis were reduced in patient fibroblasts, but other variants impaired PC synthesis with little effect on CCT levels. To explore the impact on CCT stability and function we expressed WT and mutant CCTs in COS-1 cells, which have very low endogenous CCT. Over-expression of two missense variants in the catalytic domain (V142M and P150A) generated aggregated enzymes that could not be refolded after solubilization by denaturation. Other mutations in the catalytic core that generated CCTs with reduced solubility could be purified. Five variants destabilized the catalytic domain-fold as assessed by lower transition temperatures for unfolding, and three of these manifested defects in substrate Km values. A mutation (R223S) in a signal-transducing linker between the catalytic and membrane-binding domains also impaired enzyme kinetics. E280del, a single amino acid deletion in the autoinhibitory helix increased the constitutive (lipid-independent) enzyme activity ∼4-fold. This helix also participates in membrane binding, and surprisingly E280del enhanced the enzyme's response to anionic lipid vesicles ∼4-fold. These in vitro analyses on purified mutant CCTs will complement future measurements of their impact on PC synthesis in cultured cells and in tissues with a stringent requirement for CCTα.
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Affiliation(s)
- Rosemary B Cornell
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada; Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada.
| | - Svetla G Taneva
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
| | - Melissa K Dennis
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
| | - Ronnie Tse
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
| | - Randeep K Dhillon
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
| | - Jaeyong Lee
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
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7
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Membrane lipid compositional sensing by the inducible amphipathic helix of CCT. Biochim Biophys Acta Mol Cell Biol Lipids 2015; 1861:847-861. [PMID: 26747646 DOI: 10.1016/j.bbalip.2015.12.022] [Citation(s) in RCA: 52] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2015] [Revised: 12/22/2015] [Accepted: 12/29/2015] [Indexed: 11/21/2022]
Abstract
The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.
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8
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Craddock CP, Adams N, Bryant FM, Kurup S, Eastmond PJ. Regulation of endomembrane biogenesis in arabidopsis by phospatidic acid hydrolase. PLANT SIGNALING & BEHAVIOR 2015; 10:e1065367. [PMID: 26225871 PMCID: PMC4883860 DOI: 10.1080/15592324.2015.1065367] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/01/2015] [Accepted: 06/18/2015] [Indexed: 08/29/2023]
Abstract
Coordination of membrane lipid biosynthesis is important for cell function during plant growth and development. Here we summarize our recent work on PHOSPHATIDIC ACID PHOSPHOHYDROLASE (PAH) which suggests that this enzyme is a key regulator of phosphaticylcholine (PC) biosynthesis in Arabidopsis thaliana. Disruption of PAH activity elevates phosphatidic acid (PA) levels and stimulates PC biosynthesis and biogenesis of the endoplasmic reticulum (ER). Furthermore, the activity of PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE (CCT), which is the key enzyme controlling the rate of PC biosynthesis, is directly stimulated by PA and expression of a constitutively active version of CCT replicates the effects of PAH disruption. Hence PAH activity can control the abundance of PA, which in turn can modulate CCT activity to govern the rate of PC biosynthesis. Crucially it is not yet clear how PAH activity is regulated in Arabidopsis but there is evidence that PAH1 and PAH2 are both phosphorylated and further work will be required to investigate whether this is functionally significant.
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Affiliation(s)
- Christian P Craddock
- Department of Plant Biology and Crop Science; Rothamsted Research, Harpenden; Hertfordshire, UK
| | - Nicolette Adams
- Department of Plant Biology and Crop Science; Rothamsted Research, Harpenden; Hertfordshire, UK
| | - Fiona M Bryant
- Department of Plant Biology and Crop Science; Rothamsted Research, Harpenden; Hertfordshire, UK
| | - Smita Kurup
- Department of Plant Biology and Crop Science; Rothamsted Research, Harpenden; Hertfordshire, UK
| | - Peter J Eastmond
- Department of Plant Biology and Crop Science; Rothamsted Research, Harpenden; Hertfordshire, UK
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9
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Cornell RB, Ridgway ND. CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis. Prog Lipid Res 2015; 59:147-71. [PMID: 26165797 DOI: 10.1016/j.plipres.2015.07.001] [Citation(s) in RCA: 91] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2015] [Revised: 07/07/2015] [Accepted: 07/07/2015] [Indexed: 12/12/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.
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Affiliation(s)
- Rosemary B Cornell
- Department of Molecular Biology and Biochemistry and the Department of Chemistry, Simon Fraser University, Burnaby, B.C. V5A-1S6, Canada.
| | - Neale D Ridgway
- Departments of Pediatrics, and Biochemistry and Molecular Biology, Atlantic Research Centre, Dalhousie University, Halifax, Nova Scotia B3H-4H7, Canada
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10
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Craddock CP, Adams N, Bryant FM, Kurup S, Eastmond PJ. PHOSPHATIDIC ACID PHOSPHOHYDROLASE Regulates Phosphatidylcholine Biosynthesis in Arabidopsis by Phosphatidic Acid-Mediated Activation of CTP:PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE Activity. THE PLANT CELL 2015; 27:1251-64. [PMID: 25862304 PMCID: PMC4558698 DOI: 10.1105/tpc.15.00037] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/15/2015] [Accepted: 03/19/2015] [Indexed: 05/04/2023]
Abstract
Regulation of membrane lipid biosynthesis is critical for cell function. We previously reported that disruption of PHOSPHATIDIC ACID PHOSPHOHYDROLASE1 (PAH1) and PAH2 stimulates net phosphatidylcholine (PC) biosynthesis and proliferation of the endoplasmic reticulum (ER) in Arabidopsis thaliana. Here, we show that this response is caused specifically by a reduction in the catalytic activity of the protein and positively correlates with an accumulation of its substrate, phosphatidic acid (PA). The accumulation of PC in pah1 pah2 is suppressed by disruption of CTP:PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE1 (CCT1), which encodes a key enzyme in the nucleotide pathway for PC biosynthesis. The activity of recombinant CCT1 is stimulated by lipid vesicles containing PA. Truncation of CCT1, to remove the predicted C-terminal amphipathic lipid binding domain, produced a constitutively active enzyme. Overexpression of native CCT1 in Arabidopsis has no significant effect on PC biosynthesis or ER morphology, but overexpression of the truncated constitutively active version largely replicates the pah1 pah2 phenotype. Our data establish that membrane homeostasis is regulated by lipid composition in Arabidopsis and reveal a mechanism through which the abundance of PA, mediated by PAH activity, modulates CCT activity to govern PC content.
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Affiliation(s)
- Christian P Craddock
- School of Life Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - Nicolette Adams
- School of Life Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - Fiona M Bryant
- Department of Plant Biology and Crop Science, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom
| | - Smita Kurup
- Department of Plant Biology and Crop Science, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom
| | - Peter J Eastmond
- Department of Plant Biology and Crop Science, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom
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11
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Yamamoto G, Baratela W, Almeida T, Lazar M, Afonso C, Oyamada M, Suzuki L, Oliveira L, Ramos E, Kim C, Passos-Bueno M, Bertola D. Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy. Am J Hum Genet 2014; 94:113-9. [PMID: 24387991 DOI: 10.1016/j.ajhg.2013.11.022] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2013] [Accepted: 11/22/2013] [Indexed: 01/31/2023] Open
Abstract
Spondylometaphyseal dysplasia with cone-rod dystrophy is a rare autosomal-recessive disorder characterized by severe short stature, progressive lower-limb bowing, flattened vertebral bodies, metaphyseal involvement, and visual impairment caused by cone-rod dystrophy. Whole-exome sequencing of four individuals affected by this disorder from two Brazilian families identified two previously unreported homozygous mutations in PCYT1A. This gene encodes the alpha isoform of the phosphate cytidylyltransferase 1 choline enzyme, which is responsible for converting phosphocholine into cytidine diphosphate-choline, a key intermediate step in the phosphatidylcholine biosynthesis pathway. A different enzymatic defect in this pathway has been previously associated with a muscular dystrophy with mitochondrial structural abnormalities that does not have cartilage and/or bone or retinal involvement. Thus, the deregulation of the phosphatidylcholine pathway may play a role in multiple genetic diseases in humans, and further studies are necessary to uncover its precise pathogenic mechanisms and the entirety of its phenotypic spectrum.
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12
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Huang HKH, Taneva SG, Lee J, Silva LP, Schriemer DC, Cornell RB. The membrane-binding domain of an amphitropic enzyme suppresses catalysis by contact with an amphipathic helix flanking its active site. J Mol Biol 2012; 425:1546-64. [PMID: 23238251 DOI: 10.1016/j.jmb.2012.12.003] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2012] [Revised: 11/24/2012] [Accepted: 12/03/2012] [Indexed: 11/29/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CCT), the regulatory enzyme in the synthesis of phosphatidylcholine, is activated by binding membranes using a lipid-induced amphipathic helix (domain M). Domain M functions to silence catalysis when CCT is not membrane engaged. The silencing mechanism is unknown. We used photo-cross-linking and mass spectrometry to identify contacts between domain M and other CCT domains in its soluble form. Each of four sites in domain M forged cross-links to the same set of peptides that flank the active site and overlap at helix αE at the base of the active site. These cross-links were broken in the presence of activating lipid vesicles. Mutagenesis of domain M revealed that multiple hydrophobic residues within a putative auto-inhibitory (AI) motif contribute to the contact with helix αE and silencing. Helix αE was confirmed as the docking site for domain M by deuterium exchange analysis. We compared the dynamics and fold stability of CCT domains by site-directed fluorescence anisotropy and urea denaturation. The results suggest a bipartite structure for domain M: a disordered N-terminal portion and an ordered C-terminal AI motif with an unfolding transition identical with that of helix αE. Reduction in hydrophobicity of the AI motif decreased its order and fold stability, as did deletion of the catalytic domain. These results support a model in which catalytic silencing is mediated by the docking of an amphipathic AI motif onto the amphipathic helices αE. An unstructured leash linking αE with the AI motif may facilitate both the silencing contact and its membrane-triggered disruption.
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Affiliation(s)
- Harris K-H Huang
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
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Ding Z, Taneva SG, Huang HKH, Campbell SA, Semenec L, Chen N, Cornell RB. A 22-mer segment in the structurally pliable regulatory domain of metazoan CTP: phosphocholine cytidylyltransferase facilitates both silencing and activating functions. J Biol Chem 2012; 287:38980-91. [PMID: 22988242 DOI: 10.1074/jbc.m112.402081] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT), an amphitropic enzyme that regulates phosphatidylcholine synthesis, is composed of a catalytic head domain and a regulatory tail. The tail region has dual functions as a regulator of membrane binding/enzyme activation and as an inhibitor of catalysis in the unbound form of the enzyme, suggesting conformational plasticity. These functions are well conserved in CCTs across diverse phyla, although the sequences of the tail regions are not. CCT regulatory tails of diverse origins are composed of a long membrane lipid-inducible amphipathic helix (m-AH) followed by a highly disordered segment, reminiscent of the Parkinson disease-linked protein, α-synuclein, which we show shares a novel sequence motif with vertebrate CCTs. To unravel features required for silencing, we created chimeric enzymes by fusing the catalytic domain of rat CCTα to the regulatory tail of CCTs from Drosophila, Caenorhabditis elegans, or Saccharomyces cerevisiae or to α-synuclein. Only the tail domains of the two invertebrate CCTs were competent for both suppression of catalytic activity and for activation by lipid vesicles. Thus, both silencing and activating functions of the m-AH can tolerate significant changes in length and sequence. We identified a highly amphipathic 22-residue segment in the m-AH with features conserved among animal CCTs but not yeast CCT or α-synuclein. Deletion of this segment from rat CCT increased the lipid-independent V(max) by 10-fold, equivalent to the effect of deleting the entire tail, and severely weakened membrane binding affinity. However, membrane binding was required for additional increases in catalytic efficiency. Thus, full activation of CCT may require not only loss of a silencing conformation in the m-AH but a gain of an activating conformation, promoted by membrane binding.
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Affiliation(s)
- Ziwei Ding
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
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Dennis MK, Taneva SG, Cornell RB. The intrinsically disordered nuclear localization signal and phosphorylation segments distinguish the membrane affinity of two cytidylyltransferase isoforms. J Biol Chem 2011; 286:12349-60. [PMID: 21303909 DOI: 10.1074/jbc.m110.201715] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Membrane phosphatidylcholine homeostasis is maintained in part by a sensing device in the key regulatory enzyme, CTP:phosphocholine cytidylyltransferase (CCT). CCT responds to decreases in membrane phosphatidylcholine content by reversible membrane binding and activation. Two prominent isoforms, CCTα and -β2, have nearly identical catalytic domains and very similar membrane binding amphipathic helical (M) domains but have divergent and structurally disordered N-terminal (N) and C-terminal phosphorylation (P) regions. We found that the binding affinity of purified CCTβ2 for anionic membranes was weaker than CCTα by more than an order of magnitude. Using chimeric CCTs, insertion/deletion mutants, and truncated CCTs, we show that the stronger affinity of CCTα can be attributed in large part to the electrostatic membrane binding function of the polybasic nuclear localization signal (NLS) motif, present in the unstructured N-terminal segment of CCTα but lacking in CCTβ2. The membrane partitioning of CCTβ2 in cells enriched with the lipid activator, oleic acid, was also weaker than that of CCTα and was elevated by incorporation of the NLS motif. Thus, the polybasic NLS can function as a secondary membrane binding motif not only in vitro but in the context of cell membranes. A comparison of phosphorylated, dephosphorylated, and region P-truncated forms showed that the in vitro membrane affinity of CCTβ2 is more sensitive than CCTα to phosphorylation status, which antagonizes membrane binding of both isoforms. These data provide a model wherein the primary membrane binding motif, an amphipathic helical domain, works in collaboration with other intrinsically disordered segments that modulate membrane binding strength. The NLS reinforces, whereas the phosphorylated tail antagonizes the attraction of domain M for anionic membranes.
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Affiliation(s)
- Melissa K Dennis
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada
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15
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Braker JD, Hodel KJ, Mullins DR, Friesen JA. Identification of hydrophobic amino acids required for lipid activation of C. elegans CTP:phosphocholine cytidylyltransferase. Arch Biochem Biophys 2009; 492:10-6. [PMID: 19836342 DOI: 10.1016/j.abb.2009.10.004] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2009] [Revised: 10/09/2009] [Accepted: 10/10/2009] [Indexed: 11/18/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CCT), critical for phosphatidylcholine biosynthesis, is activated by translocation to the membrane surface. The lipid activation region of Caenorhabditis elegans CCT is between residues 246 and 266 of the 347 amino acid polypeptide, a region proposed to form an amphipathic alpha helix. When leucine 246, tryptophan 249, isoleucine 256, isoleucine 257, or phenylalanine 260, on the hydrophobic face of the helix, were changed individually to serine low activity was observed in the absence of lipid vesicles, similar to wild-type CCT, while lipid stimulated activity was reduced compared to wild-type CCT. Mutational analysis of phenylalanine 260 implicated this residue as a contributor to auto-inhibition of CCT while mutation of L246, W249, I256, and I257 simultaneously to serine resulted in significantly higher activity in the absence of lipid vesicles and an enzyme that was not lipid activated. These results support a concerted mechanism of lipid activation that requires multiple residues on the hydrophobic face of the putative amphipathic alpha helix.
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Affiliation(s)
- Jay D Braker
- Department of Chemistry, Illinois State University, Normal, IL 61790, USA
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16
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Oxysterol activation of phosphatidylcholine synthesis involves CTP:phosphocholine cytidylyltransferase α translocation to the nuclear envelope. Biochem J 2009; 418:209-17. [DOI: 10.1042/bj20081923] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
In addition to suppressing cholesterol synthesis and uptake, oxysterols also activate glycerophospholipid and SM (sphingomyelin) synthesis, possibly to buffer cells from excess sterol accumulation. In the present study, we investigated the effects of oxysterols on the CDP-choline pathway for PtdCho (phosphatidylcholine) synthesis using wild-type and sterol-resistant CHO (Chinese-hamster ovary) cells expressing a mutant of SCAP [SREBP (sterol-regulatory-element-binding protein) cleavage-activating protein] (CHO-SCAP D443N). [3H]Choline-labelling experiments showed that 25OH (25-hydroxycholesterol), 22OH (22-hydroxycholesterol) and 27OH (27-hydroxycholesterol) increased PtdCho synthesis in CHO cells as a result of CCTα (CTP:phosphocholine cytidylyltransferase α) translocation and activation at the NE (nuclear envelope). These oxysterols also activate PtdCho synthesis in J774 macrophages. in vitro, CCTα activity was stimulated 2- to 2.5-fold by liposomes containing 5 mol% 25OH, 22OH or 27OH. Inclusion of up to 5 mol% cholesterol did not further activate CCTα. 25OH activated CCTα in CHO-SCAP D443N cells leading to a transient increase in PtdCho synthesis and accumulation of CDP-choline. CCTα translocation to the NE and intranuclear tubules in CHO-SCAP D443N cells was complete after 1 h exposure to 25OH compared with only partial translocation by 4–6 h in CHO-Mock cells. These enhanced responses in CHO-D443N cells were sterol-dependent since depletion with cyclodextrin or lovastatin resulted in reduced sensitivity to 25OH. However, the lack of effect of cholesterol on in vitro CCT activity indicates an indirect relationship or involvement of other sterols or oxysterol. We conclude that translocation and activation of CCTα at nuclear membranes by side-chain hydroxylated sterols are regulated by the cholesterol status of the cell.
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17
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Gehrig K, Morton CC, Ridgway ND. Nuclear export of the rate-limiting enzyme in phosphatidylcholine synthesis is mediated by its membrane binding domain. J Lipid Res 2008; 50:966-76. [PMID: 19098306 DOI: 10.1194/jlr.m800632-jlr200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase alpha (CCTalpha), the rate-limiting enzyme in the CDP-choline pathway for phosphatidylcholine (PtdCho) synthesis, is activated by translocation to nuclear membranes. However, CCTalpha is cytoplasmic in cells with increased capacity for PtdCho synthesis and following acute activation, suggesting that nuclear export is linked to activation. The objective of this study was to identify which CCTalpha domains were involved in nuclear export in response to the lipid activators farnesol (FOH) and oleate. Imaging of CCT-green fluorescent protein (GFP) mutants expressed in CCTalpha-deficient CHO58 cells showed that FOH-mediated translocation to nuclear membranes and export to the cytoplasm required the membrane binding amphipathic helix (domain M). Nuclear export was reduced by a mutation that mimics constitutive phosphorylation of the CCT phosphorylation (P) domain. However, domain M alone was sufficient to promote translocation to the nuclear envelope and export of a nuclear-localized GFP construct in FOH- or oleate-treated CHO58 cells. In the context of acute activation with lipid mediators, nuclear export of CCT-GFP mutants correlated with in vitro activity but not PtdCho synthesis. This study describes a nuclear export pathway that is dependent on membrane interaction of an amphipathic helix, thus linking lipid-dependent activation to the nuclear/cytoplasmic distribution of CCTalpha.
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Affiliation(s)
- Karsten Gehrig
- Department of Pediatrics, and Biochemistry & Molecular Biology, Atlantic Research Centre, Dalhousie University, Halifax, Nova Scotia, Canada
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18
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Marcucci H, Elena C, Gilardoni P, Banchio C. Characterization of the murine CTP:phosphocholine cytidylyltransferase beta gene promoter. Biochim Biophys Acta Mol Cell Biol Lipids 2008; 1781:254-62. [DOI: 10.1016/j.bbalip.2008.03.006] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2007] [Revised: 03/12/2008] [Accepted: 03/14/2008] [Indexed: 10/22/2022]
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Banchio C, Lingrell S, Vance DE. Sp-1 Binds Promoter Elements That Are Regulated by Retinoblastoma and Regulate CTP:Phosphocholine Cytidylyltransferase-α Transcription. J Biol Chem 2007; 282:14827-35. [PMID: 17384411 DOI: 10.1074/jbc.m700527200] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The retinoblastoma (Rb) protein is implicated in transcriptional regulation of at least five cellular genes. Co-transfection of Rb and truncated promoter constructs has defined a discrete element (retinoblastoma control element (RCE)) within the promoters of each of these genes as being necessary for Rb-mediated transcriptional control. In the present report we demonstrate that two RCEs identified within the CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) proximal promoter are essential to promote transcription. Mutations that abolished each RCE markedly decreased CTalpha transcription. Co-transfection of Rb and truncated promoter constructs demonstrated that Rb regulates CTalpha expression by different mechanisms depending on the phase of the cell cycle. The regulation of CTalpha expression by Rb required both the Sp1 and the RCEs. Maximal expression occurred when both Rb and Sp1 were overexpressed. Moreover, RCEs were required for Rb association with the DNA. This regulatory mechanism alters CTalpha activity and thereafter changes PC availability and cell physiology. This is the first report demonstrating not only that surrounding Sp1 binding sites alter regulation mediated by Rb, but also that the expression of a gene involved in PC biosynthesis shares a common regulatory pathway with genes responsible for cell growth and differentiation.
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Affiliation(s)
- Claudia Banchio
- Department of Biochemistry and Canadian Institutes of Health Research Group in Molecular and Cell Biology of Lipids, University of Alberta, Edmonton, Alberta T6G 2S2, Canada.
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20
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Fagone P, Sriburi R, Ward-Chapman C, Frank M, Wang J, Gunter C, Brewer JW, Jackowski S. Phospholipid Biosynthesis Program Underlying Membrane Expansion during B-lymphocyte Differentiation. J Biol Chem 2007; 282:7591-605. [PMID: 17213195 DOI: 10.1074/jbc.m608175200] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Stimulated B-lymphocytes differentiate into plasma cells committed to antibody production. Expansion of the endoplasmic reticulum and Golgi compartments is a prerequisite for high rate synthesis, assembly, and secretion of immunoglobulins. The bacterial cell wall component lipopolysaccharide (LPS) stimulates murine B-cells to proliferate and differentiate into antibody-secreting cells that morphologically resemble plasma cells. LPS activation of CH12 B-cells augmented phospholipid production and initiated a genetic program, including elevated expression of the genes for the synthesis, elongation, and desaturation of fatty acids that supply the phospholipid acyl moieties. Likewise, many of the genes in phospholipid biosynthesis were up-regulated, most notably those encoding Lipin1 and choline phosphotransferase. In contrast, CTP:phosphocholine cytidylyltransferase alpha (CCTalpha) protein, a key control point in phosphatidylcholine biosynthesis, increased because of stabilization of protein turnover rather than transcriptional activation. Furthermore, an elevation in cellular diacylglycerol and fatty acid correlated with enhanced allosteric activation of CCTalpha by the membrane lipids. This work defines a genetic and biochemical program for membrane phospholipid biogenesis that correlates with an increase in the phospholipid components of the endoplasmic reticulum and Golgi compartments in LPS-stimulated B-cells.
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Affiliation(s)
- Paolo Fagone
- Department of Infectious Diseases, St. Jude Children's Research Hospital, Memphis, Tennessee 38105-2794, USA
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21
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Banchio C, Lingrell S, Vance DE. Role of Histone Deacetylase in the Expression of CTP:Phosphocholine Cytidylyltransferase α. J Biol Chem 2006; 281:10010-5. [PMID: 16484221 DOI: 10.1074/jbc.m513503200] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Histone acetylation plays an important role in chromatin remodeling and gene expression. The molecular mechanisms involved in cell-specific expression of CTP:phosphocholine cytidylyltransferase alpha (CTalpha) are not fully understood. In this study, we investigated whether or not histone deacetylation is involved in repression of CTalpha expression in quiescent C3H10T1/2 mouse embryo fibroblasts. We have examined the contributions of the Sp1 and E2F binding sites in the repression of CTalpha gene expression. Immunoprecipitation experiments showed that histone deacetylase 1 (HDAC1) and HDAC activity are associated with Sp1 in serum-starved cells or during serum stimulation. However, HDAC1 association with E2F was only detected in serum-starved cells. By chromatin immunoprecipitation assays, we detected both direct and indirect association of HDAC1 with the CTalpha promoter. Treatment with the HDAC inhibitor trichostatin A induced CTalpha expression. Our data suggest that HDAC1 plays a critical role in CTalpha repression and that Sp1 and E2F may serve as key targets for HDAC1-mediated CTalpha repression in fibroblasts.
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Affiliation(s)
- Claudia Banchio
- Department of Biochemistry and Canadian Institutes of Health Research Group in Molecular and Cell Biology of Lipids, University of Alberta, Edmonton, Alberta T6G 2S2, Canada.
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22
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Ryan AJ, Andrews M, Zhou J, Mallampalli RK. c-Jun N-terminal kinase regulates CTP:phosphocholine cytidylyltransferase. Arch Biochem Biophys 2006; 447:23-33. [PMID: 16466687 DOI: 10.1016/j.abb.2006.01.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2005] [Revised: 01/09/2006] [Accepted: 01/11/2006] [Indexed: 11/19/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CCTalpha) is a rate-regulatory enzyme required for phosphatidylcholine (PtdCho) synthesis. CCTalpha is also a phosphoenzyme, but the physiologic role of kinases on enzyme function remains unclear. We report high-level expression of two major isoforms of the c-Jun N-terminal kinase family (JNK1 and JNK2) in murine lung epithelia. Further, JNK1 and JNK2 phosphorylated purified CCTalpha in vitro, and this was associated with a dose-dependent decrease (approximately 40%) in CCT activity. To evaluate JNK in vivo, lung epithelial cells were infected with a replication defective adenoviral vector encoding murine JNK2 (Adv-JNK2) or an empty vector. Adv-JNK2 infection, unlike the empty vector, markedly increased JNK2 expression concomitant with increased incorporation of [32P]orthophosphate into endogenous CCTalpha. Although Adv-JNK2 infection only modestly reduced CCT activity, it reduced PtdCho synthesis by approximately 30% in cells. These observations suggest a role for JNK kinases as negative regulators of phospholipid synthesis in murine lung epithelia.
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Affiliation(s)
- Alan J Ryan
- Department of Internal Medicine, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA.
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Agassandian M, Zhou J, Tephly LA, Ryan AJ, Carter AB, Mallampalli RK. Oxysterols inhibit phosphatidylcholine synthesis via ERK docking and phosphorylation of CTP:phosphocholine cytidylyltransferase. J Biol Chem 2005; 280:21577-87. [PMID: 15788406 DOI: 10.1074/jbc.m412409200] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Surfactant deficiency contributes to acute lung injury and may result from the elaboration of bioactive lipids such as oxysterols. We observed that the oxysterol 22-hydroxycholesterol (22-HC) in combination with its obligate partner, 9-cis-retinoic acid (9-cis-RA), decreased surfactant phosphatidylcholine (PtdCho) synthesis by increasing phosphorylation of the regulatory enzyme CTP:phosphocholine cytidylyltransferase-alpha (CCTalpha). Phosphorylation of CCTalpha decreased its activity. 22-HC/9-cis-RA inhibition of PtdCho synthesis was blocked by PD98059 or dominant-negative ERK (p42 kinase). Overexpression of constitutively active MEK1, the kinase upstream of p42 kinase, increased CCTalpha phosphorylation. Expression of truncated CCTalpha mutants lacking proline-directed sites within the C-terminal phosphorylation domain partially blocked oxysterol-mediated inhibition of PtdCho synthesis. Mutagenesis of Ser315 within CCTalpha was both required and sufficient to confer significant resistance to 22-HC/9-cis-RA inhibition of PtdCho synthesis. A novel putative ERK-docking domain N-terminal to this phosphoacceptor site was mapped within the CCTalpha membrane-binding domain (residues 287-300). The results are the first demonstration of a physiologically relevant phosphorylation site and docking domain within CCTalpha that serve as targets for ERKs, resulting in inhibition of surfactant synthesis.
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Affiliation(s)
- Marianna Agassandian
- Department of Internal Medicine, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City 52242, USA
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Bogan MJ, Agnes GR, Pio F, Cornell RB. Interdomain and membrane interactions of CTP:phosphocholine cytidylyltransferase revealed via limited proteolysis and mass spectrometry. J Biol Chem 2005; 280:19613-24. [PMID: 15713672 DOI: 10.1074/jbc.m414028200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) is a multi-domain enzyme that regulates phosphatidylcholine synthesis. It converts to an active form upon binding cell membranes, and interdomain dissociations have been hypothesized to accompany this process. To identify these interdomain and membrane interactions, the tertiary structures of three forms of CCTalpha were probed by monitoring accessibility to proteases. Time-limited digestion with chymotrypsin or arginine C of soluble CCTalpha (CCT(sol)), phospholipid vesicle-bound CCT (CCT(mem)), and a soluble constitutively active CCT truncated at amino acid 236 generated complex mixtures of peptides that were resolved and identified by gel electrophoresis/immunoblotting and by matrix-assisted laser desorption/ionization-mass spectrometry, with or without coupling to capillary liquid chromatography. Identification of cleavage sites enabled assembly of peptide bond accessibility maps for each CCT form. Our results reveal a approximately 80-residue core within the catalytic domain (domain C) as the most inaccessible region in all three forms and the C-terminal phosphorylation domain as the most accessible. Membrane binding has little effect on the protease accessibility of these domains. To map the protease sites onto the catalytic domain, its three-dimensional structure was modeled from the atomic coordinates of glycerol-phosphate cytidylyltransferase (Protein Data Bank code 1COZ). The protease inaccessibility of most sites in domain C could be explained by burial or location within secondary structural elements. The accessibility of the N-terminal domain (domain N) was enhanced upon membrane binding. Residues Phe(234)-Leu(303) were inaccessible in CCT(mem), suggesting burial in the membrane. Surprisingly, residues Leu(274)-Leu(303) of this domain were also inaccessible in CCT(sol). We propose that this region is buried by interdomain contacts with domain N in CCT(sol). Membrane binding and burial of domain M in the lipid bilayer may disrupt this interaction, leading to increased exposure of sites in domain N.
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Affiliation(s)
- Michael J Bogan
- Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada
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Kent C. Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective. Biochim Biophys Acta Mol Cell Biol Lipids 2005; 1733:53-66. [PMID: 15749057 DOI: 10.1016/j.bbalip.2004.12.008] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2004] [Revised: 12/14/2004] [Accepted: 12/17/2004] [Indexed: 12/22/2022]
Abstract
Phosphatidylcholine is a prominent constituent of eukaryotic and some prokaryotic membranes. This Perspective focuses on the two enzymes that regulate its biosynthesis, choline kinase and CTP:phosphocholine cytidylyltransferase. These enzymes are discussed with respect to their molecular properties, isoforms, enzymatic activities, and structures, and the possible molecular mechanisms by which they participate in regulation of phosphatidylcholine levels in the cell.
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26
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Lagace TA, Ridgway ND. The rate-limiting enzyme in phosphatidylcholine synthesis regulates proliferation of the nucleoplasmic reticulum. Mol Biol Cell 2005; 16:1120-30. [PMID: 15635091 PMCID: PMC551478 DOI: 10.1091/mbc.e04-10-0874] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The nucleus contains a network of tubular invaginations of the nuclear envelope (NE), termed the nucleoplasmic reticulum (NR), implicated in transport, gene expression, and calcium homeostasis. Here, we show that proliferation of the NR, measured by the frequency of NE invaginations and tubules, is regulated by CTP:phosphocholine cytidylyltransferase-alpha (CCTalpha), the nuclear and rate-limiting enzyme in the CDP-choline pathway for phosphatidylcholine (PtdCho) synthesis. In Chinese hamster ovary (CHO)-K1 cells, fatty acids triggered activation and translocation of CCTalpha onto intranuclear tubules characteristic of the NR. This was accompanied by a twofold increase in NR tubules quantified by immunostaining for lamin A/C or the NE. CHO MT58 cells expressing a temperature-sensitive CCTalpha allele displayed reduced PtdCho synthesis and CCTalpha expression and minimal proliferation of the NR in response to oleate compared with CHO MT58 cells stably expressing CCTalpha. Expression of CCTalpha mutants in CHO58 cells revealed that both enzyme activity and membrane binding promoted NR proliferation. In support of a direct role for membrane binding in NR tubule formation, recombinant CCTalpha caused the deformation of liposomes into tubules in vitro. This demonstrates that a key nuclear enzyme in PtdCho synthesis coordinates lipid synthesis and membrane deformation to promote formation of a dynamic nuclear-cytoplasmic interface.
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Affiliation(s)
- Thomas A Lagace
- Atlantic Research Center, Departments of Pediatrics and Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7
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27
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Ridsdale R, Tseu I, Roth-Kleiner M, Wang J, Post M. Increased Phosphatidylcholine Production but Disrupted Glycogen Metabolism in Fetal Type II Cells of Mice That Overexpress CTP:Phosphocholine Cytidylyltransferase. J Biol Chem 2004; 279:55946-57. [PMID: 15498769 DOI: 10.1074/jbc.m407670200] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) is a rate-determining enzyme in the de novo synthesis of phosphatidylcholine (PtdCho). Alveolar type II cells synthesize large quantities of disaturated PtdCho, the surface-active agent of pulmonary surfactant, particularly at late gestation when the lung prepares itself for postnatal air breathing. To clarify the role of CCTalpha in lung surfactant maturation, we overexpressed CCTalpha(1-367) using the surfactant protein-C promoter. Lungs of transgenic mice were analyzed at day 18 of gestation (term = 19 days). Overexpression of CCTalpha(1-367) increased the synthesis and content of PtdCho in fetal type II cells isolated from the transgenic mice. Also, PtdCho content of fetal lung fluid was increased. No changes in surfactant protein content were detected. Interestingly, fetal type II cells of transgenic mice contained more glycogen than control cells. Incorporation studies with [U-(14)C]glucose demonstrated that overexpression of CCTalpha(1-367) in fetal type II cells increased glycogen synthesis without affecting glycogen breakdown. To determine which domain contributes to this glycogen phenotype, two additional transgenes were created overexpressing either CCTalpha(1-239) or CCTalpha(239-367). Glycogen synthesis and content were increased in fetal type II cells expressing CCTalpha(239-367) but not CCTalpha(1-239)(.) We conclude that overexpression of CCTalpha increases surfactant PtdCho synthesis without affecting surfactant protein levels but that it disrupts glycogen metabolism in differentiating type II cells via its regulatory domain.
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Affiliation(s)
- Ross Ridsdale
- Canadian Institutes of Health Research Group in Lung Development, Hospital for Sick Children Research Institute and Institute of Medical Sciences, University of Toronto, 555 University Avenue, Toronto M5G 1X8, Canada
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28
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Ng MNP, Kitos TE, Cornell RB. Contribution of lipid second messengers to the regulation of phosphatidylcholine synthesis during cell cycle re-entry. Biochim Biophys Acta Mol Cell Biol Lipids 2004; 1686:85-99. [PMID: 15522825 DOI: 10.1016/j.bbalip.2004.09.001] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2004] [Revised: 07/29/2004] [Accepted: 09/01/2004] [Indexed: 10/26/2022]
Abstract
During entry into the cell cycle a phosphatidylcholine (PC) metabolic cycle is activated. We have examined the hypothesis that PC synthesis during the G(0) to G(1) transition is controlled by one or more lipid products of PC turnover acting directly on the rate-limiting enzyme in the synthesis pathway, CTP: phosphocholine cytidylyltransferase (CCT). The acceleration of PC synthesis was two- to threefold during the first hour after addition of serum to quiescent IIC9 fibroblasts. The rate increased to approximately 15-fold above the basal rate during the second hour. The production of arachidonic acid, diacylglycerol (DAG), and phosphatidic acid (PA) preceded the second, rapid phase of PC synthesis. However, an increase in the cellular content of these lipid mediators was detected only for DAG. CCT activation and translocation to membranes accompanied the second phase of the PC synthesis acceleration. Bromoenol lactone (BEL), an inhibitor of calcium-independent phospholipase A(2) and PA phosphatase, blocked production of fatty acids and DAG, inhibited both phases of the PC synthesis response to serum, and reduced CCT activity and membrane affinity. The effect of BEL on PC synthesis was partially reversed by in situ generation of DAG via exogenous PC-specific phospholipase C to generate approximately 2-fold elevation in PC-derived DAG. Exogenous arachidonic acid also partially reversed the inhibition by BEL, but only at a concentration that generated a supra-physiological cellular content of free fatty acid. 1-Butanol, which blocks PA production, had no effect on DAG generation, or on PC synthesis. We conclude that fatty acids and DAG could contribute to the initial slow phase of the PC synthesis response. DAG is the most likely lipid regulator of CCT activity and the rapid phase of PC synthesis. However, processes other than direct activation of CCT by lipid mediators likely contribute to the highly accelerated phase during entry into the cell cycle.
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Affiliation(s)
- Michael N P Ng
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6
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29
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Helmink BA, Friesen JA. Characterization of a lipid activated CTP:phosphocholine cytidylyltransferase from Drosophila melanogaster. Biochim Biophys Acta Mol Cell Biol Lipids 2004; 1683:78-88. [PMID: 15238222 DOI: 10.1016/j.bbalip.2004.04.008] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2003] [Revised: 04/28/2004] [Accepted: 04/30/2004] [Indexed: 11/28/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) is an enzyme critical for cellular phosphatidylcholine (PC) synthesis, converting phosphocholine and cytidine 5'-triphosphate (CTP) to CDP-choline. We have isolated a cDNA encoding an isoform of CCT from Drosophila melanogaster and expressed the recombinant native and 6 x -His-tagged forms using a baculovirus expression system in Spodoptera frugiperda (Sf9) insect cells. Immunoblot using anti-phospho amino acid antibodies reveals the enzyme is phosphorylated on serine and threonine residues, but not tyrosine. The purified native enzyme exhibits a V(max) value of 1352+/-159 nmol CDP-choline/min/mg, a K(m) value of 0.50+/-0.09 mM for phosphocholine, and a K' (Hill constant) value of 0.72+/-0.10 mM for CTP. The 6 x -His-tagged enzyme has similar properties with a V(max) value of 2254+/-253 nmol CDP-choline/min/mg, a K(m) value of 0.63+/-0.13 mM for phosphocholine and a K' for CTP equal to 0.81+/-0.20 mM. Each form of the enzyme was activated to a similar extent by synthetic PC vesicles containing 50 mol% oleate. The efficiency of lipid activation was greatest using PC vesicles containing diphosphatidylglycerol (DPG), significantly less efficient activation was seen when phosphatidylserine (PS) and phosphatidylinositol (PI) were incorporated into vesicles, and PC alone or PC vesicles containing phosphatidylethanolamine were the least efficient enzyme activators.
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Affiliation(s)
- Beth Ann Helmink
- Department of Chemistry, Illinois State University, Normal, IL 61790, USA
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Banchio C, Schang LM, Vance DE. Phosphorylation of Sp1 by cyclin-dependent kinase 2 modulates the role of Sp1 in CTP:phosphocholine cytidylyltransferase alpha regulation during the S phase of the cell cycle. J Biol Chem 2004; 279:40220-6. [PMID: 15247247 DOI: 10.1074/jbc.m406468200] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
Phosphatidylcholine is the major lipid component in mammalian membranes. Phosphatidylcholine synthesis increases in C3H10T1/2 fibroblasts during the G(1) and S phases of the cell cycle. Previous studies demonstrated that the mRNA encoding CTP:phosphocholine cytidylyltransferase alpha (CTalpha) increases during S phase (Golfman, L. S., Bakovic, M., and Vance, D. E. (2001) J. Biol. Chem. 276, 43688-43692) and that this activation is driven by increased binding of Sp1 to the CTalpha promoter (Banchio, C., Schang, L. M., and Vance, D. E. (2003) J. Biol. Chem. 278, 32457-32464). We now demonstrate that cyclin-dependent kinase 2 (CDK2) phosphorylation of Sp1 activates CTalpha transcription during S phase. Sp1 binds in a phosphorylated state to the CTalpha promoter. Sp1 binding is enhanced by association with cyclin A/E and CDK2, both in vivo and in vitro. In cells that overexpress Sp1, co-expression of cyclin A and CDK2 induces a high and constant level of CTalpha expression, whereas reduction in the expression of cyclin A, cyclin E, and CDK2 eliminates the induction of CTalpha expression in S phase. Furthermore, CTalpha expression is decreased in cells overexpressing a dominant-negative form of CDK2 and in cells treated with the CDK2 kinase inhibitors roscovitine and olomoucine. These results enhance our understanding of the regulatory mechanisms involved in the expression of CTalpha in preparation for cell division.
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Affiliation(s)
- Claudia Banchio
- Department of Biochemistry, Canadian Institutes of Health Research Group in Molecular and Cell Biology of Lipids, University of Alberta, Edmonton, Alberta T6G 2S2, Canada
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Xie M, Smith JL, Ding Z, Zhang D, Cornell RB. Membrane Binding Modulates the Quaternary Structure of CTP:Phosphocholine Cytidylyltransferase. J Biol Chem 2004; 279:28817-25. [PMID: 15069071 DOI: 10.1074/jbc.m403311200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT), a key enzyme that controls phosphatidylcholine synthesis, is regulated by reversible interactions with membranes containing anionic lipids. Previous work demonstrated that CCT is a homodimer. In this work we show that the structure of the dimer interface is altered upon encountering membranes that activate CCT. Chemical cross-linking reactions were established which captured intradimeric interactions but not random CCT dimer collisions. The efficiency of capturing covalent cross-links with four different reagents was diminished markedly upon presentation of activating anionic lipid vesicles but not zwitterionic vesicles. Experiments were conducted to show that the anionic vesicles did not interfere with the chemistry of the cross-linking reactions and did not sequester available cysteine sites on CCT for reaction with the cysteine-directed cross-linking reagent. Thus, the loss of cross-linking efficiency suggested that contact sites at the dimer interface had increased distance or reduced flexibility upon binding of CCT to membranes. The regions of the enzyme involved in dimerization were mapped using three approaches: 1) limited proteolysis followed by cross-linking of fragments, 2) yeast two-hybrid analysis of interactions between select domains, and 3) disulfide bonding potential of CCTs with individual cysteine to serine substitutions for the seven native cysteines. We found that the N-terminal domain (amino acids 1-72) is an important participant in forming the dimer interface, in addition to the catalytic domain (amino acids 73-236). We mapped the intersubunit disulfide bond to the cystine 37 pair in domain N and showed that this disulfide is sensitive to anionic vesicles, implicating this specific region in the membrane-sensitive dimer interface.
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Affiliation(s)
- Mingtang Xie
- Department of Molecular Biology and Biochemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada
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Jackowski S, Rehg JE, Zhang YM, Wang J, Miller K, Jackson P, Karim MA. Disruption of CCTbeta2 expression leads to gonadal dysfunction. Mol Cell Biol 2004; 24:4720-33. [PMID: 15143167 PMCID: PMC416414 DOI: 10.1128/mcb.24.11.4720-4733.2004] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
There are two mammalian genes that encode isoforms of CTP:phosphocholine cytidylyltransferase (CCT), a key rate-controlling step in membrane phospholipid biogenesis. Quantitative determination of the CCT transcripts reveals that CCTalpha is ubiquitously expressed and is found at the highest levels in the testis and lung, with lower levels in the liver and ovary. CCTbeta2 is a very minor isoform in most tissues but is significantly expressed in the brain, lung, and gonads. CCTbeta3 is the third isoform recently discovered in mice and is expressed in the same tissues as CCTbeta2, with its highest level in testes. We investigated the role(s) of CCTbeta2 by generating knockout mice. The brains and lungs of mice lacking CCTbeta2 expression did not exhibit any overt defects. On the other hand, a large percentage of the CCTbeta2(-/-) females were sterile and their ovaries exhibited defective ovarian follicle development. The proportion of female CCTbeta2(-/-) mice with defective ovaries increased as the animals aged. The rare litters born from CCTbeta2(-/-) x CCTbeta2(-/0) matings had the normal number of pups. The abnormal ovarian histopathology was characterized by disorganization of the tissue in young adult mice and absence of follicles and ova in older mice, along with interstitial stromal cell hyperplasia which culminated in the emergence of tubulostromal ovarian tumors by 16 months of age. Grossly defective CCTbeta2(-/-) ovaries were associated with high follicle-stimulating (FSH) and luteinizing (LH) hormone levels. Male CCTbeta2(-/0) mice exhibited progressive multifocal testicular degeneration and reduced fertility but had normal FSH and LH levels. Thus, the most notable phenotype of CCTbeta2 knockout mice was gonad degeneration and reproductive deficiency. The results indicate that although CCTbeta2 is expressed at very low levels compared to the alpha-isoform, loss of CCTbeta2 expression causes a breakdown in the gonadal response to hormonal stimulation.
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Affiliation(s)
- Suzanne Jackowski
- Protein Science Division, Department of Infectious Diseases, St. Jude Children's Research Hospital, 332 N. Lauderdale, Memphis, TN 38105-2794, USA.
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Larvor MP, Cerdan R, Gumila C, Maurin L, Seta P, Roustan C, Vial H. Characterization of the lipid-binding domain of the Plasmodium falciparum CTP:phosphocholine cytidylyltransferase through synthetic-peptide studies. Biochem J 2003; 375:653-61. [PMID: 12901716 PMCID: PMC1223718 DOI: 10.1042/bj20031011] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2003] [Accepted: 08/05/2003] [Indexed: 11/17/2022]
Abstract
Phospholipid biosynthesis plays a key role in malarial infection and is regulated by CCT (CTP:phosphocholine cytidylyltransferase). This enzyme belongs to the group of amphitropic proteins which are regulated by reversible membrane interaction. To assess the role of the putative membrane-binding domain of Plasmodium falciparum CCT (PfCCT), we synthesized three peptides, K21, V20 and K54 corresponding to residues 274-294, 308-327 and 274-327 of PfCCT respectively. Conformational behaviour of the peptides, their ability to bind to liposomes and to destabilize lipid bilayers, and their insertion properties were investigated by different biophysical techniques. The intercalation mechanisms of the peptides were refined further by using surface-pressure measurements on various monolayers at the air/water interface. In the present study, we show that the three studied peptides are able to bind to anionic and neutral phospholipids, and that they present an alpha-helical conformation upon lipid binding. Peptides V20 and the full-length K54 intercalate their hydrophobic parts into an anionic bilayer and, to a lesser extent, a neutral one for V20. Peptide K21 interacts only superficially with both types of phospholipid vesicles. Adsorption experiments performed at the air/water interface revealed that peptide K54 is strongly surface-active in the absence of lipid. Peptide V20 presents an atypical behaviour in the presence of phosphatidylserine. Whatever the initial surface pressure of a phosphatidylserine film, peptide V20 and phosphatidylserine entities seem linked together in a special organization involving electrostatic and hydrophobic interactions. We showed that PfCCT presents different lipid-dependence properties from other studied CCTs. Although the lipid-binding domain seems to be located in the C-terminal region of the enzyme, as with the mammalian counterpart, the membrane anchorage, which plays a key role in the enzyme regulation, is driven by two alpha-helices, which behave differently from one another.
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Affiliation(s)
- Marie-Pierre Larvor
- CNRS (UMR 5539) Laboratoire de Dynamique Moléculaire des Interactions Membranaires, Université Montpellier II, cc 107, Place Eugène Bataillon, 34095 Montpellier, cedex 5, France
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Zhou J, Ryan AJ, Medh J, Mallampalli RK. Oxidized lipoproteins inhibit surfactant phosphatidylcholine synthesis via calpain-mediated cleavage of CTP:phosphocholine cytidylyltransferase. J Biol Chem 2003; 278:37032-40. [PMID: 12857760 PMCID: PMC2768468 DOI: 10.1074/jbc.m304316200] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We investigated effects of pro-atherogenic oxidized lipoproteins on phosphatidylcholine (PtdCho) biosynthesis in murine lung epithelial cells (MLE-12). Cells surface-bound, internalized, and degraded oxidized low density lipoproteins (Ox-LDL). Ox-LDL significantly reduced [3H]choline incorporation into PtdCho in cells by selectively inhibiting the activity of the rate-regulatory enzyme, CTP:phosphocholine cytdylyltransferase (CCT). Ox-LDL coordinately increased the cellular turnover of CCTalpha protein as determined by [35S]methionine pulse-chase studies by inducing the calcium-activated proteinase, calpain. Forced expression of calpain or exposure of cells to the calcium ionophore, A23187, increased CCTalpha degradation, whereas overexpression of the endogenous calpain inhibitor, calpastatin, attenuated Ox-LDL-induced CCTalpha degradation. The effects of Ox-LDL on CCTalpha breakdown were attenuated in calpain-deficient cells. In vitro calpain digestion of CCTalpha isolated from cells transfected with truncated or internal deletion mutants indicated multiple cleavage sites within the CCTalpha primary structure, leading to the generation of a 26-kDa (p26) fragment. Calpain hydrolysis of purified CCTalpha generated p26, which upon NH2-terminal sequencing localized a calpain attack site within the CCTalpha amino terminus. Expression of a CCTalpha mutant where the amino-terminal cleavage site and a putative carboxyl-terminal hydrolysis region were modified resulted in an enzyme that was significantly less sensitive to proteolytic cleavage and restored the ability of cells to synthesize surfactant PtdCho after Ox-LDL treatment. Thus, these results provide a critical link between proatherogenic lipoproteins and their metabolic target, CCTalpha, resulting in impaired surfactant metabolism.
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Affiliation(s)
- Jiming Zhou
- Department of Internal Medicine, The University of Iowa College of Medicine, Iowa City, Iowa 52242
| | - Alan J. Ryan
- Department of Internal Medicine, The University of Iowa College of Medicine, Iowa City, Iowa 52242
- Department of Veterans Affairs Medical Center, The University of Iowa College of Medicine, Iowa City, Iowa 52242
| | - Jheem Medh
- Department of Internal Medicine, The University of Iowa College of Medicine, Iowa City, Iowa 52242
| | - Rama K. Mallampalli
- Department of Internal Medicine, The University of Iowa College of Medicine, Iowa City, Iowa 52242
- Department of Biochemistry, The University of Iowa College of Medicine, Iowa City, Iowa 52242
- Department of Veterans Affairs Medical Center, The University of Iowa College of Medicine, Iowa City, Iowa 52242
- To whom correspondence should be addressed: Division of Pulmonary Diseases, Critical Care, and Occupational Medicine, Dept. of Internal Medicine, University of Iowa College of Medicine, Iowa City, IA 52242. Tel.: 319-356-1265; Fax: 319-353-6406;
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Karim M, Jackson P, Jackowski S. Gene structure, expression and identification of a new CTP:phosphocholine cytidylyltransferase beta isoform. BIOCHIMICA ET BIOPHYSICA ACTA 2003; 1633:1-12. [PMID: 12842190 DOI: 10.1016/s1388-1981(03)00067-2] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) is a key regulatory enzyme in phosphatidylcholine (PtdCho) biosynthesis, and in mammals, there are two distinct genes that encode enzymes that catalyze this reaction. This work defines the structures of both the murine CCT genes (Pcyt1a and Pcyt1b) and identifies a new CCT protein, CCTbeta3, with a unique amino terminus that arises from an alternate initiation exon. CCTalpha is expressed in all tissues, and is most abundant in liver, kidney and heart. A second CCTalpha transcript is described that initiates from a separate untranslated exon that is most highly expressed in testis. The CCTbeta isoforms are most highly expressed in brain and reproductive tissues. CCTbeta3 is not expressed in embryonic brain tissues, but is a significant transcript in the adult. These data suggest unique roles for the CCT protein isoforms in the differential regulation of PtdCho biosynthesis in specific tissues.
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Affiliation(s)
- Mohammad Karim
- Protein Science Division, Department of Infectious Diseases, St. Jude Children's Research Hospital, Memphis, TN 38105-2794, USA
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Ridsdale R, Tseu I, Wang J, Post M. CTP:phosphocholine cytidylyltransferase alpha is a cytosolic protein in pulmonary epithelial cells and tissues. J Biol Chem 2001; 276:49148-55. [PMID: 11583989 DOI: 10.1074/jbc.m103566200] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) is a rate-determining enzyme in de novo synthesis of phosphatidylcholine (PC). The lung requires a steady synthesis of PC for lung surfactant of which disaturated PC is the essential active agent. Surfactant synthesis occurs in alveolar type II cells. Studies with non-pulmonary cells have suggested that CCT is both a nuclear and cytoplasmic protein. The unusual requirements of the lung for PC synthesis and, therefore, CCT activity suggest a unique mechanism of regulation and possibly localization of CCT. The localization of CCT alpha in lung epithelial cells and, of greater consequence, lung tissues are yet unknown. Three isoforms of CCT have been identified. Herein we investigated the localization of the ubiquitously expressed CCT alpha isoform. To ascertain CCT alpha localization in lungs and lung-related epithelial cells, we employed a number of localization methods. Immunogold electron microscopy using polyclonal antibodies raised to either the carboxyl terminus, catalytic domain, or amino terminus of CCT alpha localized CCT alpha mostly to the exterior plasma membrane or regions of the endoplasmic reticulum (ER) in both A549 and MLE-15 epithelial lung cell lines and primary cultures of fetal rat lung epithelial cells. In contrast to other studies, little or no nuclear labeling was observed. Indirect immunofluorescence of these cells with anti-CCT alpha antibodies resulted in a similar distribution. Indirect visualization of both hemagglutinin- and FLAG-tagged CCT alpha as well as direct visualization of enhanced green fluorescence protein-CCT alpha fusion protein corroborated a cytoplasmic localization of CCT alpha in pulmonary cells. Moreover, analysis of lung tissue from fetal and adult mouse by either immunogold electron microscopy or indirect immunofluorescence yielded a strong cytoplasmic CCT alpha signal with virtually no nuclear localization in epithelial cells lining the airways. The cytoplasmic localization of CCT alpha in type II cells was further substantiated with transgenic mice overexpressing FLAG-tagged CCT alpha using the lung-specific human surfactant protein C (SP-C) promoter. We conclude that CCT alpha does not localize to the nucleus in pulmonary tissues, and, therefore, nuclear localization of CCT alpha is not a universal event.
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Affiliation(s)
- R Ridsdale
- Canadian Institutes for Health Research Group in Lung Development, Programme in Lung Biology Research, Hospital for Sick Children Research Institute, Institute of Medical Science, University of Toronto, Toronto M5G 1X8, Canada
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Friesen JA, Liu MF, Kent C. Cloning and characterization of a lipid-activated CTP:phosphocholine cytidylyltransferase from Caenorhabditis elegans: identification of a 21-residue segment critical for lipid activation. BIOCHIMICA ET BIOPHYSICA ACTA 2001; 1533:86-98. [PMID: 11566446 DOI: 10.1016/s1388-1981(01)00145-7] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
The genome of the nematode Caenorhabditis elegans contains several genes that appear to encode proteins similar to CTP:phosphocholine cytidylyltransferase (CCT). We have isolated a 1044-nucleotide cDNA clone from a C. elegans cDNA library that encodes the 347-amino acid version of CCT that is most similar to previously-identified CCTs. Native and His-tagged forms were expressed and purified using a baculovirus expression system. The enzyme was maximally activated by 5 microM phosphatidylcholine:oleate (50:50) vesicles with a k(cat) value in the presence of lipid 37-fold greater than the k(cat) value in the absence of lipid. To localize the region of C. elegans CCT critical for lipid activation, a series of C-terminal truncation mutants was analyzed. CCT truncated after amino acids 225 or 245 was quite active in the absence of lipids and not further activated in the presence of lipids, supporting the concept that the lipid-activation segment is inhibitory to catalysis in the absence of lipids. CCT truncated after amino acids 266, 281, or 319 was activated by lipid similar to wild-type enzyme. Kinetic analysis in the absence of lipid revealed the lipid-independent CCT truncated after amino acid 245 to have a k(cat) value 15-fold greater than either full-length CCT or CCT truncated after amino acid 266. We conclude that elements critical for activation of C. elegans CCT by lipids are contained within amino acids 246-266, that this region is inhibitory in the absence of lipids, and that the inhibition is relieved by the association of the enzyme with lipid.
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Affiliation(s)
- J A Friesen
- Department of Biological Chemistry, 4417 Medical Science I, University of Michigan Medical Center, Ann Arbor, MI 48109-0606, USA
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Leppimäki P, Mattinen J, Slotte JP. Sterol-induced upregulation of phosphatidylcholine synthesis in cultured fibroblasts is affected by the double-bond position in the sterol tetracyclic ring structure. EUROPEAN JOURNAL OF BIOCHEMISTRY 2000; 267:6385-94. [PMID: 11029581 DOI: 10.1046/j.1432-1327.2000.01726.x] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
We have examined how a specific enrichment of cultured fibroblasts with various sterols (cholesterol, lathosterol, 7-dehydrocholesterol, allocholesterol and dihydrocholesterol) regulate synthesis de novo of phosphatidylcholine, cholesterol and cholesteryl (or steryl) esters in human skin fibroblasts. When human skin fibroblasts were incubated for 1 h with 130 microM cholesterol/CyD complexes, the mass of cellular free cholesterol increased by 100 nmol.mg-1 protein (from 90 nmol.mg-1 to 190 nmol.mg-1 protein). A similar exposure of cells to different sterol/CyD complexes increased the cell sterol content between 38 and 181 nmol sterol per mg cell protein. In cholesterol-enriched cells, the rate of phosphatidylcholine synthesis was doubled compared to control cells, irrespective of the type of precursor used ([3H]choline, [3H]palmitic acid, or [14C]glycerol). Enrichment of fibroblasts with 7-dehydrocholesterol, allocholesterol, or dihydrocholesterol also upregulated phosphatidylcholine synthesis, whereas cells enriched with lathosterol failed to upregulate their phosphatidylcholine synthesis. The activity of membrane-bound CTP:phosphocholine cytidylyltransferase, the rate-limiting enzyme, was increased by 47 +/- 4% in cholesterol-enriched cells whereas its activity was unchanged in lathosterol-enriched cells. Sterol enrichment with all tested sterols (including lathosterol) down-regulated acetate-incorporation into cholesterol, and upregulated sterol esterification in the sterol-enriched fibroblasts. Using 31P-NMR to measure the lamellar-to-hexagonal (Lalpha-HII) phase transition in multilamellar lipid dispersions, lathosterol-containing membranes underwent their transition at significantly higher temperatures compared to membranes containing any of the other sterols. In a system with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine and either cholesterol or lathosterol (70:30 mol/mol), differential scanning calorimetry also revealed that the Lalpha-HII-transition occurred at a higher temperature with lathosterol compared to either cholesterol, allocholesterol, or dihydrocholesterol. These findings together suggest that there may exist a correlation between the propensity of a sterol to stabilize the Lalpha-HII-transition and its capacity to upregulate the activity of CTP:phosphocholine cytidylyltransferase in cells.
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Affiliation(s)
- P Leppimäki
- Department of Biochemistry, Abo Akademi University, Turku, Finland
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Lykidis A, Jackowski S. Regulation of mammalian cell membrane biosynthesis. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 2000; 65:361-93. [PMID: 11008493 DOI: 10.1016/s0079-6603(00)65010-9] [Citation(s) in RCA: 89] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
This review explores current information on the interrelationship between phospholipid biochemistry and cell biology. Phosphatidylcholine is the most abundant phospholipid and it biosynthesis has been studied extensively. The choline cytidylyltransferase regulates phosphatidylcholine production, and recent advances in our understanding of the mechanisms that govern cytidylyltransferase include the discovery of multiple isoforms and a more complete understanding of the lipid regulation of enzyme activity. Similarities between phosphatidylcholine formation and the phosphatidylethanolamine and phosphatidylinositol biosynthetic pathways are discussed, together with current insight into control mechanisms. Membrane phospholipid doubling during cell cycle progression is a function of periodic biosynthesis and degradation. Membrane homeostasis is maintained by a phospholipase A-mediated degradation of excess phospholipid, whereas insufficient phosphatidylcholine triggers apoptosis in cells.
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Affiliation(s)
- A Lykidis
- Department of Biochemistry, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
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DeLong CJ, Qin L, Cui Z. Nuclear localization of enzymatically active green fluorescent protein-CTP:phosphocholine cytidylyltransferase alpha fusion protein is independent of cell cycle conditions and cell types. J Biol Chem 2000; 275:32325-30. [PMID: 10918057 DOI: 10.1074/jbc.m004644200] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To address the recent controversy about the subcellular localization of CTP:phosphocholine cytidylyltransferase alpha (CTalpha), this study was designed to visualize green fluorescent protein (GFP). CTalpha fusion proteins directly and continuously under different conditions of cell cycling and in various cell lines. The GFP. CTalpha fusion proteins were enzymatically active and capable of rescuing mutant cells with a temperature-sensitive CT. The expressed GFP.CTalpha fusion protein was localized to the nucleus in all cell lines and required the N-terminal nuclear targeting sequence. Serum depletion/replenishment did not cause shuttling of CTalpha between the nucleus and cytoplasm. Moreover, the subcellular localization of CTalpha was examined continuously through all stages of the cell cycle in synchronized cells. No shuttling of CTalpha between the nucleus and cytoplasm was observed at any stage of the cell cycle. Stimulation of cells with oleate had no effect on the localization of CTalpha. The GFP.CTalpha lacking the nuclear targeting sequence stayed exclusively in the cytoplasm. Regardless of their localization, the GFP.CTalpha fusion proteins were equally active for phosphatidylcholine synthesis and mutant rescue. We conclude that the nuclear localization of CTalpha is a biological event independent of cell cycle in most mammalian cells and is unrelated to activation of phosphatidylcholine synthesis.
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Affiliation(s)
- C J DeLong
- Department of Biochemistry, Wake Forest University, School of Medicine, Winston-Salem, North Carolina 27157, USA
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41
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Cornell RB, Northwood IC. Regulation of CTP:phosphocholine cytidylyltransferase by amphitropism and relocalization. Trends Biochem Sci 2000; 25:441-7. [PMID: 10973058 DOI: 10.1016/s0968-0004(00)01625-x] [Citation(s) in RCA: 173] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Phosphatidylcholine (PC) synthesis in animal cells is generally controlled by cytidine 5'-triphosphate (CTP):phosphocholine cytidylyltransferase (CCT). This enzyme is amphitropic, that is, it can interconvert between a soluble inactive form and a membrane-bound active form. The membrane-binding domain of CCT is a long amphipathic alpha helix that responds to changes in the physical properties of PC-deficient membranes. Binding of this domain to membranes activates CCT by relieving an inhibitory constraint in the catalytic domain. This leads to stimulation of PC synthesis and maintenance of membrane PC content. Surprisingly, the major isoform, CCT alpha, is localized in the nucleus of many cells. Recently, a new level of its regulation has emerged with the discovery that signals that stimulate PC synthesis recruit CCT alpha from an inactive nuclear reservoir to a functional site on the endoplasmic reticulum.
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Affiliation(s)
- R B Cornell
- Dept of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, BC, Canada V5A 1S6.
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Bernstein LS, Grillo AA, Loranger SS, Linder ME. RGS4 binds to membranes through an amphipathic alpha -helix. J Biol Chem 2000; 275:18520-6. [PMID: 10764749 DOI: 10.1074/jbc.m000618200] [Citation(s) in RCA: 107] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
RGS4, a mammalian GTPase-activating protein for G protein alpha subunits, requires its N-terminal 33 amino acids for plasma membrane localization and biological activity (Srinivasa, S. P., Bernstein, L. S., Blumer, K. J., and Linder, M. E. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 5584-5589). In this study, we tested the hypothesis that the N-terminal domain mediates membrane binding by forming an amphipathic alpha-helix. RGS4 bound to liposomes containing anionic phospholipids in a manner dependent on the first 33 amino acids. Circular dichroism spectroscopy of a peptide corresponding to amino acids 1-31 of RGS4 revealed that the peptide adopted an alpha-helical conformation in the presence of anionic phospholipids. Point mutations that either neutralized positive charges on the hydrophilic face or substituted polar residues on the hydrophobic face of the model helix disrupted plasma membrane targeting and biological activity of RGS4 expressed in yeast. Recombinant mutant proteins were active as GTPase-activating proteins in solution but exhibited diminished binding to anionic liposomes. Peptides corresponding to mutants with the most pronounced phenotypes were also defective in forming an alpha-helix as measured by circular dichroism spectroscopy. These results support a model for direct interaction of RGS4 with membranes through hydrophobic and electrostatic interactions of an N-terminal alpha-helix.
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Affiliation(s)
- L S Bernstein
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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Lykidis A, Baburina I, Jackowski S. Distribution of CTP:phosphocholine cytidylyltransferase (CCT) isoforms. Identification of a new CCTbeta splice variant. J Biol Chem 1999; 274:26992-7001. [PMID: 10480912 DOI: 10.1074/jbc.274.38.26992] [Citation(s) in RCA: 118] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase is a major regulator of phosphatidylcholine biosynthesis. A single isoform, CCTalpha, has been studied extensively and a second isoform, CCTbeta, was recently identified. We identify and characterize a third cDNA, CCTbeta2, that differs from CCTbeta1 at the carboxyl-terminal end and is predicted to arise as a splice variant of the CCTbeta gene. Like CCTalpha, CCTbeta2 is heavily phosphorylated in vivo, in contrast to CCTbeta1. CCTbeta1 and CCTbeta2 mRNAs were differentially expressed by the human tissues examined, whereas CCTalpha was more uniformly represented. Using isoform-specific antibodies, both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells, in contrast to CCTalpha which resided in the nucleus in addition to associating with the endoplasmic reticulum. CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells, just as CCTalpha and CCTbeta1 supporting proliferation at the nonpermissive conditions. Overexpression experiments did not reveal discrete physiological functions for the three isoforms that catalyze the same biochemical reaction; however, the differential cellular localization and tissue-specific distribution suggest that CCTbeta1 and CCTbeta2 may play a role that is distinct from ubiquitously expressed CCTalpha.
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Affiliation(s)
- A Lykidis
- Department of Biochemistry, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
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Johnson JE, Cornell RB. Amphitropic proteins: regulation by reversible membrane interactions (review). Mol Membr Biol 1999; 16:217-35. [PMID: 10503244 DOI: 10.1080/096876899294544] [Citation(s) in RCA: 221] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/16/2022]
Abstract
What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.
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Affiliation(s)
- J E Johnson
- Institute of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, BC, Canada
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Friesen JA, Campbell HA, Kent C. Enzymatic and cellular characterization of a catalytic fragment of CTP:phosphocholine cytidylyltransferase alpha. J Biol Chem 1999; 274:13384-9. [PMID: 10224101 DOI: 10.1074/jbc.274.19.13384] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To probe the mechanism of lipid activation of CTP:phosphocholine cytidylyltransferase (CCTalpha), we have characterized a catalytic fragment of the enzyme that lacks the membrane-binding segment. The kinetic properties of the purified fragment, CCTalpha236, were characterized, as well as the effects of expressing the fragment in cultured cells. CCTalpha236 was truncated after residue 236, which corresponds to the end of the highly conserved catalytic domain. The activity of purified CCTalpha236 was independent of lipids and about 50-fold higher than the activity of wild-type CCTalpha assayed in the absence of lipids, supporting a model in which the membrane-binding segment functions as an inhibitor of the catalytic domain. The kcat/Km values for CCTalpha236 were only slightly lower than those for lipid-activated CCTalpha. The importance of the membrane-binding segment in vivo was tested by expression of CCTalpha236 in CHO58 cells, a cell line that is temperature-sensitive for growth and CCTalpha activity. Expression of wild-type CCTalpha in these cells complemented the defective growth phenotype when the cells were cultured in complete or delipidated fetal bovine serum. Expression of CCTalpha236, however, did not complement the growth phenotype in the absence of serum lipids. These cells were capable of making phosphatidylcholine in the delipidated medium, so the inability of the cells to grow was not due to defective phosphatidylcholine synthesis. Supplementation of the delipidated medium with an unsaturated fatty acid allowed growth of CHO58 cells expressing CCTalpha236. These results indicate that the membrane-binding segment of CCTalpha has an important role in cellular lipid metabolism.
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Affiliation(s)
- J A Friesen
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan 48109-0606, USA
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Clement JM, Kent C. CTP:phosphocholine cytidylyltransferase: insights into regulatory mechanisms and novel functions. Biochem Biophys Res Commun 1999; 257:643-50. [PMID: 10208837 DOI: 10.1006/bbrc.1999.0512] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
A key regulatory enzyme in phosphatidylcholine biosynthesis, CTP:cholinephosphate cytidylyltransferase (CCT), catalyzes the formation of CDP-choline. This review discusses the essential features of CCT and addresses intriguing new insights into the catalytic and regulatory properties of this complex enzyme. Characterization of a lipid-binding segment in rat CCT is described and the role of lipids in CCT activation is discussed. An analysis of the phosphorylation domain is presented and possible physiological rationales for reversible phosphorylation of CCT are discussed. The nuclear localization of CCT is examined in the context of multiple CCT isoforms, as is recent evidence establishing a potential link between CCT activity and vesicular transport.
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Affiliation(s)
- J M Clement
- Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan, 48109, USA
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47
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Tang W, Walsh A, Tabas I. Macrophage-targeted CTP:phosphocholine cytidylyltransferase (1-314) transgenic mice. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1437:301-16. [PMID: 10101264 DOI: 10.1016/s1388-1981(99)00023-2] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CT) is a rate-limiting and complexly regulated enzyme in phosphatidylcholine (PC) biosynthesis and is important in the adaptation of macrophages to cholesterol loading. The goal of the present study was to use transgenesis to study the CT reaction in differentiated macrophages in vivo. We successfully created macrophage-targeted transgenic mice that overexpress a truncated form of CT, called CT-314. Sonicated homogenates of peritoneal macrophages overexpressing CT-314 protein demonstrated a two-fold increase in CT activity in vitro compared with homogenates from nontransgenic macrophages. CT-314 macrophages, however, demonstrated no increase in CT activity or PC biosynthesis in vivo. This finding could not be explained simply by intracellular mistargeting of CT-314, by the inability of CT-314 to associate with cellular membranes, or by substrate limitation. To further probe the mechanism, an in vitro assay using intact nuclei was developed in an attempt to preserve interactions between CT, which is primarily a nuclear enzyme in macrophages, and other nuclear molecules. This intact-nuclei assay faithfully reproduced the situation observed in living macrophages, namely, no significant increase in CT activity despite increased CT-314 protein. In contrast, CT activity in sonicated nuclei from CT-314 macrophages was substantially higher than that from nontransgenic macrophages. Thus, a sonication-sensitive interaction between excess CT and one or more nuclear molecules may be responsible for the limitation of CT activity in CT-314 macrophages. These data represent the first report of a CT transgenic animal and the first study of a differentiated cell type with excess CT.
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Affiliation(s)
- W Tang
- The Departments of Medicine, and Anatomy and Cell Biology, College of Physicians and Surgeons, Columbia University, 630 W. 168th Street, New York, NY 10032, USA
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48
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Weinhold PA, Barrett D. Studies on the regulation of CTP:phosphocholine cytidylyltransferase using permeabilized HEP G2 cells: evidence that both active and inactive enzyme are membrane-bound. BIOCHIMICA ET BIOPHYSICA ACTA 1998; 1391:307-19. [PMID: 9555069 DOI: 10.1016/s0005-2760(97)00206-3] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
To obtain more insight into the mechanisms regulating CTP:phosphocholine cytidylyltransferase (CT), we determined the effect of oleate treatment on the rate of CT release from permeabilized Hep G2 cells and the distribution of the CT remaining in the permeabilized cells. When we permeabilized untreated cells in pH 7.5 buffer containing 0.15 M KCl, the rate of CT release was much slower than the release of lactate dehydrogenase. Oleate treatment caused a further decrease in CT release from cells. In untreated cells, 70-80% of the CT remaining in cells 10 min after permeabilization was recovered as soluble CT. Oleate treatment increased the amount of bound CT but over 50% of the CT in cells 10 min after permeabilization was recovered as soluble CT. In both control and oleate-treated cells, the increase in CT release with time correlated with a decrease in the amount of CT recovered from permeabilized cells as soluble CT. These results suggested that CT existed in a form that was not immediately available for release from permeabilized cells, but was recovered in the soluble fraction after cell disruption. When cells were permeabilized in 10 mM imidazole-20% glycerol-5 mM Mg2+ pH 6.5, over 80% of CT in control and over 90% of CT in oleate-treated cells was recovered bound to the particulate fraction. Essentially no CT was released from the cells. The recovery of CT in the particulate fraction required Mg2+ to be present when permeabilization was initiated. The addition of Mg2+, after cells were disrupted, did not increase CT in the particulate fraction. In untreated cells, 50% of bound CT was active. Oleate treatment increased the amount of active CT in the particulate fraction to over 70% of total. About 50% of particulate CT in untreated cells but only 15% in oleate-treated cells was extracted with 0.15 M KCl. Inactive CT was preferentially extracted by KCl. The bound CT was recovered in isolated nuclei. Overall, the results suggested that both inactive and active CT are bound to nuclear membranes, and that the activation of CT involves conversion of CT loosely bound to membrane to a form more tightly bound to membranes perhaps by hydrophobic interaction with phospholipids. This model does not involve translocation from a soluble pool.
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Affiliation(s)
- P A Weinhold
- Veterans Affairs Medical Center and Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, MI, USA.
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Abstract
CTP:phosphocholine cytidylyltransferase (CCT) catalyzes the synthesis of CDP-choline and is regulatory for phosphatidylcholine biosynthesis. This review focuses on recent developments in understanding the catalytic and regulatory mechanisms of this enzyme. Evidence for the nuclear localization of the enzyme is discussed, as well as evidence suggesting cytoplasmic localization. A comparison of the catalytic domains of CCTs from a wide variety of organisms is presented, highlighting a large number of completely conserved residues. Work implying a role for the conserved HXGH sequence in catalysis is described. The membrane-binding domain in rat CCT has been defined, and the role of lipids in activating the enzyme is discussed. The identification of the phosphorylation domain is described, as well as approaches to understand the role of phosphorylation in enzyme activity. Other possible control mechanisms such as enzyme degradation and gene expression are presented.
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Affiliation(s)
- C Kent
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor 48109-0606, USA.
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50
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Yeo HJ, Larvor MP, Ancelin ML, Vial HJ. Plasmodium falciparum CTP:phosphocholine cytidylyltransferase expressed in Escherichia coli: purification, characterization and lipid regulation. Biochem J 1997; 324 ( Pt 3):903-10. [PMID: 9210415 PMCID: PMC1218507 DOI: 10.1042/bj3240903] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The Plasmodium falciparum CTP:phosphocholine cytidylyltransferase (PfCCT) has been isolated from an overexpressing strain of Escherichia coli. The plasmid pETPfCCT mediated the overexpression of the full-length polypeptide directly. The recombinant protein corresponded to 6-9% of the total cellular proteins and was found essentially in the insoluble membrane fraction. Urea at 6 M was used to solubilize the recombinant protein from the insoluble fraction. The CCT activity was restored upon the removal of urea, and the protein was subsequently purified to homogeneity on a Q-Sepharose column. Approx. 1.4 mg of pure enzyme was obtained from a 250 ml culture of E. coli. Biochemical properties, including in vitro substrate specificity and enzymic characterization, were assessed. The lipid regulation of the recombinant plasmodial CCT activity was characterized for the first time. The Km values were 0.49+/-0.03 mM (mean+/-S.E.M.) for phosphocholine and 10.9+/-0.5 mM for CTP in the presence of lipid activators (oleic acid/egg phosphatidylcholine vesicles), whereas the Km values were 0.66+/-0.07 mM for phosphocholine and 28.9+/-0.8 mM for CTP in the absence of lipid activators. The PfCCT activity was stimulated to the same extent in response to egg phosphatidylcholine vesicles containing anionic lipids, such as oleic acid, cardiolipin and phosphatidylglycerol, and was insensitive or slightly sensitive to PC vesicles containing neutral lipids, such as diacylglycerol and monoacylglycerol. Furthermore, the stimulated enzyme activity by oleic acid was antagonized by the cationic aminolipid sphingosine. These lipid-dependence properties place the parasite enzyme intermediately between the mammalian enzymes and the yeast enzyme.
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Affiliation(s)
- H J Yeo
- Laboratoire de Dynamique Moléculaire des Interactions Membranaires, CNRS UMR 5539, Université Montpellier II, case 107, 34095 Montpellier Cedex 5, France
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