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Nurmi H, Backman AE, Halin J, Lönnfors M, Blom T, Roos-Mattjus P, Mattjus P. Glycolipid transfer protein knockout disrupts vesicle trafficking to the plasma membrane. J Biol Chem 2023; 299:104607. [PMID: 36924944 PMCID: PMC10140181 DOI: 10.1016/j.jbc.2023.104607] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2022] [Revised: 02/16/2023] [Accepted: 03/06/2023] [Indexed: 03/17/2023] Open
Abstract
The glycolipid transfer protein (GLTP) has been linked to many cellular processes aside from its best-known in vitro function as a lipid transport protein. It has been proposed to act as a sensor and regulator of glycosphingolipid homeostasis in cells. Furthermore, through its previously determined interaction with the endoplasmic reticulum (ER) membrane protein VAP-A (vesicle-associated membrane protein-associated protein A), GLTP may also be involved in facilitating vesicular transport in cells. In this study, we characterized the phenotype of CRISPR/Cas9-mediated GLTP knockout HeLa cells. We showed that motility, three-dimensional growth, and cellular metabolism were all altered by GLTP knockout. Expression of a GLTP mutant incapable of binding VAP disrupted cell spheroid formation, indicating that the GLTP-VAP interaction is linked to cellular adhesion, cohesion, and three-dimensional growth. Most notably, we found evidence that GLTP, through its interaction with VAP-A, affects vesicular trafficking, marking the first cellular process discovered to be directly impacted by a change in GLTP expression.
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Affiliation(s)
- Henrik Nurmi
- Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Artillerigatan 6A, III, 20520 Turku, Finland
| | - AndersP E Backman
- Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Artillerigatan 6A, III, 20520 Turku, Finland
| | - Josefin Halin
- Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Artillerigatan 6A, III, 20520 Turku, Finland
| | - Max Lönnfors
- Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Artillerigatan 6A, III, 20520 Turku, Finland
| | - Tomas Blom
- Department of Anatomy and Research Programs Unit, Faculty of Medicine, University of Helsinki, Helsinki, Finland
| | - Pia Roos-Mattjus
- Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Artillerigatan 6A, III, 20520 Turku, Finland
| | - Peter Mattjus
- Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Artillerigatan 6A, III, 20520 Turku, Finland.
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GLTP Is a Potential Prognostic Biomarker and Correlates with Immunotherapy Efficacy in Cervical Cancer. DISEASE MARKERS 2022; 2022:9109365. [PMID: 35845139 PMCID: PMC9282991 DOI: 10.1155/2022/9109365] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/11/2022] [Revised: 06/22/2022] [Accepted: 06/24/2022] [Indexed: 11/21/2022]
Abstract
Cervical cancer (CC) is the fourth most commonly diagnosed cancer in women worldwide. The prognosis of CC patients remains poor. The objective of our study was to explore the potential of glycolipid transfer protein (GLTP) in predicting the prognosis of CC and patients' response to immunotherapy. The expression of GLTP was determined using TCGA and GEO datasets. The prognostic value of GLTP in CC patients was analyzed using Kaplan-Meier analysis and multivariate analysis. The relationships between BTBD10 and immunological checkpoints, immune checkpoint genes, and ferroptosis-related genes were analyzed to explore the impact of GLTP on CC immunotherapy. According to the dysregulated expressions of BTBD10, the IC50 distribution of various targeted medicines was studied. In this study, we found that GLTP expression was distinctly upregulated in CC specimens. However, Kaplan-Meier assays showed that CC patients with low GLTP expressions tended to exhibit a shorter overall survival. Importantly, multivariate assays revealed that GLTP expression was an independent prognostic factor for CC patients. Moreover, we observed that GLTP expression was related to CD4+ T cells, macrophages, and dendritic cells (DCs). Meanwhile, GLTP expressions were associated with those of immune checkpoints, ferroptosis-related genes, and m6A-related genes. The IC50 of Cisplatin, Docetaxel, and Paclitaxel was lower in the high-GLTP-expressing group. Taken together, GLTP was expected to be a prognostic and immunotherapeutic marker for CC.
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Gao YG, McDonald J, Malinina L, Patel DJ, Brown RE. Ceramide-1-phosphate transfer protein promotes sphingolipid reorientation needed for binding during membrane interaction. J Lipid Res 2022; 63:100151. [PMID: 34808193 PMCID: PMC8953657 DOI: 10.1016/j.jlr.2021.100151] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2021] [Revised: 11/15/2021] [Accepted: 11/16/2021] [Indexed: 12/16/2022] Open
Abstract
Lipid transfer proteins acquire and release their lipid cargoes by interacting transiently with source and destination biomembranes. In the GlycoLipid Transfer Protein (GLTP) superfamily, the two-layer all-α-helical GLTP-fold defines proteins that specifically target sphingolipids (SLs) containing either sugar or phosphate headgroups via their conserved but evolutionarily-modified SL recognitions centers. Despite comprehensive structural insights provided by X-ray crystallography, the conformational dynamics associated with membrane interaction and SL uptake/release by GLTP superfamily members have remained unknown. Herein, we report insights gained from molecular dynamics (MD) simulations into the conformational dynamics that enable ceramide-1-phosphate transfer proteins (CPTPs) to acquire and deliver ceramide-1-phosphate (C1P) during interaction with 1-palmitoyl-2-oleoyl phosphatidylcholine bilayers. The focus on CPTP reflects this protein's involvement in regulating pro-inflammatory eicosanoid production and autophagy-dependent inflammasome assembly that drives interleukin (IL-1β and IL-18) production and release by surveillance cells. We found that membrane penetration by CPTP involved α-6 helix and the α-2 helix N-terminal region, was confined to one bilayer leaflet, and was relatively shallow. Large-scale dynamic conformational changes were minimal for CPTP during membrane interaction or C1P uptake except for the α-3/α-4 helices connecting loop, which is located near the membrane interface and interacts with certain phosphoinositide headgroups. Apart from functioning as a shallow membrane-docking element, α-6 helix was found to adeptly reorient membrane lipids to help guide C1P hydrocarbon chain insertion into the interior hydrophobic pocket of the SL binding site.These findings support a proposed 'hydrocarbon chain-first' mechanism for C1P uptake, in contrast to the 'lipid polar headgroup-first' uptake used by most lipid-transfer proteins.
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Affiliation(s)
- Yong-Guang Gao
- Hormel Institute, University of Minnesota, Austin, MN, USA.
| | | | - Lucy Malinina
- Hormel Institute, University of Minnesota, Austin, MN, USA
| | - Dinshaw J Patel
- Structural Biology Program, Memorial Sloan Kettering Cancer Center, New York, NY, USA
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Zhang Y, Zhang X, Lu M, Zou X. Ceramide-1-phosphate and its transfer proteins in eukaryotes. Chem Phys Lipids 2021; 240:105135. [PMID: 34499882 DOI: 10.1016/j.chemphyslip.2021.105135] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2021] [Revised: 07/31/2021] [Accepted: 09/02/2021] [Indexed: 02/07/2023]
Abstract
Ceramide-1-phosphate (C1P) is a bioactive phosphorylated sphingolipid (SL), produced through the direct phosphorylation of ceramide by ceramide kinase. It plays important roles in regulating cell survival, migration, apoptosis and autophagy and is involved in inflammasome assembly/activation, which can stimulate group IVA cytosolic phospholipase A2α and subsequently increase the levels of arachidonic acid and pro-inflammatory cytokines. Human C1P transfer protein (CPTP) can selectively transport C1P from the Golgi apparatus to specific cellular sites through a non-vesicular mechanism. Human CPTP also affects specific SL levels, thus regulating cell SL homeostasis. In addition, human CPTP plays a crucial role in the regulation of autophagy, inflammation and cell death; thus, human CPTP is closely associated with autophagy and inflammation-related diseases such as cardiovascular and neurodegenerative diseases, and cancers. Therefore, illustrating the functions and mechanisms of human CPTP is important for providing the research foundations for targeted therapy. The key human CPTP residues for C1P recognition and binding are highly conserved in eukaryotic orthologs, while the human CPTP homolog in Arabidopsis (accelerated cell death 11) also exhibits selective inter-membrane transfer of phyto-C1P. These results demonstrate that C1P transporters play fundamental roles in SL metabolism in cells. The present review summarized novel findings of C1P and its TPs in eukaryotes.
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Affiliation(s)
- Yanqun Zhang
- Department of Oncology, Xiangya Hospital, Central South University, Changsha, Hunan, 410008, PR China
| | - Xiangyu Zhang
- Affiliated Stomatology Hospital of Guilin Medical University, Guilin, 541004, PR China
| | - Mengyun Lu
- Affiliated Stomatology Hospital of Guilin Medical University, Guilin, 541004, PR China
| | - Xianqiong Zou
- Affiliated Stomatology Hospital of Guilin Medical University, Guilin, 541004, PR China; College of Biotechnology, Guilin Medical University, Guilin, 541100, PR China.
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Mishra SK, Gao YG, Zou X, Stephenson DJ, Malinina L, Hinchcliffe EH, Chalfant CE, Brown RE. Emerging roles for human glycolipid transfer protein superfamily members in the regulation of autophagy, inflammation, and cell death. Prog Lipid Res 2020; 78:101031. [PMID: 32339554 DOI: 10.1016/j.plipres.2020.101031] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2019] [Revised: 02/27/2020] [Accepted: 02/27/2020] [Indexed: 12/14/2022]
Abstract
Glycolipid transfer proteins (GLTPs) were first identified over three decades ago as ~24kDa, soluble, amphitropic proteins that specifically accelerate the intermembrane transfer of glycolipids. Upon discovery that GLTPs use a unique, all-α-helical, two-layer 'sandwich' architecture (GLTP-fold) to bind glycosphingolipids (GSLs), a new protein superfamily was born. Structure/function studies have provided exquisite insights defining features responsible for lipid headgroup selectivity and hydrophobic 'pocket' adaptability for accommodating hydrocarbon chains of differing length and unsaturation. In humans, evolutionarily-modified GLTP-folds have been identified with altered sphingolipid specificity, e. g. ceramide-1-phosphate transfer protein (CPTP), phosphatidylinositol 4-phosphate adaptor protein-2 (FAPP2) which harbors a GLTP-domain and GLTPD2. Despite the wealth of structural data (>40 Protein Data Bank deposits), insights into the in vivo functional roles of GLTP superfamily members have emerged slowly. In this review, recent advances are presented and discussed implicating human GLTP superfamily members as important regulators of: i) pro-inflammatory eicosanoid production associated with Group-IV cytoplasmic phospholipase A2; ii) autophagy and inflammasome assembly that drive surveillance cell release of interleukin-1β and interleukin-18 inflammatory cytokines; iii) cell cycle arrest and necroptosis induction in certain colon cancer cell lines. The effects exerted by GLTP superfamily members appear linked to their ability to regulate sphingolipid homeostasis by acting in either transporter and/or sensor capacities. These timely findings are opening new avenues for future cross-disciplinary, translational medical research involving GLTP-fold proteins in human health and disease. Such avenues include targeted regulation of specific GLTP superfamily members to alter sphingolipid levels as a therapeutic means for combating viral infection, neurodegenerative conditions and circumventing chemo-resistance during cancer treatment.
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Affiliation(s)
- Shrawan K Mishra
- Hormel Institute, University of Minnesota, Austin, MN 55912, USA
| | - Yong-Guang Gao
- Hormel Institute, University of Minnesota, Austin, MN 55912, USA
| | - Xianqiong Zou
- Hormel Institute, University of Minnesota, Austin, MN 55912, USA
| | - Daniel J Stephenson
- Department of Biochemistry and Molecular Biology, Virginia Commonwealth University Medical Center, Richmond, VA 23298-0614, USA; Department of Cell Biology, Microbiology and Molecular Biology, University of South Florida, Tampa, FL 33620, USA
| | - Lucy Malinina
- Hormel Institute, University of Minnesota, Austin, MN 55912, USA
| | | | - Charles E Chalfant
- Department of Cell Biology, Microbiology and Molecular Biology, University of South Florida, Tampa, FL 33620, USA; Research Service, James A. Haley Veterans Hospital, Tampa, FL 33612, USA; The Moffitt Cancer Center, Tampa, FL 33620, USA
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Mishra SK, Stephenson DJ, Chalfant CE, Brown RE. Upregulation of human glycolipid transfer protein (GLTP) induces necroptosis in colon carcinoma cells. Biochim Biophys Acta Mol Cell Biol Lipids 2019; 1864:158-167. [PMID: 30472325 PMCID: PMC6448591 DOI: 10.1016/j.bbalip.2018.11.002] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2018] [Revised: 11/03/2018] [Accepted: 11/10/2018] [Indexed: 02/06/2023]
Abstract
Human GLTP on chromosome 12 (locus 12q24.11) encodes a 24 kD amphitropic lipid transfer protein (GLTP) that mediates glycosphingolipid (GSL) intermembrane trafficking and regulates GSL homeostatic levels within cells. Herein, we provide evidence that GLTP overexpression inhibits the growth of human colon carcinoma cells (HT-29; HCT-116), but spares normal colonic cells (CCD-18Co). Mechanistic studies reveal that GLTP overexpression arrested the cell cycle at the G1/S checkpoint via upregulation of cyclin-dependent kinase inhibitor-1B (Kip1/p27) and cyclin-dependent kinase inhibitor 1A (Cip1/p21) at the protein and mRNA levels, and downregulation of cyclin-dependent kinase-2 (CDK2), cyclin-dependent kinase-4 (CDK4), cyclin E and cyclin D1 protein levels. Assessment of the biological fate of HCT-116 cells overexpressing GLTP indicated no increase in cell death suggesting induction of quiescence. However, HT-29 cells overexpressing GLTP underwent cell death by necroptosis as revealed by phosphorylation of human mixed lineage kinase domain-like protein (pMLKL) via receptor-interacting protein kinase-3 (RIPK-3), elevated cytosolic calcium, and plasma membrane permeabilization by pMLKL oligomerization. Overexpression of W96A-GLTP, an ablated GSL binding site mutant, failed to arrest the cell cycle or induce necroptosis. Sphingolipid assessment (ceramide, monohexosylceramide, sphingomyelin, ceramide-1-phosphate, sphingosine, and sphingosine-1-phosphate) of HT-29 cells overexpressing GLTP revealed large decreases (>5-fold) in sphingosine-1-phosphate with minimal change in 16:0-ceramide, tipping the 'sphingolipid rheostat' (S1P/16:0-Cer ratio) towards cell death. Depletion of RIPK-3 or MLKL abrogated necroptosis induced by GLTP overexpression. Our findings establish GLTP upregulation as a previously unknown suppressor of human colon carcinoma HT-29 cells via interference with cell cycle progression and induction of necroptosis.
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Affiliation(s)
| | - Daniel J Stephenson
- Department of Biochemistry and Molecular Biology, Virginia Commonwealth University Medical Center, Richmond, VA 23298-0614, USA; Department of Cell Biology, Microbiology and Molecular Biology, University of South Florida, Tampa, FL 33620, USA
| | - Charles E Chalfant
- Department of Cell Biology, Microbiology and Molecular Biology, University of South Florida, Tampa, FL 33620, USA; Research Service, James A. Haley Veterans Hospital, Tampa, FL 33612, USA; The Moffitt Cancer Center, Tampa, FL 33620, USA
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Youneszadeh-Fashalami M, Salati AP, Keyvanshokooh S. Comparison of proteomic profiles in the ovary of Sterlet sturgeon (Acipenser ruthenus) during vitellogenic stages. COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY D-GENOMICS & PROTEOMICS 2018; 27:23-29. [PMID: 29738886 DOI: 10.1016/j.cbd.2018.04.006] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/24/2018] [Revised: 04/22/2018] [Accepted: 04/27/2018] [Indexed: 11/29/2022]
Abstract
One of the challenges of sturgeon aquaculture is that sturgeon takes an extended amount of time to reach sexual maturity. The pattern of the protein expression in relation to the late maturity of sturgeon can help to better understand changes in sexual maturity. 17β-estradiol (E2), testosterone (T) and vitellogenin (Vtg) levels were examined at all stages of sexual maturation in Sterlet sturgeon (Acipenser ruthenus). Two-dimensional gel electrophoresis and mass spectrometry analysis were used to show the pattern of the ovarian proteins. The T levels increased from the previtellogenic to the postvitellogenic stages (P < 0.05) and Vtg showed a decremental pattern in pre- and postvitellogenic, and atresia (not significantly). The analysis showed 900 protein spots, 19 of which were successfully identified and had significant differences between the previtellogenic and the vitellogenic groups (P < 0.05). Among the identified proteins, 40% involved in cell defense (heat shock protein, Glutathione peroxidase, natural killer enhancing factor, peroxiredoxin-2), 30% in transcription and translation (constitutive photomorphogenesis 9 and Ybx2), 20% in metabolism and energy production (triose-phosphate isomerase (TPI)) and 10% in transport (glycolipid transfer protein). In the vitellogenic stage, the proteins were related to metabolism and energy production (TPI, ES1, creatin kinase, enolase, nucleoside diphosphate kinase, 50%), cell defense (thioredoxin and dislophid isomerase, 20%) and transport (fatty acid binding protein, 10%). Our findings show changes in protein expression pattern from cell defense to metabolism during egg development.
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Affiliation(s)
- Mohammad Youneszadeh-Fashalami
- Department of Fisheries, Faculty of marine Natural Resources, Khorramshahr University of Marine Science and Technology, Iran; South Iranian Aquaculture Research Center, Ahwaz, Iran
| | - Amir Parviz Salati
- Department of Fisheries, Faculty of marine Natural Resources, Khorramshahr University of Marine Science and Technology, Iran.
| | - Saeed Keyvanshokooh
- Department of Fisheries, Faculty of marine Natural Resources, Khorramshahr University of Marine Science and Technology, Iran
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9
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Functional evaluation of tryptophans in glycolipid binding and membrane interaction by HET-C2, a fungal glycolipid transfer protein. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2018; 1860:1069-1076. [PMID: 29305831 DOI: 10.1016/j.bbamem.2018.01.001] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/05/2017] [Revised: 12/16/2017] [Accepted: 01/01/2018] [Indexed: 02/07/2023]
Abstract
HET-C2 is a fungal glycolipid transfer protein (GLTP) that uses an evolutionarily-modified GLTP-fold to achieve more focused transfer specificity for simple neutral glycosphingolipids than mammalian GLTPs. Only one of HET-C2's two Trp residues is topologically identical to the three Trp residues of mammalian GLTP. Here, we provide the first assessment of the functional roles of HET-C2 Trp residues in glycolipid binding and membrane interaction. Point mutants HET-C2W208F, HET-C2W208A and HET-C2F149Y all retained >90% activity and 80-90% intrinsic Trp fluorescence intensity; whereas HET-C2F149A transfer activity decreased to ~55% but displayed ~120% intrinsic Trp emission intensity. Thus, neither W208 nor F149 is absolutely essential for activity and most Trp emission intensity (~85-90%) originates from Trp109. This conclusion was supported by HET-C2W109Y/F149Y which displayed ~8% intrinsic Trp intensity and was nearly inactive. Incubation of the HET-C2 mutants with 1-palmitoyl-2-oleoyl-phosphatidylcholine vesicles containing different monoglycosylceramides or presented by lipid ethanol-injection decreased Trp fluorescence intensity and blue-shifted the Trp λmax by differing amounts compared to wtHET-C2. With HET-C2 mutants for Trp208, the emission intensity decreases (~30-40%) and λmax blue-shifts (~12nm) were more dramatic than for wtHET-C2 or F149 mutants and closely resembled human GLTP. When Trp109 was mutated, the glycolipid induced changes in HET-C2 emission intensity and λmax blue-shift were nearly nonexistent. Our findings indicate that the HET-C2 Trp λmax blue-shift is diagnostic for glycolipid binding; whereas the emission intensity decrease reflects higher environmental polarity encountered upon nonspecific interaction with phosphocholine headgroups comprising the membrane interface and specific interaction with the hydrated glycolipid sugar.
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Malinina L, Patel DJ, Brown RE. How α-Helical Motifs Form Functionally Diverse Lipid-Binding Compartments. Annu Rev Biochem 2017; 86:609-636. [PMID: 28375742 DOI: 10.1146/annurev-biochem-061516-044445] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Lipids are produced site-specifically in cells and then distributed nonrandomly among membranes via vesicular and nonvesicular trafficking mechanisms. The latter involves soluble amphitropic proteins extracting specific lipids from source membranes to function as molecular solubilizers that envelope their insoluble cargo before transporting it to destination sites. Lipid-binding and lipid transfer structural motifs range from multi-β-strand barrels, to β-sheet cups and baskets covered by α-helical lids, to multi-α-helical bundles and layers. Here, we focus on how α-helical proteins use amphipathic helical layering and bundling to form modular lipid-binding compartments and discuss the functional consequences. Preformed compartments generally rely on intramolecular disulfide bridging to maintain conformation (e.g., albumins, nonspecific lipid transfer proteins, saposins, nematode polyprotein allergens/antigens). Insights into nonpreformed hydrophobic compartments that expand and adapt to accommodate a lipid occupant are few and provided mostly by the three-layer, α-helical ligand-binding domain of nuclear receptors. The simple but elegant and nearly ubiquitous two-layer, α-helical glycolipid transfer protein (GLTP)-fold now further advances understanding.
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Affiliation(s)
- Lucy Malinina
- The Hormel Institute, University of Minnesota, Austin, Minnesota 55912; ,
| | - Dinshaw J Patel
- Structural Biology Program, Memorial Sloan Kettering Cancer Center, New York, NY 10065;
| | - Rhoderick E Brown
- The Hormel Institute, University of Minnesota, Austin, Minnesota 55912; ,
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Zhai X, Gao YG, Mishra SK, Simanshu DK, Boldyrev IA, Benson LM, Bergen HR, Malinina L, Mundy J, Molotkovsky JG, Patel DJ, Brown RE. Phosphatidylserine Stimulates Ceramide 1-Phosphate (C1P) Intermembrane Transfer by C1P Transfer Proteins. J Biol Chem 2016; 292:2531-2541. [PMID: 28011644 DOI: 10.1074/jbc.m116.760256] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2016] [Revised: 12/21/2016] [Indexed: 11/06/2022] Open
Abstract
Genetic models for studying localized cell suicide that halt the spread of pathogen infection and immune response activation in plants include Arabidopsis accelerated-cell-death 11 mutant (acd11). In this mutant, sphingolipid homeostasis is disrupted via depletion of ACD11, a lipid transfer protein that is specific for ceramide 1-phosphate (C1P) and phyto-C1P. The C1P binding site in ACD11 and in human ceramide-1-phosphate transfer protein (CPTP) is surrounded by cationic residues. Here, we investigated the functional regulation of ACD11 and CPTP by anionic phosphoglycerides and found that 1-palmitoyl-2-oleoyl-phosphatidic acid or 1-palmitoyl-2-oleoyl-phosphatidylglycerol (≤15 mol %) in C1P source vesicles depressed C1P intermembrane transfer. By contrast, replacement with 1-palmitoyl-2-oleoyl-phosphatidylserine stimulated C1P transfer by ACD11 and CPTP. Notably, "soluble" phosphatidylserine (dihexanoyl-phosphatidylserine) failed to stimulate C1P transfer. Also, none of the anionic phosphoglycerides affected transfer action by human glycolipid lipid transfer protein (GLTP), which is glycolipid-specific and has few cationic residues near its glycolipid binding site. These findings provide the first evidence for a potential phosphoglyceride headgroup-specific regulatory interaction site(s) existing on the surface of any GLTP-fold and delineate new differences between GLTP superfamily members that are specific for C1P versus glycolipid.
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Affiliation(s)
- Xiuhong Zhai
- From the Hormel Institute, University of Minnesota, Austin, Minnesota 55912,
| | - Yong-Guang Gao
- From the Hormel Institute, University of Minnesota, Austin, Minnesota 55912
| | - Shrawan K Mishra
- From the Hormel Institute, University of Minnesota, Austin, Minnesota 55912
| | - Dhirendra K Simanshu
- the Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10065
| | - Ivan A Boldyrev
- the Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, 117997 Moscow, Russia
| | - Linda M Benson
- the Medical Genomic Facility-Proteomics Core, Mayo Foundation, Rochester, Minnesota 55905, and
| | - H Robert Bergen
- the Medical Genomic Facility-Proteomics Core, Mayo Foundation, Rochester, Minnesota 55905, and
| | - Lucy Malinina
- From the Hormel Institute, University of Minnesota, Austin, Minnesota 55912
| | - John Mundy
- the Department of Biology, BioCenter, University of Copenhagen, DK-2200 Copenhagen N, Denmark
| | - Julian G Molotkovsky
- the Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, 117997 Moscow, Russia
| | - Dinshaw J Patel
- the Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10065
| | - Rhoderick E Brown
- From the Hormel Institute, University of Minnesota, Austin, Minnesota 55912,
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Malinina L, Simanshu DK, Zhai X, Samygina VR, Kamlekar R, Kenoth R, Ochoa-Lizarralde B, Malakhova ML, Molotkovsky JG, Patel DJ, Brown RE. Sphingolipid transfer proteins defined by the GLTP-fold. Q Rev Biophys 2015; 48:281-322. [PMID: 25797198 PMCID: PMC4691851 DOI: 10.1017/s003358351400016x] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Glycolipid transfer proteins (GLTPs) originally were identified as small (~24 kDa), soluble, amphitropic proteins that specifically accelerate the intermembrane transfer of glycolipids. GLTPs and related homologs now are known to adopt a unique, helically dominated, two-layer 'sandwich' architecture defined as the GLTP-fold that provides the structural underpinning for the eukaryotic GLTP superfamily. Recent advances now provide exquisite insights into structural features responsible for lipid headgroup selectivity as well as the adaptability of the hydrophobic compartment for accommodating hydrocarbon chains of differing length and unsaturation. A new understanding of the structural versatility and evolutionary premium placed on the GLTP motif has emerged. Human GLTP-motifs have evolved to function not only as glucosylceramide binding/transferring domains for phosphoinositol 4-phosphate adaptor protein-2 during glycosphingolipid biosynthesis but also as selective binding/transfer proteins for ceramide-1-phosphate. The latter, known as ceramide-1-phosphate transfer protein, recently has been shown to form GLTP-fold while critically regulating Group-IV cytoplasmic phospholipase A2 activity and pro-inflammatory eicosanoid production.
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Affiliation(s)
- Lucy Malinina
- The Hormel Institute, University of Minnesota, Austin, MN 55912, USA
- Structural Biology Unit, CICbioGUNE, Technology Park of Bizkaia, 48160 Derio-Bilbao, Spain
| | - Dhirendra K. Simanshu
- Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065, USA
| | - Xiuhong Zhai
- The Hormel Institute, University of Minnesota, Austin, MN 55912, USA
| | - Valeria R. Samygina
- Structural Biology Unit, CICbioGUNE, Technology Park of Bizkaia, 48160 Derio-Bilbao, Spain
| | | | - Roopa Kenoth
- The Hormel Institute, University of Minnesota, Austin, MN 55912, USA
| | - Borja Ochoa-Lizarralde
- Structural Biology Unit, CICbioGUNE, Technology Park of Bizkaia, 48160 Derio-Bilbao, Spain
| | | | - Julian G. Molotkovsky
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - Dinshaw J. Patel
- Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065, USA
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Tuuf J, Mattjus P. Membranes and mammalian glycolipid transferring proteins. Chem Phys Lipids 2013; 178:27-37. [PMID: 24220498 DOI: 10.1016/j.chemphyslip.2013.10.013] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2013] [Revised: 10/29/2013] [Accepted: 10/30/2013] [Indexed: 01/04/2023]
Abstract
Glycolipids are synthesized in and on various organelles throughout the cell. Their trafficking inside the cell is complex and involves both vesicular and protein-mediated machineries. Most important for the bulk lipid transport is the vesicular system, however, lipids moved by transfer proteins are also becoming more characterized. Here we review the latest advances in the glycolipid transfer protein (GLTP) and the phosphoinositol 4-phosphate adaptor protein-2 (FAPP2) field, from a membrane point of view.
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Affiliation(s)
- Jessica Tuuf
- Biochemistry, Department of Biosciences, Åbo Akademi University, Turku, Finland
| | - Peter Mattjus
- Biochemistry, Department of Biosciences, Åbo Akademi University, Turku, Finland.
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Simanshu DK, Kamlekar RK, Wijesinghe DS, Zou X, Zhai X, Mishra SK, Molotkovsky JG, Malinina L, Hinchcliffe EH, Chalfant CE, Brown RE, Patel DJ. Non-vesicular trafficking by a ceramide-1-phosphate transfer protein regulates eicosanoids. Nature 2013; 500:463-7. [PMID: 23863933 PMCID: PMC3951269 DOI: 10.1038/nature12332] [Citation(s) in RCA: 146] [Impact Index Per Article: 13.3] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2012] [Accepted: 05/22/2013] [Indexed: 12/20/2022]
Abstract
Phosphorylated sphingolipids [ceramide-1-phosphate (C1P) and sphingosine-1-phosphate (S1P)] have emerged as key regulators of cell growth, survival, migration, and inflammation1–5. C1P (Fig. 1a) produced by ceramide kinase is an activator of group IVA cytosolic phospholipase A2α (cPLA2α), the rate-limiting releaser of arachidonic acid used for pro-inflammatory eicosanoid production3,6–9, which contributes to disease pathogenesis in asthma/airway hyper-responsiveness, cancer, atherosclerosis, and thrombosis. To modulate eicosanoid action and avoid the damaging effects of chronic inflammation, cells require efficient targeting, trafficking, and presentation of C1P to specific cellular sites. Vesicular trafficking is likely10 but nonvesicular mechanisms for C1P sensing, transfer, and presentation remain unexplored11,12. Moreover, the molecular basis for selective recognition and binding among signaling lipids with phosphate headgroups, namely C1P, phosphatidic acid (PA) or their lyso-derivatives, remains unclear. Herein, an ubiquitously-expressed lipid transfer protein (CPTP) is shown to specifically transfer C1P between membranes. Crystal structures establish C1P binding via a novel surface-localized, phosphate headgroup recognition center connected to an interior hydrophobic pocket that adaptively expands to ensheath differing-length lipid chains using a cleft-like gating mechanism. The two-layer, α-helically-dominated ‘sandwich’ topology identifies CPTP as the prototype for a new GLTP-fold13 subfamily. CPTP resides in the cell cytosol but associates with the trans-Golgi/TGN, nucleus, and plasma membrane. RNAi-induced CPTP depletion elevates C1P steady-state levels and alters Golgi cisternae stack morphology. The resulting C1P decrease in plasma membranes and increase in the Golgi complex stimulates cPLA2α release of arachidonic acid, triggering pro-inflammatory eicosanoid generation.
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Affiliation(s)
- Dhirendra K Simanshu
- Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10065, USA
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15
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Zhai X, Momsen WE, Malakhov DA, Boldyrev IA, Momsen MM, Molotkovsky JG, Brockman HL, Brown RE. GLTP-fold interaction with planar phosphatidylcholine surfaces is synergistically stimulated by phosphatidic acid and phosphatidylethanolamine. J Lipid Res 2013; 54:1103-13. [PMID: 23369752 DOI: 10.1194/jlr.m034744] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Among amphitropic proteins, human glycolipid transfer protein (GLTP) forms a structurally-unique fold that translocates on/off membranes to specifically transfer glycolipids. Phosphatidylcholine (PC) bilayers with curvature-induced packing stress stimulate much faster glycolipid intervesicular transfer than nonstressed PC bilayers raising questions about planar cytosol-facing biomembranes being viable sites for GLTP interaction. Herein, GLTP-mediated desorption kinetics of fluorescent glycolipid (tetramethyl-boron dipyrromethene (BODIPY)-label) from lipid monolayers are assessed using a novel microfluidics-based surface balance that monitors lipid lateral packing while simultaneously acquiring surface fluorescence data. At biomembrane-like packing (30-35 mN/m), GLTP uptake of BODIPY-glycolipid from POPC monolayers was nearly nonexistent but could be induced by reducing surface pressure to mirror packing in curvature-stressed bilayers. In contrast, 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) matrices supported robust BODIPY-glycolipid uptake by GLTP at both high and low surface pressures. Unexpectedly, negatively-charged cytosol-facing lipids, i.e., phosphatidic acid and phosphatidylserine, also supported BODIPY-glycolipid uptake by GLTP at high surface pressure. Remarkably, including both 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (5 mol%) and POPE (15 mol%) in POPC synergistically activated GLTP at high surface pressure. Our study shows that matrix lipid headgroup composition, rather than molecular packing per se, is a key regulator of GLTP-fold function while demonstrating the novel capabilities of the microfluidics-based film balance for investigating protein-membrane interfacial interactions.
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Affiliation(s)
- Xiuhong Zhai
- The Hormel Institute, University of Minnesota, Austin, MN, USA
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The glycolipid transfer protein (GLTP) domain of phosphoinositol 4-phosphate adaptor protein-2 (FAPP2): structure drives preference for simple neutral glycosphingolipids. Biochim Biophys Acta Mol Cell Biol Lipids 2012; 1831:417-27. [PMID: 23159414 DOI: 10.1016/j.bbalip.2012.10.010] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2012] [Revised: 10/23/2012] [Accepted: 10/29/2012] [Indexed: 11/23/2022]
Abstract
Phosphoinositol 4-phosphate adaptor protein-2 (FAPP2) plays a key role in glycosphingolipid (GSL) production using its C-terminal domain to transport newly synthesized glucosylceramide away from the cytosol-facing glucosylceramide synthase in the cis-Golgi for further anabolic processing. Structural homology modeling against human glycolipid transfer protein (GLTP) predicts a GLTP-fold for FAPP2 C-terminal domain, but no experimental support exists to warrant inclusion in the GLTP superfamily. Here, the biophysical properties and glycolipid transfer specificity of FAPP2-C-terminal domain have been characterized and compared with other established GLTP-folds. Experimental evidence for a GLTP-fold includes: i) far-UV circular dichroism (CD) showing secondary structure with high alpha-helix content and a low thermally-induced unfolding transition (~41°C); ii) near-UV-CD indicating only subtle tertiary conformational change before/after interaction with membranes containing/lacking glycolipid; iii) Red-shifted tryptophan (Trp) emission wavelength maximum (λ(max)~352nm) for apo-FAPP2-C-terminal domain consistent with surface exposed intrinsic Trp residues; iv) 'signature' GLTP-fold Trp fluorescence response, i.e., intensity decrease (~30%) accompanied by strongly blue-shifted λ(max) (~14nm) upon interaction with membranes containing glycolipid, supporting direct involvement of Trp in glycolipid binding and enabling estimation of partitioning affinities. A structurally-based preference for other simple uncharged GSLs, in addition to glucosylceramide, makes human FAPP2-GLTP more similar to fungal HET-C2 than to plant AtGLTP1 (glucosylceramide-specific) or to broadly GSL-selective human GLTP. These findings along with the distinct mRNA exon/intron organizations originating from single-copy genes on separate human chromosomes suggest adaptive evolutionary divergence by these two GLTP-folds.
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Zou X, Gao Y, Ruvolo VR, Gardner TL, Ruvolo PP, Brown RE. Human glycolipid transfer protein gene (GLTP) expression is regulated by Sp1 and Sp3: involvement of the bioactive sphingolipid ceramide. J Biol Chem 2010; 286:1301-11. [PMID: 20974858 DOI: 10.1074/jbc.m110.127837] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Glycolipid transfer protein (GLTP) accelerates glycolipid intermembrane transfer via a unique lipid transfer/binding fold (GLTP fold) that defines the GLTP superfamily and is the prototype for functional GLTP-like domains in larger proteins, i.e. FAPP2. Human GLTP is encoded by the single-copy GLTP gene on chromosome 12 (12q24.11 locus), but regulation of GLTP gene expression remains completely unexplored. Herein, the ability of glycosphingolipids (and their sphingolipid metabolites) to regulate the transcriptional expression of GLTP via its promoter has been evaluated. Using luciferase and GFP reporters in concert with deletion mutants, the constitutive and basal (225 bp; ∼78% G+C) human GLTP promoters have been defined along with adjacent regulatory elements. Despite high G+C content, translational regulation was not evident by the mammalian target of rapamycin pathway. Four GC-boxes were shown to be functional Sp1/Sp3 transcription factor binding sites. Mutation of one GC-box was particularly detrimental to GLTP transcriptional activity. Sp1/Sp3 RNA silencing and mithramycin A treatment significantly inhibited GLTP promoter activity. Among tested sphingolipid analogs of glucosylceramide, sulfatide, ganglioside GM1, ceramide 1-phosphate, sphingosine 1-phosphate, dihydroceramide, sphingosine, only ceramide, a nonglycosylated precursor metabolite unable to bind to GLTP protein, induced GLTP promoter activity and raised transcript levels in vivo. Ceramide treatment partially blocked promoter activity decreases induced by Sp1/Sp3 knockdown. Ceramide treatment also altered the in vivo binding affinity of Sp1 and Sp3 for the GLTP promoter and decreased Sp3 acetylation. This study represents the first characterization of any Gltp gene promoter and links human GLTP expression to sphingolipid homeostasis through ceramide.
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Affiliation(s)
- Xianqiong Zou
- The Hormel Institute, University of Minnesota, Austin, Minnesota 55912, USA
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18
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Kamlekar RK, Gao Y, Kenoth R, Molotkovsky JG, Prendergast FG, Malinina L, Patel DJ, Wessels WS, Venyaminov SY, Brown RE. Human GLTP: Three distinct functions for the three tryptophans in a novel peripheral amphitropic fold. Biophys J 2010; 99:2626-35. [PMID: 20959104 PMCID: PMC2955354 DOI: 10.1016/j.bpj.2010.08.038] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2010] [Revised: 07/27/2010] [Accepted: 08/10/2010] [Indexed: 01/17/2023] Open
Abstract
Human glycolipid transfer protein (GLTP) serves as the GLTP-fold prototype, a novel, to our knowledge, peripheral amphitropic fold and structurally unique lipid binding motif that defines the GLTP superfamily. Despite conservation of all three intrinsic Trps in vertebrate GLTPs, the Trp functional role(s) remains unclear. Herein, the issue is addressed using circular dichroism and fluorescence spectroscopy along with an atypical Trp point mutation strategy. Far-ultraviolet and near-ultraviolet circular dichroism spectroscopic analyses showed that W96F-W142Y-GLTP and W96Y-GLTP retain their native conformation and stability, whereas W85Y-W96F-GLTP is slightly altered, in agreement with relative glycolipid transfer activities of >90%, ∼85%, and ∼45%, respectively. In silico three-dimensional modeling and acrylamide quenching of Trp fluorescence supported a nativelike folding conformation. With the Trp⁹⁶-less mutants, changes in emission intensity, wavelength maximum, lifetime, and time-resolved anisotropy decay induced by phosphoglyceride membranes lacking or containing glycolipid and by excitation at different wavelengths along the absorption-spectrum red edge indicated differing functions for W142 and W85. The data suggest that W142 acts as a shallow-penetration anchor during docking with membrane interfaces, whereas the buried W85 indole helps maintain proper folding and possibly regulates membrane-induced transitioning to a glycolipid-acquiring conformation. The findings illustrate remarkable versatility for Trp, providing three distinct intramolecular functions in the novel amphitropic GLTP fold.
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Affiliation(s)
| | - Yongguang Gao
- The Hormel Institute, University of Minnesota, Austin, Minnesota
| | - Roopa Kenoth
- The Hormel Institute, University of Minnesota, Austin, Minnesota
| | - Julian G. Molotkovsky
- Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | | | - Lucy Malinina
- Structural Biology, Centro de Investigación Cooperativa BioGUNE, Derio, Spain
| | - Dinshaw J. Patel
- Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, New York
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Zhai X, Malakhova ML, Pike HM, Benson LM, Bergen HR, Sugár IP, Malinina L, Patel DJ, Brown RE. Glycolipid acquisition by human glycolipid transfer protein dramatically alters intrinsic tryptophan fluorescence: insights into glycolipid binding affinity. J Biol Chem 2009; 284:13620-13628. [PMID: 19270338 PMCID: PMC2679463 DOI: 10.1074/jbc.m809089200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2008] [Revised: 02/23/2009] [Indexed: 01/28/2023] Open
Abstract
Glycolipid transfer proteins (GLTPs) are small, soluble proteins that selectively accelerate the intermembrane transfer of glycolipids. The GLTP fold is conformationally unique among lipid binding/transfer proteins and serves as the prototype and founding member of the new GLTP superfamily. In the present study, changes in human GLTP tryptophan fluorescence, induced by membrane vesicles containing glycolipid, are shown to reflect glycolipid binding when vesicle concentrations are low. Characterization of the glycolipid-induced "signature response," i.e. approximately 40% decrease in Trp intensity and approximately 12-nm blue shift in emission wavelength maximum, involved various modes of glycolipid presentation, i.e. microinjection/dilution of lipid-ethanol solutions or phosphatidylcholine vesicles, prepared by sonication or extrusion and containing embedded glycolipids. High resolution x-ray structures of apo- and holo-GLTP indicate that major conformational alterations are not responsible for the glycolipid-induced GLTP signature response. Instead, glycolipid binding alters the local environment of Trp-96, which accounts for approximately 70% of total emission intensity of three Trp residues in GLTP and provides a stacking platform that aids formation of a hydrogen bond network with the ceramide-linked sugar of the glycolipid headgroup. The changes in Trp signal were used to quantitatively assess human GLTP binding affinity for various lipids including glycolipids containing different sugar headgroups and homogenous acyl chains. The presence of the glycolipid acyl chain and at least one sugar were essential for achieving a low-to-submicromolar dissociation constant that was only slightly altered by increased sugar headgroup complexity.
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Affiliation(s)
- Xiuhong Zhai
- The Hormel Institute, University of Minnesota, Austin, Minnesota 55912
| | | | - Helen M Pike
- The Hormel Institute, University of Minnesota, Austin, Minnesota 55912
| | - Linda M Benson
- Mayo Proteomics Research Center, Mayo Clinic College of Medicine, Rochester, Minnesota 55905
| | - H Robert Bergen
- Mayo Proteomics Research Center, Mayo Clinic College of Medicine, Rochester, Minnesota 55905
| | - István P Sugár
- Department of Neurology, Mount Sinai School of Medicine, New York, New York 10029
| | - Lucy Malinina
- Structural Biology, CIC bioGUNE, Parque Tecnológico de Vizcaya, Ed. 800, Derio 48160, Spain.
| | - Dinshaw J Patel
- Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10065.
| | - Rhoderick E Brown
- The Hormel Institute, University of Minnesota, Austin, Minnesota 55912.
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20
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Mattjus P. Glycolipid transfer proteins and membrane interaction. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2008; 1788:267-72. [PMID: 19007748 DOI: 10.1016/j.bbamem.2008.10.003] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/27/2008] [Revised: 10/07/2008] [Accepted: 10/07/2008] [Indexed: 12/17/2022]
Abstract
The glycolipid transfer protein is found from animals and fungi to plants and red micro-alga. Some eukaryotes that do not encode the glucosylceramide synthase like the yeast Schizosaccharomyces pombe and Saccharomyces cerevisiae do neither produce glycolipid transfer like proteins. On the other hand yeast like Eremothecium gossypii that do synthesize glucosylceramide also express glycolipid transfer protein. Based on this novel genetic relationship it is not far fetched to assume that there must be a strong correlation between the synthesis of the glycolipid precursor and the glycolipid transfer protein. Because the glycolipid transfer protein is localized in the cytosol it is unlikely that it would participate in events associated with lipid rafts or caveolar structures, since they are found on the outer leaflet of the plasma membrane. Rather, GLTP is likely to be involved in events at the cytosolic side of the plasma membrane or the endoplasmic reticulum, maybe function as a reporter or sensor of glycolipid levels. A similar function has been proposed for other proteins with affinity for lipids like the oxysterol binding proteins and phosphatidylinositol transfer proteins that are thought to be able act as lipid sensors. Recent discoveries in the glycolipid transfer protein field are discussed.
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Affiliation(s)
- Peter Mattjus
- Department of Biochemistry and Pharmacy, Abo Akademi University, Turku, Finland.
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