1
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Cooke AL, Morris J, Melchior JT, Street SE, Jerome WG, Huang R, Herr AB, Smith LE, Segrest JP, Remaley AT, Shah AS, Thompson TB, Davidson WS. A thumbwheel mechanism for APOA1 activation of LCAT activity in HDL. J Lipid Res 2018; 59:1244-1255. [PMID: 29773713 DOI: 10.1194/jlr.m085332] [Citation(s) in RCA: 50] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2018] [Revised: 05/08/2018] [Indexed: 01/28/2023] Open
Abstract
APOA1 is the most abundant protein in HDL. It modulates interactions that affect HDL's cardioprotective functions, in part via its activation of the enzyme, LCAT. On nascent discoidal HDL, APOA1 comprises 10 α-helical repeats arranged in an anti-parallel stacked-ring structure that encapsulates a lipid bilayer. Previous chemical cross-linking studies suggested that these APOA1 rings can adopt at least two different orientations, or registries, with respect to each other; however, the functional impact of these structural changes is unknown. Here, we placed cysteine residues at locations predicted to form disulfide bonds in each orientation and then measured APOA1's ability to adopt the two registries during HDL particle formation. We found that most APOA1 oriented with the fifth helix of one molecule across from fifth helix of the other (5/5 helical registry), but a fraction adopted a 5/2 registry. Engineered HDLs that were locked in 5/5 or 5/2 registries by disulfide bonds equally promoted cholesterol efflux from macrophages, indicating functional particles. However, unlike the 5/5 registry or the WT, the 5/2 registry impaired LCAT cholesteryl esterification activity (P < 0.001), despite LCAT binding equally to all particles. Chemical cross-linking studies suggest that full LCAT activity requires a hybrid epitope composed of helices 5-7 on one APOA1 molecule and helices 3-4 on the other. Thus, APOA1 may use a reciprocating thumbwheel-like mechanism to activate HDL-remodeling proteins.
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Affiliation(s)
- Allison L Cooke
- Departments of Pathology and Laboratory Medicine University of Cincinnati, Cincinnati, OH 45237
| | - Jamie Morris
- Departments of Pathology and Laboratory Medicine University of Cincinnati, Cincinnati, OH 45237
| | - John T Melchior
- Departments of Pathology and Laboratory Medicine University of Cincinnati, Cincinnati, OH 45237
| | - Scott E Street
- Departments of Pathology and Laboratory Medicine University of Cincinnati, Cincinnati, OH 45237
| | - W Gray Jerome
- Departments of Pathology, Microbiology, and Immunology, Vanderbilt University Medical Center, Nashville, TN 37232
| | - Rong Huang
- Departments of Pathology and Laboratory Medicine University of Cincinnati, Cincinnati, OH 45237
| | - Andrew B Herr
- Division of Immunobiology and Center for Systems Immunology Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229
| | - Loren E Smith
- Anesthesiology, Vanderbilt University Medical Center, Nashville, TN 37232
| | - Jere P Segrest
- Medicine, Vanderbilt University Medical Center, Nashville, TN 37232
| | - Alan T Remaley
- Lipoprotein Metabolism Section, Cardiovascular-Pulmonary Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892
| | - Amy S Shah
- Division of Endocrinology, Department of Pediatrics, Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229
| | - Thomas B Thompson
- Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati, Cincinnati, OH 45237
| | - W Sean Davidson
- Departments of Pathology and Laboratory Medicine University of Cincinnati, Cincinnati, OH 45237
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2
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Gorshkova IN, Mei X, Atkinson D. Arginine 123 of apolipoprotein A-I is essential for lecithin:cholesterol acyltransferase activity. J Lipid Res 2017; 59:348-356. [PMID: 29208698 DOI: 10.1194/jlr.m080986] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2017] [Revised: 11/29/2017] [Indexed: 01/10/2023] Open
Abstract
ApoA-I activates LCAT that converts lipoprotein cholesterol to cholesteryl ester (CE). Molecular dynamic simulations suggested earlier that helices 5 of two antiparallel apoA-I molecules on discoidal HDL form an amphipathic tunnel for migration of acyl chains and unesterified cholesterol to the active sites of LCAT. Our recent crystal structure of Δ(185-243)apoA-I showed the tunnel formed by helices 5/5, with two positively charged residues arginine 123 positioned at the edge of the hydrophobic tunnel. We hypothesized that these uniquely positioned residues Arg123 are poised for interaction with fatty acids produced by LCAT hydrolysis of the sn-2 chains of phosphatidylcholine, thus positioning the fatty acids for esterification to cholesterol. To test the importance of Arg123 for LCAT phospholipid hydrolysis and CE formation, we generated apoA-I[R123A] and apoA-I[R123E] mutants and made discoidal HDL with the mutants and WT apoA-I. Neither mutation of Arg123 changed the particle composition or size, or the protein conformation or stability. However, both mutations of Arg123 significantly reduced LCAT catalytic efficiency and the apparent Vmax for CE formation without affecting LCAT phospholipid hydrolysis. A control mutation, apoA-I[R131A], did not affect LCAT phospholipid hydrolysis or CE formation. These data suggest that Arg123 of apoA-I on discoidal HDL participates in LCAT-mediated cholesterol esterification.
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Affiliation(s)
- Irina N Gorshkova
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118
| | - Xiaohu Mei
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118
| | - David Atkinson
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118
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3
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Pollard RD, Fulp B, Sorci-Thomas MG, Thomas MJ. High-Density Lipoprotein Biogenesis: Defining the Domains Involved in Human Apolipoprotein A-I Lipidation. Biochemistry 2016; 55:4971-81. [PMID: 27501467 DOI: 10.1021/acs.biochem.6b00347] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
The first step in removing cholesterol from a cell is the ATP-binding cassette transporter 1 (ABCA1)-driven transfer of cholesterol to lipid-free or lipid-poor apolipoprotein A-I (apoA-I), which yields cholesterol-rich nascent high-density lipoprotein (nHDL) that then matures in plasma to spherical, cholesteryl ester-rich HDL. However, lipid-free apoA-I has a three-dimensional (3D) conformation that is significantly different from that of lipidated apoA-I on nHDL. By comparing the lipid-free apoA-I 3D conformation of apoA-I to that of 9-14 nm diameter nHDL, we formulated the hypothetical helical domain transitions that might drive particle formation. To test the hypothesis, ten apoA-I mutants were prepared that contained two strategically placed cysteines several of which could form intramolecular disulfide bonds and others that could not form these bonds. Mass spectrometry was used to identify amino acid sequence and intramolecular disulfide bond formation. Recombinant HDL (rHDL) formation was assessed with this group of apoA-I mutants. ABCA1-driven nHDL formation was measured in four mutants and wild-type apoA-I. The mutants contained cysteine substitutions in one of three regions: the N-terminus, amino acids 34 and 55 (E34C to S55C), central domain amino acids 104 and 162 (F104C to H162C), and the C-terminus, amino acids 200 and 233 (L200C to L233C). Mutants were studied in the locked form, with an intramolecular disulfide bond present, or unlocked form, with the cysteine thiol blocked by alkylation. Only small amounts of rHDL or nHDL were formed upon locking the central domain. We conclude that both the N- and C-terminal ends assist in the initial steps in lipid acquisition, but that opening of the central domain was essential for particle formation.
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Affiliation(s)
- Ricquita D Pollard
- Section on Molecular Medicine, Department of Internal Medicine, Wake Forest School of Medicine , Winston-Salem, North Carolina 27101, United States
| | - Brian Fulp
- Department of Biochemistry, Wake Forest School of Medicine , Winston-Salem, North Carolina 27101, United States
| | - Mary G Sorci-Thomas
- Departments of Medicine, Division of Endocrinology, Pharmacology and Toxicology, and Blood Research Institute, BloodCenter of Wisconsin, Medical College of Wisconsin , Milwaukee, Wisconsin 53226, United States
| | - Michael J Thomas
- Department of Pharmacology and Toxicology, Medical College of Wisconsin , 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226, United States
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4
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Pan L, Segrest JP. Computational studies of plasma lipoprotein lipids. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:2401-2420. [PMID: 26969087 DOI: 10.1016/j.bbamem.2016.03.010] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/25/2016] [Revised: 03/03/2016] [Accepted: 03/04/2016] [Indexed: 12/27/2022]
Abstract
Plasma lipoproteins are macromolecular assemblies of proteins and lipids found in the blood. The lipid components of lipoproteins are amphipathic lipids such as phospholipids (PLs), and unesterified cholesterols (UCs) and hydrophobic lipids such as cholesteryl esters (CEs) and triglycerides (TGs). Since lipoproteins are soft matter supramolecular assemblies easily deformable by thermal fluctuations and they also exist in varying densities and protein/lipid components, a detailed understanding of their structure/function is experimentally difficult. Molecular dynamics (MD) simulation has emerged as a particularly promising way to explore the structure and dynamics of lipoproteins. The purpose of this review is to survey the current status of computational studies of the lipid components of the lipoproteins. Computational studies aim to explore three levels of complexity for the 3-dimensional structural dynamics of lipoproteins at various metabolic stages: (i) lipoprotein particles consist of protein with minimal lipid; (ii) lipoprotein particles consist of PL-rich discoidal bilayer-like lipid particles; (iii) mature circulating lipoprotein particles consist of CE-rich or TG-rich spheroidal lipid-droplet-like particles. Due to energy barriers involved in conversion between these species, other biomolecules also participate in lipoprotein biological assembly. For example: (i) lipid-poor apolipoprotein A-I (apoA-I) interacts with ATP-binding cassette transporter A1 (ABCA1) to produce nascent discoidal high density lipoprotein (dHDL) particles; (ii) lecithin-cholesterol acyltransferase (LCAT) mediates the conversion of UC to CE in dHDL, driving spheroidal HDL (sHDL) formation; (iii) transfer proteins, cholesterol ester transfer protein (CETP) and phospholipid transfer protein (PLTP), transfer both CE and TG and PL, respectively, between lipoprotein particles. Computational studies have the potential to explore different lipoprotein particles at each metabolic stage in atomistic detail. This review discusses the current status of computational methods including all-atom MD (AAMD), coarse-grain MD (CGMD), and MD-simulated annealing (MDSA) and their applications in lipoprotein structural dynamics and biological assemblies. Results from MD simulations are discussed and compared across studies in order to identify key findings, controversies, issues and future directions. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.
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Affiliation(s)
- Lurong Pan
- Division of Gerontology, Geriatrics, & Palliative Care, School of Medicine, University of Alabama at Birmingham, Birmingham, AL 35294, United States
| | - Jere P Segrest
- Division of Gerontology, Geriatrics, & Palliative Care, School of Medicine, University of Alabama at Birmingham, Birmingham, AL 35294, United States.
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5
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Mizrachi D, Chen Y, Liu J, Peng HM, Ke A, Pollack L, Turner RJ, Auchus RJ, DeLisa MP. Making water-soluble integral membrane proteins in vivo using an amphipathic protein fusion strategy. Nat Commun 2015; 6:6826. [PMID: 25851941 PMCID: PMC4403311 DOI: 10.1038/ncomms7826] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2014] [Accepted: 03/03/2015] [Indexed: 11/30/2022] Open
Abstract
Integral membrane proteins (IMPs) play crucial roles in all cells and represent attractive pharmacological targets. However, functional and structural studies of IMPs are hindered by their hydrophobic nature and the fact that they are generally unstable following extraction from their native membrane environment using detergents. Here we devise a general strategy for in vivo solubilization of IMPs in structurally relevant conformations without the need for detergents or mutations to the IMP itself, as an alternative to extraction and in vitro solubilization. This technique, called SIMPLEx (solubilization of IMPs with high levels of expression), allows the direct expression of soluble products in living cells by simply fusing an IMP target with truncated apolipoprotein A-I, which serves as an amphipathic proteic ‘shield' that sequesters the IMP from water and promotes its solubilization. The study of integral membrane proteins (IMPs) is hampered by yields and the difficulty in retaining activity once they have been solubilized. Here Mizrachi et al. develop a strategy for in vivo expression and solubilization of IMPs in functionally relevant states by fusing them to truncated apolipoprotein A-I.
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Affiliation(s)
- Dario Mizrachi
- School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, USA
| | - Yujie Chen
- School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
| | - Jiayan Liu
- Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan 48109, USA
| | - Hwei-Ming Peng
- Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan 48109, USA
| | - Ailong Ke
- Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York 14850, USA
| | - Lois Pollack
- School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
| | - Raymond J Turner
- Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4
| | - Richard J Auchus
- Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan 48109, USA
| | - Matthew P DeLisa
- School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, USA
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6
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Zhang X, Lei D, Zhang L, Rames M, Zhang S. A model of lipid-free apolipoprotein A-I revealed by iterative molecular dynamics simulation. PLoS One 2015; 10:e0120233. [PMID: 25793886 PMCID: PMC4368682 DOI: 10.1371/journal.pone.0120233] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2014] [Accepted: 01/29/2015] [Indexed: 01/06/2023] Open
Abstract
Apolipoprotein A-I (apo A-I), the major protein component of high-density lipoprotein, has been proven inversely correlated to cardiovascular risk in past decades. The lipid-free state of apo A-I is the initial stage which binds to lipids forming high-density lipoprotein. Molecular models of lipid-free apo A-I have been reported by methods like X-ray crystallography and chemical cross-linking/mass spectrometry (CCL/MS). Through structural analysis we found that those current models had limited consistency with other experimental results, such as those from hydrogen exchange with mass spectrometry. Through molecular dynamics simulations, we also found those models could not reach a stable equilibrium state. Therefore, by integrating various experimental results, we proposed a new structural model for lipid-free apo A-I, which contains a bundled four-helix N-terminal domain (1–192) that forms a variable hydrophobic groove and a mobile short hairpin C-terminal domain (193–243). This model exhibits an equilibrium state through molecular dynamics simulation and is consistent with most of the experimental results known from CCL/MS on lysine pairs, fluorescence resonance energy transfer and hydrogen exchange. This solution-state lipid-free apo A-I model may elucidate the possible conformational transitions of apo A-I binding with lipids in high-density lipoprotein formation.
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Affiliation(s)
- Xing Zhang
- Department of Applied Physics, Xi'an Jiaotong University, Xi'an, Shaanxi, People's Republic of China
- Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States of America
| | - Dongsheng Lei
- Department of Applied Physics, Xi'an Jiaotong University, Xi'an, Shaanxi, People's Republic of China
- Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States of America
| | - Lei Zhang
- Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States of America
| | - Matthew Rames
- Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States of America
| | - Shengli Zhang
- Department of Applied Physics, Xi'an Jiaotong University, Xi'an, Shaanxi, People's Republic of China
- * E-mail:
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7
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Kontush A, Lindahl M, Lhomme M, Calabresi L, Chapman MJ, Davidson WS. Structure of HDL: particle subclasses and molecular components. Handb Exp Pharmacol 2015; 224:3-51. [PMID: 25522985 DOI: 10.1007/978-3-319-09665-0_1] [Citation(s) in RCA: 162] [Impact Index Per Article: 18.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Abstract
A molecular understanding of high-density lipoprotein (HDL) will allow a more complete grasp of its interactions with key plasma remodelling factors and with cell-surface proteins that mediate HDL assembly and clearance. However, these particles are notoriously heterogeneous in terms of almost every physical, chemical and biological property. Furthermore, HDL particles have not lent themselves to high-resolution structural study through mainstream techniques like nuclear magnetic resonance and X-ray crystallography; investigators have therefore had to use a series of lower resolution methods to derive a general structural understanding of these enigmatic particles. This chapter reviews current knowledge of the composition, structure and heterogeneity of human plasma HDL. The multifaceted composition of the HDL proteome, the multiple major protein isoforms involving translational and posttranslational modifications, the rapidly expanding knowledge of the HDL lipidome, the highly complex world of HDL subclasses and putative models of HDL particle structure are extensively discussed. A brief history of structural studies of both plasma-derived and recombinant forms of HDL is presented with a focus on detailed structural models that have been derived from a range of techniques spanning mass spectrometry to molecular dynamics.
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Affiliation(s)
- Anatol Kontush
- National Institute for Health and Medical Research (INSERM), UMR-ICAN 1166, Paris, France,
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8
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Kim SH, Kothari S, Patel AB, Bielicki JK, Narayanaswami V. A pyrene based fluorescence approach to study conformation of apolipoprotein E3 in macrophage-generated nascent high density lipoprotein. Biochem Biophys Res Commun 2014; 450:124-8. [PMID: 24866239 DOI: 10.1016/j.bbrc.2014.05.071] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2014] [Accepted: 05/18/2014] [Indexed: 12/18/2022]
Abstract
Apolipoprotein E3 (apoE3) is an anti-atherogenic apolipoprotein with the ability to exist in lipid-free and lipoprotein-associated states. During atherosclerosis, its function in promoting cholesterol efflux from macrophages via the ATP-binding cassette transporter A1 (ABCA1) takes a prominent role, leading to generation of nascent high density lipoprotein (nHDL) particles. The objective of this study is to understand the conformation adopted by apoE3 in macrophage-generated nHDL using a fluorescence spectroscopic approach involving pyrene. Pyrene-labeled recombinant human apoE3 displayed a robust ability to stimulate ABCA1-mediated cholesterol efflux from cholesterol-loaded J774 macrophages (which do not express apoE), comparable to that elicited by unlabeled apoE3. The nHDL recovered from the conditioned medium revealed the presence of apoE3 by immunoblot analysis. A heterogeneous population of nHDL bearing exogenously added apoE3 was generated with particle size varying from ∼12 to ∼19 nm in diameter, corresponding to molecular mass of ∼450 to ∼700 kDa. The lipid: apoE3 ratio varied from ∼60:1 to 10:1. A significant extent of pyrene excimer emission was noted in nHDL, indicative of spatial proximity between Cys112 on neighboring apoE3 molecules similar to that noted in reconstituted HDL. Cross-linking analysis using Cys-specific cross-linkers revealed the predominant presence of dimers. Taken together the data indicate a double belt arrangement of apoE molecules on nHDL. A similar organization of the C-terminal tail of apoE on nHDL was noted when pyrene-apoEA277C(201-299) was used as the cholesterol acceptor. These studies open up the possibility of using exogenously labeled apoE3 to generate nHDL for structural and conformational analysis.
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Affiliation(s)
- Sea H Kim
- Department of Chemistry & Biochemistry, California State University Long Beach, Long Beach, CA 90840, USA
| | - Shweta Kothari
- Department of Chemistry & Biochemistry, California State University Long Beach, Long Beach, CA 90840, USA
| | - Arti B Patel
- Department of Chemistry & Biochemistry, California State University Long Beach, Long Beach, CA 90840, USA
| | - John K Bielicki
- Life Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, Berkeley, CA 94720, USA
| | - Vasanthy Narayanaswami
- Department of Chemistry & Biochemistry, California State University Long Beach, Long Beach, CA 90840, USA.
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9
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Pollard RD, Fulp B, Samuel MP, Sorci-Thomas MG, Thomas MJ. The conformation of lipid-free human apolipoprotein A-I in solution. Biochemistry 2013; 52:9470-81. [PMID: 24308268 DOI: 10.1021/bi401080k] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Apolipoprotein AI (apoA-I) is the principal acceptor of lipids from ATP-binding cassette transporter A1, a process that yields nascent high density lipoproteins. Analysis of lipidated apoA-I conformation yields a belt or twisted belt in which two strands of apoA-I lie antiparallel to one another. In contrast, biophysical studies have suggested that a part of lipid-free apoA-I was arranged in a four-helix bundle. To understand how lipid-free apoA-I opens from a bundle to a belt while accepting lipid it was necessary to have a more refined model for the conformation of lipid-free apoA-I. This study reports the conformation of lipid-free human apoA-I using lysine-to-lysine chemical cross-linking in conjunction with disulfide cross-linking achieved using selective cysteine mutations. After proteolysis, cross-linked peptides were verified by sequencing using tandem mass spectrometry. The resulting structure is compact with roughly four helical regions, amino acids 44-186, bundled together. C- and N-terminal ends, amino acids 1-43 and 187-243, respectively, are folded such that they lie close to one another. An unusual feature of the molecule is the high degree of connectivity of lysine40 with six other lysines, lysines that are close, for example, lysine59, to distant lysines, for example, lysine239, that are at the opposite end of the primary sequence. These results are compared and contrasted with other reported conformations for lipid-free human apoA-I and an NMR study of mouse apoA-I.
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Affiliation(s)
- Ricquita D Pollard
- Department of Biochemistry and ‡Department of Pathology, Section on Lipid Sciences, Wake Forest School of Medicine , Medical Center Blvd, Winston-Salem, North Carolina 27157-1016, United States
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10
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Oda MN, Budamagunta MS, Geier EG, Chandradas SH, Shao B, Heinecke JW, Voss JC, Cavigiolio G. Conservation of apolipoprotein A-I's central domain structural elements upon lipid association on different high-density lipoprotein subclasses. Biochemistry 2013; 52:6766-78. [PMID: 23984834 DOI: 10.1021/bi4007012] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The antiatherogenic properties of apolipoprotein A-I (apoA-I) are derived, in part, from lipidation-state-dependent structural elements that manifest at different stages of apoA-I's progression from lipid-free protein to spherical high-density lipoprotein (HDL). Previously, we reported the structure of apoA-I's N-terminus on reconstituted HDLs (rHDLs) of different sizes. We have now investigated at the single-residue level the conformational adaptations of three regions in the central domain of apoA-I (residues 119-124, 139-144, and 164-170) upon apoA-I lipid binding and HDL formation. An important function associated with these residues of apoA-I is the activation of lecithin:cholesterol acyltransferase (LCAT), the enzyme responsible for catalyzing HDL maturation. Structural examination was performed by site-directed tryptophan fluorescence and spin-label electron paramagnetic resonance spectroscopies for both the lipid-free protein and rHDL particles 7.8, 8.4, and 9.6 nm in diameter. The two methods provide complementary information about residue side chain mobility and molecular accessibility, as well as the polarity of the local environment at the targeted positions. The modulation of these biophysical parameters yielded new insight into the importance of structural elements in the central domain of apoA-I. In particular, we determined that the loosely lipid-associated structure of residues 134-145 is conserved in all rHDL particles. Truncation of this region completely abolished LCAT activation but did not significantly affect rHDL size, reaffirming the important role of this structural element in HDL function.
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Affiliation(s)
- Michael N Oda
- Children's Hospital Oakland Research Institute , Oakland, California 94609, United States
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11
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Miyazaki M, Tajima Y, Ishihama Y, Handa T, Nakano M. Effect of phospholipid composition on discoidal HDL formation. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1828:1340-6. [DOI: 10.1016/j.bbamem.2013.01.012] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/20/2012] [Revised: 01/12/2013] [Accepted: 01/17/2013] [Indexed: 10/27/2022]
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12
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Gursky O. Crystal structure of Δ(185-243)ApoA-I suggests a mechanistic framework for the protein adaptation to the changing lipid load in good cholesterol: from flatland to sphereland via double belt, belt buckle, double hairpin and trefoil/tetrafoil. J Mol Biol 2013; 425:1-16. [PMID: 23041415 PMCID: PMC3534807 DOI: 10.1016/j.jmb.2012.09.027] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2012] [Revised: 09/23/2012] [Accepted: 09/29/2012] [Indexed: 12/18/2022]
Abstract
Apolipoprotein A-I (apoA-I) is the major protein of plasma high-density lipoproteins (HDLs), macromolecular assemblies of proteins and lipids that remove cell cholesterol and protect against atherosclerosis. HDL heterogeneity, large size (7.7-12 nm), and ability to exchange proteins have prevented high-resolution structural analysis. Low-resolution studies showed that two apoA-I molecules form an antiparallel α-helical "double belt" around an HDL particle. The atomic-resolution structure of the C-terminal truncated lipid-free Δ(185-243)apoA-I, determined recently by Mei and Atkinson, provides unprecedented new insights into HDL structure-function. It allows us to propose a molecular mechanism for the adaptation of the full-length protein to increasing lipid load during cholesterol transport. ApoA-I conformations on small, midsize, and large HDLs are proposed based on the tandem α-helical repeats and the crystal structure of Δ(185-243)apoA-I and are validated by comparison with extensive biophysical data reported by many groups. In our models, the central half of the double belt ("constant" segment 66-184) is structurally conserved while the N- and C-terminal half ("variable" segments 1-65 and 185-243) rearranges upon HDL growth. This includes incremental unhinging of the N-terminal bundle around two flexible regions containing G39 and G65 to elongate the belt, along with concerted swing motion of the double belt around G65-P66 and G185-G186 hinges that are aligned on various-size particles, to confer two-dimensional surface curvature to spherical HDLs. The proposed conformational ensemble integrates and improves several existing HDL models. It helps provide a structural framework necessary to understand functional interactions with over 60 other HDL-associated proteins and, ultimately, improve the cardioprotective function of HDL.
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Affiliation(s)
- Olga Gursky
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118, USA.
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13
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Sorci-Thomas MG, Owen JS, Fulp B, Bhat S, Zhu X, Parks JS, Shah D, Jerome WG, Gerelus M, Zabalawi M, Thomas MJ. Nascent high density lipoproteins formed by ABCA1 resemble lipid rafts and are structurally organized by three apoA-I monomers. J Lipid Res 2012; 53:1890-909. [PMID: 22750655 PMCID: PMC3413229 DOI: 10.1194/jlr.m026674] [Citation(s) in RCA: 97] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2012] [Revised: 06/29/2012] [Indexed: 12/29/2022] Open
Abstract
This report details the lipid composition of nascent HDL (nHDL) particles formed by the action of the ATP binding cassette transporter A1 (ABCA1) on apolipoprotein A-I (apoA-I). nHDL particles of different size (average diameters of ∼ 12, 10, 7.5, and <6 nm) and composition were purified by size-exclusion chromatography. Electron microscopy suggested that the nHDL were mostly spheroidal. The proportions of the principal nHDL lipids, free cholesterol, glycerophosphocholine, and sphingomyelin were similar to that of lipid rafts, suggesting that the lipid originated from a raft-like region of the cell. Smaller amounts of glucosylceramides, cholesteryl esters, and other glycerophospholipid classes were also present. The largest particles, ∼ 12 nm and 10 nm diameter, contained ∼ 43% free cholesterol, 2-3% cholesteryl ester, and three apoA-I molecules. Using chemical cross-linking chemistry combined with mass spectrometry, we found that three molecules of apoA-I in the ∼ 9-14 nm nHDL adopted a belt-like conformation. The smaller (7.5 nm diameter) spheroidal nHDL particles carried 30% free cholesterol and two molecules of apoA-I in a twisted, antiparallel, double-belt conformation. Overall, these new data offer fresh insights into the biogenesis and structural constraints involved in forming nascent HDL from ABCA1.
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Affiliation(s)
- Mary G Sorci-Thomas
- Department of Pathology, Section on Lipid Sciences, Wake Forest Baptist Medical Center, Winston-Salem, NC 27157, USA.
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14
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Li L, Li S, Jones MK, Segrest JP. Rotational and hinge dynamics of discoidal high density lipoproteins probed by interchain disulfide bond formation. Biochim Biophys Acta Mol Cell Biol Lipids 2011; 1821:481-9. [PMID: 22063273 DOI: 10.1016/j.bbalip.2011.10.013] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2011] [Revised: 10/10/2011] [Accepted: 10/13/2011] [Indexed: 11/30/2022]
Abstract
To develop a detailed double belt model for discoidal HDL, we previously scored inter-helical salt bridges between all possible registries of two stacked antiparallel amphipathic helical rings of apolipoprotein (apo) A-I. The top score was the antiparallel apposition of helix 5 with 5 followed closely by appositions of helix 5 with 4 and helix 5 with 6. The rationale for the current study is that, for each of the optimal scores, a pair of identical residues can be identified in juxtaposition directly on the contact edge between the two antiparallel helical belts of apoA-I. Further, these residues are always in the '9th position' in one of the eighteen 11-mer repeats that make up the lipid-associating domain of apoA-I. To illustrate our terminology, 129j (LL5/5) refers to the juxtaposition of the Cα atoms of G129 (in a '9th position') in the pairwise helix 5 domains. We reasoned that if identical residues in the double belt juxtapositions were mutated to a cysteine and kept under reducing conditions during disc formation, we would have a precise method for determining registration in discoidal HDL by formation of a disulfide-linked apoA-I homodimer. Using this approach, we conclude that 129j (LL5/5) is the major rotamer orientation for double belt HDL and propose that the small ubiquitous gap between the pairwise helix 5 portions of the double belt in larger HDL discoidal particles is significantly dynamic to hinge off the disc edge under certain conditions, e.g., in smaller particles or perhaps following binding of the enzyme LCAT. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).
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Affiliation(s)
- Ling Li
- Department of Medicine and Atherosclerosis Research Unit, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
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15
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Jones MK, Gu F, Catte A, Li L, Segrest JP. "Sticky" and "promiscuous", the yin and yang of apolipoprotein A-I termini in discoidal high-density lipoproteins: a combined computational-experimental approach. Biochemistry 2011; 50:2249-63. [PMID: 21329368 DOI: 10.1021/bi101301g] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Apolipoprotein (apo) A-I-containing lipoproteins in the form of high-density lipoproteins (HDL) are inversely correlated with atherosclerosis. Because HDL is a soft form of condensed matter easily deformable by thermal fluctuations, the molecular mechanisms for HDL remodeling are not well understood. A promising approach to understanding HDL structure and dynamics is molecular dynamics (MD). In the present study, two computational strategies, MD simulated annealing (MDSA) and MD temperature jump, were combined with experimental particle reconstitution to explore molecular mechanisms for phospholipid- (PL-) rich HDL particle remodeling. The N-terminal domains of full-length apoA-I were shown to be "sticky", acting as a molecular latch largely driven by salt bridges, until, at a critical threshold of particle size, the associated domains released to expose extensive hydrocarbon regions of the PL to solvent. The "sticky" N-termini also associate with other apoA-I domains, perhaps being involved in N-terminal loops suggested by other laboratories. Alternatively, the overlapping helix 10 C-terminal domains of apoA-I were observed to be extremely mobile or "promiscuous", transiently exposing limited hydrocarbon regions of PL. Based upon these models and reconstitution studies, we propose that separation of the N-terminal domains, as particles exceed a critical size, triggers fusion between particles or between particles and membranes, while the C-terminal domains of apoA-I drive the exchange of polar lipids down concentration gradients between particles. This hypothesis has significant biological relevance since lipid exchange and particle remodeling are critically important processes during metabolism of HDL particles at every step in the antiatherogenic process of reverse cholesterol transport.
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Affiliation(s)
- Martin K Jones
- Department of Medicine and Atherosclerosis Research Unit, University of Alabama at Birmingham, Birmingham, Alabama 35294, United States
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16
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Reconstitution of G protein-coupled receptors into a model bilayer system: reconstituted high-density lipoprotein particles. Methods Mol Biol 2011; 756:167-82. [PMID: 21870225 DOI: 10.1007/978-1-61779-160-4_8] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Reconstituted high-density lipoprotein particles (rHDL) are powerful platforms used as a model phospholipid bilayer system to study membrane proteins. They consist of a discoidal-shaped planar bilayer of phospholipids that is surrounded by a dimer of apolipoprotein A-I (apoA-I). The amphipathic nature of apoA-1 shields the hydrophobic acyl chains of the lipids from solvent and keeps the particles soluble in aqueous environments. These monodispersed, nanoscale discoidal HDL particles are approximately 10-11 nm in diameter with a thickness that is dependent on the length of the phospholipid acyl chain. Reconstituted HDL particles can be assembled in vitro using purified apoA-1 and purified lipids. Investigators have utilized this model bilayer system to co-reconstitute membrane proteins, and take advantage of the small size and its monodispersion. Our laboratory and others have utilized the rHDL approach to study the behavior of G protein-coupled receptors. In this chapter, we describe strategies for the preparation of rHDL particles containing GPCRs in their monomeric form and discuss various methodologies used to analyze the reconstituted receptor function.
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17
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Prieto ED, Garda HA. Membrane insertion topology of the central apolipoprotein A-I region. Fluorescence studies using single tryptophan mutants. Biochemistry 2010; 50:466-79. [PMID: 21141907 DOI: 10.1021/bi1009634] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
Apolipoprotein A-I (apoAI) contains several amphipathic α-helices. To carry out its function, it exchanges between lipid-free and different lipidated states as bound to membranes or to lipoprotein complexes of different morphology, size, and composition. When bound to membranes or to spherical lipoprotein surfaces, it is thought that most α-helices arrange with their long axis parallel to the membrane surface. However, we previously found that a central region spanning residues 87-112 is exclusively labeled by photoactivable reagents deeply located into the membrane (Córsico et al. (2001) J. Biol. Chem. 276, 16978-16985). A pair of amphipathic α-helical repeats with a particular charge distribution is predicted in this region. In order to study their insertion topology, three single tryptophan mutants, each one containing the tryptophan residue at a selected position in the hydrophobic face of the central Y-helices (W@93, W@104, and W@108), were used. From the accessibility to quenchers located at different membrane depths, distances from the bilayer center of 13.4, 10.5, and 15.7 Å were estimated for positions 93, 104, and 108, respectively. Reported data also indicate that distances between homologous positions (in particular for W@93 and W@104) are very short in dimers in aqueous solution, but they are larger in membrane-bound dimers. Data indicate that an intermolecular central Y-helix bundle would penetrate the membrane perpendicularly to the membrane surface. Intermolecular helix-helix interactions would occur through the hydrophilic helix faces in the membrane-bound bundle but through the hydrophobic faces in the case of dimers in solution.
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Affiliation(s)
- Eduardo D Prieto
- Instituto de Investigaciones Bioquímicas de La Plata, Consejo Nacional de Investigaciones Científicas y Técnicas, Universidad Nacional de La Plata, Facultad de Ciencias Médicas, Calles 60 y 120, La Plata, Argentina
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18
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Bashtovyy D, Jones MK, Anantharamaiah GM, Segrest JP. Sequence conservation of apolipoprotein A-I affords novel insights into HDL structure-function. J Lipid Res 2010; 52:435-50. [PMID: 21159667 DOI: 10.1194/jlr.r012658] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
We performed alignment of apolipoprotein A-I (apoA-I) sequences from 31 species of animals. We found there is specific conservation of salt bridge-forming residues in the first 30 residues of apoA-I and general conservation of a variety of residue types in the central domain, helix 2/3 to helix 7/8. In the lipid-associating domain, helix 7 and helix 10 are the most and least conserved helixes, respectively. Furthermore, eight residues are completely conserved: P66, R83, P121, E191, and P220, and three of seven Tyr residues in human apoA-I, Y18, Y115, and Y192, are conserved. Residue Y18 appears to be important for assembly of HDL. E191-Y192 represents the only completely conserved pair of adjacent residues in apoA-I; Y192 is a preferred target for site-specific oxidative modification within atheroma, and molecular dynamic simulations suggest that the conserved pair E191-Y192 is in a solvent-exposed loop-helix-loop. Molecular dynamics testing of human apoA-I showed that M112 and M148 interact with Y115, a microenvironment unique to human apoA-I. Finally, conservation of Arg residues in the α11/3 helical wheel position 7 supports several possibilities: interactions with adjacent phospholipid molecules and/or oxidized lipids and/or binding of antioxidant enzymes through cation-π orbital interactions. We conclude that sequence alignment of apoA-I provides unique insights into apoA-I structure-function relationship.
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Affiliation(s)
- Denys Bashtovyy
- Department of Medicine, Atherosclerosis Research Unit, University of Alabama at Birmingham, Birmingham, AL 35294, USA
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19
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Lagerstedt JO, Cavigiolio G, Budamagunta MS, Pagani I, Voss JC, Oda MN. Structure of apolipoprotein A-I N terminus on nascent high density lipoproteins. J Biol Chem 2010; 286:2966-75. [PMID: 21047795 DOI: 10.1074/jbc.m110.163097] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoproteins (HDL) and a critical element of cholesterol metabolism. To better elucidate the role of the apoA-I structure-function in cholesterol metabolism, the conformation of the apoA-I N terminus (residues 6-98) on nascent HDL was examined by electron paramagnetic resonance (EPR) spectroscopic analysis. A series of 93 apoA-I variants bearing single nitroxide spin label at positions 6-98 was reconstituted onto 9.6-nm HDL particles (rHDL). These particles were subjected to EPR spectral analysis, measuring regional flexibility and side chain solvent accessibility. Secondary structure was elucidated from side-chain mobility and molecular accessibility, wherein two major α-helical domains were localized to residues 6-34 and 50-98. We identified an unstructured segment (residues 35-39) and a β-strand (residues 40-49) between the two helices. Residues 14, 19, 34, 37, 41, and 58 were examined by EPR on 7.8, 8.4, and 9.6 nm rHDL to assess the effect of particle size on the N-terminal structure. Residues 14, 19, and 58 showed no significant rHDL size-dependent spectral or accessibility differences, whereas residues 34, 37, and 41 displayed moderate spectral changes along with substantial rHDL size-dependent differences in molecular accessibility. We have elucidated the secondary structure of the N-terminal domain of apoA-I on 9.6 nm rHDL (residues 6-98) and identified residues in this region that are affected by particle size. We conclude that the inter-helical segment (residues 35-49) plays a role in the adaptation of apoA-I to the particle size of HDL.
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Affiliation(s)
- Jens O Lagerstedt
- Department of Experimental Medical Science, Lund University, S-221 84 Lund, Sweden
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20
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Zhang L, Song J, Cavigiolio G, Ishida BY, Zhang S, Kane JP, Weisgraber KH, Oda MN, Rye KA, Pownall HJ, Ren G. Morphology and structure of lipoproteins revealed by an optimized negative-staining protocol of electron microscopy. J Lipid Res 2010; 52:175-84. [PMID: 20978167 PMCID: PMC2999936 DOI: 10.1194/jlr.d010959] [Citation(s) in RCA: 90] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Plasma lipoprotein levels are predictors of risk for coronary artery disease. Lipoprotein structure-function relationships provide important clues that help identify the role of lipoproteins in cardiovascular disease. The compositional and conformational heterogeneity of lipoproteins are major barriers to the identification of their structures, as discovered using traditional approaches. Although electron microscopy (EM) is an alternative approach, conventional negative staining (NS) produces rouleau artifacts. In a previous study of apolipoprotein (apo)E4-containing reconstituted HDL (rHDL) particles, we optimized the NS method in a way that eliminated rouleaux. Here we report that phosphotungstic acid at high buffer salt concentrations plays a key role in rouleau formation. We also validate our protocol for analyzing the major plasma lipoprotein classes HDL, LDL, IDL, and VLDL, as well as homogeneously prepared apoA-I-containing rHDL. High-contrast EM images revealed morphology and detailed structures of lipoproteins, especially apoA-I-containing rHDL, that are amenable to three-dimensional reconstruction by single-particle analysis and electron tomography.
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Affiliation(s)
- Lei Zhang
- Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
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21
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Bhat S, Sorci-Thomas MG, Calabresi L, Samuel MP, Thomas MJ. Conformation of dimeric apolipoprotein A-I milano on recombinant lipoprotein particles. Biochemistry 2010; 49:5213-24. [PMID: 20524691 DOI: 10.1021/bi1003734] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Apolipoprotein A-I Milano (apoA-I(Milano)) is a naturally occurring human mutation of wild-type apolipoprotein A-I (apoA-I(WT)) having cystine substituted for arginine(173). Two molecules of apo-I(WT) form disks with phospholipid having a defined relationship between the apoA-I(WT) molecules. ApoA-I(Milano) forms cystine homodimers that would not allow the protein to adopt the conformation reported for apoA-I(WT). The conformational constraints for dimeric apoA-I(Milano) recombinant high-density lipoprotein (rHDL) disks made with phospholipid were deduced from a combination of chemical cross-linking and mass spectrometry. Lysine-selective homobifunctional cross-linkers were reacted with homogeneous rHDL having diameters of 78 and 125 A. After reduction, cross-linked apoA-I(Milano) was separated from monomeric apoprotein by gel electrophoresis and then subjected to in-gel trypsin digest. Cross-linked peptides were confirmed by MS/MS sequencing. The cross-links provided distance constraints that were used to refine models of lipid-bound dimeric apoA-I(Milano). These studies suggest that a single dimeric apoA-I(Milano) on 78 A diameter rHDL girdles the edge of a phospholipid disk assuming a "belt" conformation similar to the "belt" region of apoA-I(WT) on rHDL. However, the C-terminal end of dimeric apoA-I(Milano) wraps around the periphery of the particle to shield the fatty acid chains from water rather than folding back onto the "belt" as does apoA-I(WT). The two apoA-I(Milano) dimers on a 125 A diameter rHDL do not encircle the periphery of a phospholipid disk but appear to reside on the surface of a laminar micelle.
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Affiliation(s)
- Shaila Bhat
- Department of Pathology, Center for Lipid Science, Wake Forest University Health Sciences, Medical Center Boulevard, Winston-Salem, North Carolina 27157, USA
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22
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Jones MK, Catte A, Li L, Segrest JP. Dynamics of activation of lecithin:cholesterol acyltransferase by apolipoprotein A-I. Biochemistry 2009; 48:11196-210. [PMID: 19860440 DOI: 10.1021/bi901242k] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The product of transesterification of phospholipid acyl chains and unesterified cholesterol (UC) by the enzyme lecithin:cholesterol acyltransferase (LCAT) is cholesteryl ester (CE). Activation of LCAT by apolipoprotein (apo) A-I on nascent (discoidal) high-density lipoproteins (HDL) is essential for formation of mature (spheroidal) HDL during the antiatherogenic process of reverse cholesterol transport. Here we report all-atom and coarse-grained (CG) molecular dynamics (MD) simulations of HDL particles that have major implications for mechanisms of LCAT activation. Both the all-atom and CG simulations provide support for a model in which the helix 5/5 domains of apoA-I create an amphipathic "presentation tunnel" that exposes methyl ends of acyl chains at the bilayer center to solvent. Further, CG simulations show that UC also becomes inserted with high efficiency into the amphipathic presentation tunnel with its hydroxyl moiety (UC-OH) exposed to solvent; these results are consistent with trajectory analyses of the all-atom simulations showing that UC is being concentrated in the vicinity of the presentation tunnel. Finally, consistent with known product inhibition of CE-rich HDL by CE, CG simulations of CE-rich spheroidal HDL indicate partial blockage of the amphipathic presentation tunnel by CE. These results lead us to propose the following working hypothesis. After attachment of LCAT to discoidal HDL, the helix 5/5 domains in apoA-I form amphipathic presentation tunnels for migration of hydrophobic acyl chains and amphipathic UC from the bilayer to the phospholipase A2-like and esterification active sites of LCAT, respectively. This hypothesis is currently being tested by site-directed mutagenesis.
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Affiliation(s)
- Martin K Jones
- Department of Medicine and Atherosclerosis Research Unit, University ofAlabama, Birmingham, Alabama 35294, USA
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23
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Gu F, Jones MK, Chen J, Patterson JC, Catte A, Jerome WG, Li L, Segrest JP. Structures of discoidal high density lipoproteins: a combined computational-experimental approach. J Biol Chem 2009; 285:4652-65. [PMID: 19948731 DOI: 10.1074/jbc.m109.069914] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Conversion of discoidal phospholipid (PL)-rich high density lipoprotein (HDL) to spheroidal cholesteryl ester-rich HDL is a central step in reverse cholesterol transport. A detailed understanding of this process and the atheroprotective role of apolipoprotein A-I (apoA-I) requires knowledge of the structure and dynamics of these various particles. This study, combining computation with experimentation, illuminates structural features of apoA-I allowing it to incorporate varying amounts of PL. Molecular dynamics simulated annealing of PL-rich HDL models containing unesterified cholesterol results in double belt structures with the same general saddle-shaped conformation of both our previous molecular dynamics simulations at 310 K and the x-ray structure of lipid-free apoA-I. Conversion from a discoidal to a saddle-shaped particle involves loss of helicity and formation of loops in opposing antiparallel parts of the double belt. During surface expansion caused by the temperature-jump step, the curved palmitoyloleoylphosphatidylcholine bilayer surfaces approach planarity. Relaxation back into saddle-shaped structures after cool down and equilibration further supports the saddle-shaped particle model. Our kinetic analyses of reconstituted particles demonstrate that PL-rich particles exist in discrete sizes corresponding to local energetic minima. Agreement of experimental and computational determinations of particle size/shape and apoA-I helicity provide additional support for the saddle-shaped particle model. Truncation experiments combined with simulations suggest that the N-terminal proline-rich domain of apoA-I influences the stability of PL-rich HDL particles. We propose that apoA-I incorporates increasing PL in the form of minimal surface bilayers through the incremental unwinding of an initially twisted saddle-shaped apoA-I double belt structure.
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Affiliation(s)
- Feifei Gu
- Department of Medicine and Atherosclerosis Research Unit, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA
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24
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Narayanaswami V, Kiss RS, Weers PMM. The helix bundle: a reversible lipid binding motif. Comp Biochem Physiol A Mol Integr Physiol 2009; 155:123-33. [PMID: 19770066 DOI: 10.1016/j.cbpa.2009.09.009] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2009] [Revised: 09/09/2009] [Accepted: 09/11/2009] [Indexed: 01/01/2023]
Abstract
Apolipoproteins are the protein components of lipoproteins that have the innate ability to inter convert between a lipid-free and a lipid-bound form in a facile manner, a remarkable property conferred by the helix bundle motif. Composed of a series of four or five amphipathic alpha-helices that fold to form a helix bundle, this motif allows the en face orientation of the hydrophobic faces of the alpha-helices in the protein interior in the lipid-free state. A conformational switch then permits helix-helix interactions to be substituted by helix-lipid interactions upon lipid binding interaction. This review compares the apolipoprotein high-resolution structures and the factors that trigger this switch in insect apolipophorin III and the mammalian apolipoproteins, apolipoprotein E and apolipoprotein A-I, pointing out the commonalities and key differences in the mode of lipid interaction. Further insights into the lipid-bound conformation of apolipoproteins are required to fully understand their functional role under physiological conditions.
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Affiliation(s)
- Vasanthy Narayanaswami
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach CA 90840, USA
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25
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Miyazaki M, Nakano M, Fukuda M, Handa T. Smaller Discoidal High-Density Lipoprotein Particles Form Saddle Surfaces, but Not Planar Bilayers. Biochemistry 2009; 48:7756-63. [DOI: 10.1021/bi900785x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Masakazu Miyazaki
- Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
| | - Minoru Nakano
- Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
| | - Masakazu Fukuda
- Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
| | - Tetsurou Handa
- Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
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26
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Jones MK, Catte A, Patterson JC, Gu F, Chen J, Li L, Segrest JP. Thermal stability of apolipoprotein A-I in high-density lipoproteins by molecular dynamics. Biophys J 2009; 96:354-71. [PMID: 19167289 DOI: 10.1016/j.bpj.2008.09.041] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2008] [Accepted: 09/22/2008] [Indexed: 02/06/2023] Open
Abstract
Apolipoprotein (apo) A-I is an unusually flexible protein whose lipid-associated structure is poorly understood. Thermal denaturation, which is used to measure the global helix stability of high-density lipoprotein (HDL)-associated apoA-I, provides no information about local helix stability. Here we report the use of temperature jump molecular dynamics (MD) simulations to scan the per-residue helix stability of apoA-I in phospholipid-rich HDL. When three 20 ns MD simulations were performed at 500 K on each of two particles created by MD simulations at 310 K, bilayers remained intact but expanded by 40%, and total apoA-I helicity decreased from 95% to 72%. Of significance, the conformations of the overlapping N- and C-terminal domains of apoA-I in the particles were unusually mobile, exposing hydrocarbon regions of the phospholipid to solvent; a lack of buried interhelical salt bridges in the terminal domains correlated with increased mobility. Nondenaturing gradient gels show that 40% expansion of the phospholipid surface of 100:2 particles by addition of palmitoyloleoylphosphatidylcholine exceeds the threshold of particle stability. As a unifying hypothesis, we propose that the terminal domains of apoA-I are phospholipid concentration-sensitive molecular triggers for fusion/remodeling of HDL particles. Since HDL remodeling is necessary for cholesterol transport, our model for remodeling has substantial biomedical implications.
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Affiliation(s)
- Martin K Jones
- Department of Medicine, University of Alabama at Birmingham, Birmingham, Alabama, USA
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27
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Sorci-Thomas MG, Bhat S, Thomas MJ. Activation of lecithin:cholesterol acyltransferase by HDL ApoA-I central helices. CLINICAL LIPIDOLOGY 2009; 4:113-124. [PMID: 20582235 PMCID: PMC2891274 DOI: 10.2217/17584299.4.1.113] [Citation(s) in RCA: 52] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Lecithin:cholesterol acyltransferase (LCAT) is an enzyme that first hydrolyzes the sn-2 position of phospholipids, preferentially a diacylphosphocholine, and then transfers the fatty acid to cholesterol to yield a cholesteryl ester. HDL ApoA-I is the principal catalytic activator for LCAT. Activity of LCAT on nascent or lipid-poor HDL particles composed of phospholipid, cholesterol and ApoA-I allows the maturation of HDL particles into lipid-rich spherical particles that contain a core of cholesteryl ester surrounded by phospholipid and ApoA-I on the surface. This article reviews the recent progress in elucidating structural aspects of the interaction between LCAT and ApoA-I. In the last decade, there has been considerable progress in understanding the structure of ApoA-I and the central helices 5, 6, and 7 that are known to activate LCAT. However, much less information has been forthcoming describing the 3D structure and conformation of LCAT required to catalyze two separate reactions within a single monomeric peptide.
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Affiliation(s)
- Mary G Sorci-Thomas
- Department of Pathology, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1016, USA, Tel.: +1 336 716 2147, Fax: +1 336 716 6279,
| | - Shaila Bhat
- Department of Pathology, Lipid Sciences Research Center, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA, Tel.: +1 336 716 6062, Fax: +1 336 716 6279,
| | - Michael J Thomas
- Department of Biochemistry, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA, Tel.: +1 336 716 2313, Fax: +1 336 716 6279,
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28
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Thomas MJ, Bhat S, Sorci-Thomas MG. Three-dimensional models of HDL apoA-I: implications for its assembly and function. J Lipid Res 2008; 49:1875-83. [PMID: 18515783 DOI: 10.1194/jlr.r800010-jlr200] [Citation(s) in RCA: 81] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
The purpose of this review is to highlight recent advances toward the refinement of a three-dimensional structure for lipid-bound apolipoprotein A-I (apoA-I) on recombinant HDL. Recently, X-ray crystallography has yielded a new structure for full-length, lipid-free apoA-I. Although this approach has not yet been successful in solving the three-dimensional structure of lipid-bound apoA-I, analysis of the X-ray structures has been of immense help in the interpretation of structural data obtained from other methods that yield structural information. Recent studies emphasize the use of mass spectrometry to unambiguously identify cross-linked peptides or to quantify solvent accessibility using hydrogen-deuterium exchange. The combination of mass spectrometry, molecular modeling, molecular dynamic analysis, and small-angle X-ray diffraction has provided additional structural information on apoA-I folding that complements previous approaches.
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Affiliation(s)
- Michael J Thomas
- Department of Biochemistry, Section on Lipid Sciences, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA.
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Sivashanmugam A, Yang Y, Murray V, McCullough C, Chen B, Ren X, Li Q, Wang J. Chapter 15 Structural Basis of Human High‐density Lipoprotein Formation and Assembly at Sub nanometer Resolution. Methods Cell Biol 2008; 90:327-64. [DOI: 10.1016/s0091-679x(08)00815-7] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/29/2023]
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Chapter 11 Molecular Modeling of the Structural Properties and Formation of High-Density Lipoprotein Particles. CURRENT TOPICS IN MEMBRANES 2008. [DOI: 10.1016/s1063-5823(08)00011-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register]
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31
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Aksimentiev A, Brunner R, Cohen J, Comer J, Cruz-Chu E, Hardy D, Rajan A, Shih A, Sigalov G, Yin Y, Schulten K. Computer modeling in biotechnology: a partner in development. Methods Mol Biol 2008; 474:181-234. [PMID: 19031067 DOI: 10.1007/978-1-59745-480-3_11] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
Computational modeling can be a useful partner in biotechnology, in particular, in nanodevice engineering. Such modeling guides development through nanoscale views of biomolecules and devices not available through experimental imaging methods. We illustrate the role of computational modeling, mainly of molecular dynamics, through four case studies: development of silicon bionanodevices for single molecule electrical recording, development of carbon nano-tube-biomolecular systems as in vivo sensors, development of lipoprotein nanodiscs for assays of single membrane proteins, and engineering of oxygen tolerance into the enzyme hydrogenase for photosynthetic hydrogen gas production. The four case studies show how molecular dynamics approaches were adapted to the specific technical uses through (i) multi-scale extensions, (ii) fast quantum chemical force field evaluation, (iii) coarse graining, and (iv) novel sampling methods. The adapted molecular dynamics simulations provided key information on device behavior and revealed development opportunities, arguing that the "computational microscope" is an indispensable nanoengineering tool.
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Affiliation(s)
- Aleksei Aksimentiev
- Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL, USA
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Bhat S, Sorci-Thomas MG, Tuladhar R, Samuel MP, Thomas MJ. Conformational adaptation of apolipoprotein A-I to discretely sized phospholipid complexes. Biochemistry 2007; 46:7811-21. [PMID: 17563120 PMCID: PMC2553278 DOI: 10.1021/bi700384t] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The conformational constraints for apoA-I bound to recombinant phospholipid complexes (rHDL) were attained from a combination of chemical cross-linking and mass spectrometry. Molecular distances were then used to refine models of lipid-bound apoA-I on both 80 and 96 A diameter rHDL particles. To obtain molecular constraints on the protein bound to phospholipid complexes, three different lysine-selective homo-bifunctional cross-linkers with increasing spacer arm lengths (i.e., 7.7, 12.0, and 16.1 A) were reacted with purified, homogeneous recombinant 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) apoA-I rHDL complexes of each diameter. Cross-linked dimeric apoA-I products were separated from monomeric apoprotein using 12% SDS-PAGE, then subjected to in-gel trypsin digest, and identified by MS/MS sequencing. These studies aid in the refinement of our previously published molecular model of two apoA-I molecules bound to approximately 150 molecules of POPC and suggest that the protein hydrophobic interactions at the N- and C-terminal domains decrease as the number of phospholipid molecules or "lipidation state" of apoA-I increases. Thus, it appears that these incremental changes in the interaction between the N- and C-terminal ends of apoA-I stabilize its tertiary conformation in the lipid-free state as well as allowing it to unfold and sequester discrete amounts of phospholipid molecules.
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Affiliation(s)
- Shaila Bhat
- Department of Pathology, Center for Lipid Science, Wake Forest University Medical Center, 391 Technology Way, Building A1, Winston-Salem, NC 27101
| | - Mary G Sorci-Thomas
- Department of Biochemistry, Wake Forest University Medical Center, 391 Technology Way, Building A1, Winston-Salem, NC 27101
- Department of Pathology, Center for Lipid Science, Wake Forest University Medical Center, 391 Technology Way, Building A1, Winston-Salem, NC 27101
| | - Rubina Tuladhar
- Department of Biochemistry, Wake Forest University Medical Center, 391 Technology Way, Building A1, Winston-Salem, NC 27101
| | - Michael P. Samuel
- Department of Biochemistry, Wake Forest University Medical Center, 391 Technology Way, Building A1, Winston-Salem, NC 27101
| | - Michael J. Thomas
- Department of Biochemistry, Wake Forest University Medical Center, 391 Technology Way, Building A1, Winston-Salem, NC 27101
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Davidson WS, Thompson TB. The structure of apolipoprotein A-I in high density lipoproteins. J Biol Chem 2007; 282:22249-53. [PMID: 17526499 DOI: 10.1074/jbc.r700014200] [Citation(s) in RCA: 156] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Affiliation(s)
- W Sean Davidson
- Department of Pathology and Laboratory Medicine, University of Cincinnati, OH 45237, USA.
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Zhu K, Brubaker G, Smith JD. Large disk intermediate precedes formation of apolipoprotein A-I-dimyristoylphosphatidylcholine small disks. Biochemistry 2007; 46:6299-307. [PMID: 17474718 PMCID: PMC2518397 DOI: 10.1021/bi700079w] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Small approximately 8.5 nm disks formed spontaneously when dimyristoylphosphatidylcholine (DMPC) large unilamellar vesicles (LUVs) were incubated with apolipoprotein A-I (apoA-I) (100:1 molar ratio). However, in a time course study, the transient production of approximately 11 nm large disks was detected and isolated by gel filtration. The intermediate large disks contained three apoA-I molecules and were stable over time; however, when additional apoA-I was added, they formed small disks containing two molecules of apoA-I. The reaction kinetics of apoA-I with DMPC LUVs was monitored by fluorescence resonance energy transfer, and two phases were observed, supporting the presence of the intermediate in the formation of small disks. The lipid dynamics of LUVs and disks were assayed, revealing the presence of sequestered lipid-protein domains upon apoA-I binding to DMPC LUVs. In addition, the lipids in the intermediate large disks were more constrained than those in the small disks. We propose that apoA-I binds with DMPC LUVs to form small lipid-protein domains on the LUV; then the domains are released to form large disks, which can mature in the presence of additional apoA-I to form small disks. Thus, the formation of small apoA-I lipid disks proceeds through the formation of a large disk intermediate.
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Affiliation(s)
- Keng Zhu
- Dept. of Cell Biology, Cleveland Clinic, Cleveland OH, 44195
| | | | - Jonathan D. Smith
- Dept. of Cell Biology, Cleveland Clinic, Cleveland OH, 44195
- Dept. of Cardiovascular Medicine, Cleveland Clinic, Cleveland OH, 44195
- Dept. of Molecular Medicine, Cleveland Clinic Lerner College of Medicine, Case School of Medicine, Cleveland, OH 44195,USA
- Corresponding author: Jonathan D. Smith, Department of Cell Biology, Lerner Research Institute / NC10, 9500 Euclid Avenue, Cleveland, Ohio 44195, USA, Telephone: 216-444-2248, Fax: 216-444-9404, E-mail:
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35
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Thomas MJ, Bhat S, Sorci-Thomas MG. The use of chemical cross-linking and mass spectrometry to elucidate the tertiary conformation of lipid-bound apolipoprotein A-I. Curr Opin Lipidol 2006; 17:214-20. [PMID: 16680024 DOI: 10.1097/01.mol.0000226111.05060.f4] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
PURPOSE OF REVIEW The purpose of this review is to highlight recent advances in mass spectrometry and its use for identifying the lipid-bound conformation of apolipoprotein A-I. Given the current interest in understanding the structure of HDL apolipoprotein A-I, this approach seems ideal in assessing its dual role as mediator of lipid efflux and modulator of cellular inflammation. RECENT FINDINGS A large number of different technical approaches have been employed over the past 25 years in attempts to solve the lipid-bound conformation of apolipoprotein A-I. Since the X-ray crystal structure of lipid-free Delta43 apolipoprotein A-I was reported in 1997, a 'double belt' model describing lipid-bound apolipoprotein A-I conformation for recombinant HDL has prevailed. Recent studies have focused on determining the exact helix-helix registry and salt-bridging partners found on a two apolipoprotein A-I molecule disc as well as on spherical HDL particles. Investigations are all aimed at defining the conformation of lipid-bound apolipoprotein A-I which may provide an explanation for how specific domains of apolipoprotein A-I interact with important HDL-modifying proteins that ultimately determine the apolipoprotein's fate in circulation. SUMMARY Recent advances in mass spectrometric sequencing of cross-linked peptides provide an excellent tool to help define protein tertiary structure. This approach has provided refined structural information on apolipoprotein A-I folding which had eluded all previous approaches.
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Affiliation(s)
- Michael J Thomas
- Department of Biochemistry, Wake Forest University Health Sciences, Winston-Salem, North Carolina, USA.
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Martin DDO, Budamagunta MS, Ryan RO, Voss JC, Oda MN. Apolipoprotein A-I assumes a "looped belt" conformation on reconstituted high density lipoprotein. J Biol Chem 2006; 281:20418-26. [PMID: 16698792 DOI: 10.1074/jbc.m602077200] [Citation(s) in RCA: 96] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Apolipoprotein A-I (apoA-I) plays a central role in the reverse cholesterol transport pathway; however, the structural basis for its antiatherogenic effects remains poorly understood. Here we employ EPR spectroscopy and fluorescence resonance energy transfer to elucidate the conformation and relative alignment of apoA-I monomers on discoidal (9.4 nm) reconstituted high density lipoprotein (rHDL). EPR spectroscopy provided evidence for an extended helical secondary structure. Position 139 since it was the only residue examined to display a dynamic motional character consistent with a flexible loop structure. The EPR spectra of nitroxide probes at positions 133 and 146 exhibit spin coupling, indicating that these positions are proximal to an apoA-I paired counterpart on the perimeter of rHDL. fluorescence resonance energy transfer studies employing engineered apoA-I variants possessing a single tryptophan (energy donor) and/or a single cysteine (whose thiol moiety was covalently labeled with an extrinsic energy acceptor) provided evidence that paired apoA-I molecules around the perimeter of rHDL align in an extended antiparallel conformation. Taken together with the observation that the EPR spectra of nitroxide probes positioned at intervening sequence positions (134-145) do not exhibit spin coupling, this has led us to propose a "looped belt" model, wherein residues 133-146 comprise a flexible loop segment that confers to apoA-I an intrinsic ability to adapt its structure to accommodate changing particle lipid content. Specifically, in the looped belt model, with the exception of amino acids 134-145, apoA-I aligns with its counterpart in a helix 5-helix 5 registry, centered at position 139.
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Affiliation(s)
- Dale D O Martin
- Children's Hospital Oakland Research Institute, Oakland, California 94609-1673, USA
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37
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Brubaker G, Peng DQ, Somerlot B, Abdollahian DJ, Smith JD. Apolipoprotein A-I lysine modification: effects on helical content, lipid binding and cholesterol acceptor activity. Biochim Biophys Acta Mol Cell Biol Lipids 2006; 1761:64-72. [PMID: 16495141 DOI: 10.1016/j.bbalip.2006.01.007] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2005] [Revised: 01/13/2006] [Accepted: 01/17/2006] [Indexed: 11/18/2022]
Abstract
We examined the role of the positively charged lysine residues in apoAI by chemical modification. Lysine modification by reductive methylation did not alter apoAI's net charge, secondary or tertiary structure as observed by circular dichroism and trytophan fluorescence, respectively, or have much impact on lipid binding or ABCA1-dependent cholesterol acceptor activity. Acetylation of lysine residues lowered the isoelectric point of apoAI, altered its secondary and tertiary structure, and led to a 40% decrease in cholesterol acceptor activity, while maintaining 93% of its lipid binding activity. Exhaustive lysine acetoacetylation lowered apoAI's isoelectric point, profoundly disrupted its secondary and tertiary structure, and led to 90% and 82% reductions in cholesterol acceptor and lipid binding activities, respectively. The dose-dependent acetoacetylation of an increasing proportion of apoAI lysine residues demonstrated that cholesterol acceptor activity was more sensitive to this modification than lipid binding activity, suggesting that apoAI lysine positive charges play an important role in ABCA1 mediated lipid efflux beyond the role needed to maintain alpha-helical content and lipid binding activity.
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Affiliation(s)
- Gregory Brubaker
- Department of Cell Biology, Cleveland Clinic Foundation, Cleveland, OH 44195, USA
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38
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Oda MN, Hargreaves PL, Beckstead JA, Redmond KA, van Antwerpen R, Ryan RO. Reconstituted high density lipoprotein enriched with the polyene antibiotic amphotericin B. J Lipid Res 2006; 47:260-7. [PMID: 16314670 DOI: 10.1194/jlr.d500033-jlr200] [Citation(s) in RCA: 90] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The polyene antibiotic amphotericin B (AMB) is an effective antifungal agent whose therapeutic potential is limited by poor aqueous solubility and toxicity toward host tissues. Addition of apolipoprotein A-I to a multilamellar phospholipid vesicle dispersion containing 20% (w/w) AMB induces the formation of reconstituted high density lipoprotein (rHDL), with solubilization of the antibiotic. Density gradient ultracentrifugation resulted in flotation of the complexes to a density of 1.16 g/ml, and negative stain electron microscopy revealed a population of disk-shaped particles. Native gradient polyacrylamide gel electrophoresis indicated a particle diameter of approximately 8.5 nm. Absorbance spectroscopy provided evidence for AMB integration into the lipid milieu. AMB-rHDLs were potent inhibitors of Saccharomyces cerevisiae growth, yielding 90% growth inhibition at <1 microg/ml yeast culture. In studies with pathogenic fungal species, similar growth inhibition characteristics were observed. Compared with AMB-deoxycholate micelles, AMB-rHDL displayed greatly attenuated red blood cell hemolytic activity and decreased toxicity toward cultured hepatoma cells. In in vivo studies in immunocompetent mice, AMB-rHDLs were nontoxic at 10 mg/kg, and they showed efficacy in a mouse model of candidiasis at concentrations as low as 0.25 mg/kg. These results indicate that AMB-rHDLs constitute a novel formulation that effectively solubilizes the antibiotic and elicits strong in vitro and in vivo antifungal activity with no observed toxicity at therapeutic doses.
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Affiliation(s)
- Michael N Oda
- Lipid Biology in Health and Disease Research Group, Children's Hospital Oakland Research Institute, Oakland, CA 94609, USA
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39
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Gross E, Peng DQ, Hazen SL, Smith JD. A novel folding intermediate state for apolipoprotein A-I: role of the amino and carboxy termini. Biophys J 2005; 90:1362-70. [PMID: 16326917 PMCID: PMC1367287 DOI: 10.1529/biophysj.105.075069] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Intramolecular interactions between the amino and carboxy termini of apolipoprotein A-I (apoAI) are believed to stabilize the helix bundle conformation of the protein. During lipid assembly the protein undergoes conformational changes that result in an exposure of the carboxy terminus and its insertion into the lipid phase. To determine the role of the two termini in the energetics of unfolding, we studied the guanidine-hydrochloride-induced unfolding and refolding of apoAI as well as its N-terminal deletion (del[1-43]), C-terminal deletion (del[186-243]), and the double deletion containing only the central residues 44-185. Thermodynamic analysis of the equilibrium unfolding measured by fluorescence spectroscopy revealed the presence of an intermediate unfolded state (I(equil)) in addition to the native (N) and unfolded states. Refolding kinetics of apoAI, measured by stopped-flow circular dichroism, revealed two kinetic intermediates, I(burst) and I(recovery). Computer modeling suggested that the first resembles the partially unfolded protein, whereas the second overlaps with the native state of the protein. The free energy changes for the N --> I(equil) transition of the N-terminal and double deletions were lower then that of the full-length form, whereas that for the C-terminal deletion was higher. Our findings suggest that the N-terminus of apoAI stabilizes the native state of the protein by increasing the Eyring energy barrier for the N --> I(equil) unfolding transition; whereas the carboxyl terminus destabilizes that state.
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Affiliation(s)
- Eitan Gross
- Department of Cell Biology, Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA.
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40
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Bhat S, Sorci-Thomas MG, Alexander ET, Samuel MP, Thomas MJ. Intermolecular contact between globular N-terminal fold and C-terminal domain of ApoA-I stabilizes its lipid-bound conformation: studies employing chemical cross-linking and mass spectrometry. J Biol Chem 2005; 280:33015-25. [PMID: 15972827 DOI: 10.1074/jbc.m505081200] [Citation(s) in RCA: 83] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
The structure of apoA-I on discoidal high density lipoprotein (HDL) was studied using a combination of chemical cross-linking and mass spectrometry. Recombinant HDL particles containing 145 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and two molecules of apoA-I with a 96-A diameter were treated with the lysine-specific cross-linker, dithiobis(succinimidylpropionate) at varying molar ratios from 2:1 to 200:1. At low molar ratios of dithiobis(succinimidylpropionate) to apoA-I, two products were obtained corresponding to approximately 53 and approximately 80 kDa. At high molar ratios, these two products merged, yielding a product of approximately 59 kDa, close to the theoretical molecular mass of dimeric apoA-I. To identify the intermolecular cross-links giving rise to the two different sized products, bands were excised from the gel, digested with trypsin, and then analyzed by liquid chromatography-electrospray-tandem mass spectrometry. In addition, tandem mass spectrometry of unique cross-links found in the 53- and 80-kDa products suggested that a distinct conformation exists for lipid-bound apoA-I on 96-A recombinant HDL, emphasizing the inherent flexibility and malleability of the N termini and its interaction with its C-terminal domain.
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Affiliation(s)
- Shaila Bhat
- Pathology and Biochemistry, Wake Forest University Health Sciences, Winston-Salem, North Carolina 27157, USA
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Davidson WS, Silva RAGD. Apolipoprotein structural organization in high density lipoproteins: belts, bundles, hinges and hairpins. Curr Opin Lipidol 2005; 16:295-300. [PMID: 15891390 DOI: 10.1097/01.mol.0000169349.38321.ad] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
PURPOSE OF REVIEW To summarize recent advances towards an understanding of the three-dimensional structures of the apolipoprotein components of HDL with a specific focus on high resolution models of apolipoprotein A-I. RECENT FINDINGS Since the primary sequence was first reported, various models have been advanced for the structure of apolipoprotein A-I, the major protein constituent of HDL, in its lipid-free and lipid-bound forms. Unfortunately, the generation of experimental data capable of distinguishing among the competing models has lagged far behind. However, recent experimental strategies, including X-ray crystallography, applications of resonance energy transfer and mass spectrometry, have combined with sophisticated theoretical approaches to develop three-dimensional structural models of apolipoprotein A-I with previously unavailable resolution. SUMMARY The recent synergy of sophisticated computer modeling techniques with hard experimental data has generated new models for apolipoprotein A-I in certain subclasses of HDL produced in vitro. The challenge now is to adapt and test these models in the more complex forms of HDL isolated directly from human plasma.
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Affiliation(s)
- W Sean Davidson
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio 45237-0507, USA.
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42
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Raussens V, Drury J, Forte T, Choy N, Goormaghtigh E, Ruysschaert JM, Narayanaswami V. Orientation and mode of lipid-binding interaction of human apolipoprotein E C-terminal domain. Biochem J 2005; 387:747-54. [PMID: 15588256 PMCID: PMC1135005 DOI: 10.1042/bj20041536] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2004] [Revised: 12/02/2004] [Accepted: 12/10/2004] [Indexed: 11/17/2022]
Abstract
ApoE (apolipoprotein E) is an anti-atherogenic lipid transport protein that plays an integral role in lipoprotein metabolism and cholesterol homoeostasis. Lipid association educes critical functional features of apoE, mediating reduction in plasma and cellular cholesterol levels. The 10-kDa CT (C-terminal) domain of apoE facilitates helix-helix interactions in lipid-free state to promote apoE self-association and helix-lipid interactions during binding with lipoproteins, although the mode of lipid-binding interaction is not well understood. We investigated the mode of lipid-binding interaction and orientation of apoE CT domain on reconstituted lipoproteins. Isolated recombinant human apoE CT domain (residues 201-299) possesses a strong ability to interact with phospholipid vesicles, yielding lipoprotein particles with an apparent molecular mass of approximately 600 kDa, while retaining the overall alpha-helical content. Electron microscopy and non-denaturing PAGE analysis of DMPC (dimyristoylphosphatidylcholine)--apoE CT domain lipoprotein complexes revealed discoidal complexes with a diameter of approx. 17 nm. Cross-linking apoE CT domain on discoidal particles yielded dimeric species as the major product. Attenuated total reflectance Fourier transform IR spectroscopy of phospholipid-apoE CT domain complexes reveals that the helical axis is oriented perpendicular to fatty acyl chains of the phospholipid. Fluorescence quenching analysis of DMPC-apoE CT domain discoidal complexes by spin-labelled stearic acid indicated a relatively superficial location of the native tryptophan residues with respect to the plane of the phospholipid bilayer. Taken together, we propose that apoE CT domain interacts with phospholipid vesicles, forming a long extended helix that circumscribes the discoidal bilayer lipoprotein complex.
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Key Words
- apolipoprotein e
- cross-linking
- electron microscopy
- ir spectroscopy
- lipid-bound conformation
- lipoprotein-binding surface
- apoa-i, apolipoprotein a-i
- apoe, apolipoprotein e
- atr-ftir, attenuated total reflectance fourier transform ir
- ct, c-terminal
- dmpc, dimyristoylphosphatidylcholine
- dmpg, dimyristoylphosphatidylglycerol
- 5-dsa, 5-doxyl stearic acid
- 12-dsa, 12-doxyl stearic acid
- dss, disuccinimidyl suberate
- hdl, high-density lipoprotein
- ldl, low-density lipoprotein
- nt, n-terminal
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Affiliation(s)
- Vincent Raussens
- *Structure and Function of Biological Membranes, Université Libre de Bruxelles, CP-206/2, bd. Du Triomphe, B-1050 Brussels, Belgium
| | - Jessica Drury
- †Lipid Biology in Health and Disease Research Group, Children's Hospital Oakland Research Institute, 5700 Martin Luther King Jr. Way, Oakland, CA 94609, U.S.A
| | - Trudy M. Forte
- †Lipid Biology in Health and Disease Research Group, Children's Hospital Oakland Research Institute, 5700 Martin Luther King Jr. Way, Oakland, CA 94609, U.S.A
| | - Nicole Choy
- †Lipid Biology in Health and Disease Research Group, Children's Hospital Oakland Research Institute, 5700 Martin Luther King Jr. Way, Oakland, CA 94609, U.S.A
| | - Erik Goormaghtigh
- *Structure and Function of Biological Membranes, Université Libre de Bruxelles, CP-206/2, bd. Du Triomphe, B-1050 Brussels, Belgium
| | - Jean-Marie Ruysschaert
- *Structure and Function of Biological Membranes, Université Libre de Bruxelles, CP-206/2, bd. Du Triomphe, B-1050 Brussels, Belgium
| | - Vasanthy Narayanaswami
- †Lipid Biology in Health and Disease Research Group, Children's Hospital Oakland Research Institute, 5700 Martin Luther King Jr. Way, Oakland, CA 94609, U.S.A
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Li L, Chen J, Mishra VK, Kurtz JA, Cao D, Klon AE, Harvey SC, Anantharamaiah GM, Segrest JP. Double belt structure of discoidal high density lipoproteins: molecular basis for size heterogeneity. J Mol Biol 2004; 343:1293-311. [PMID: 15491614 DOI: 10.1016/j.jmb.2004.09.017] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2004] [Revised: 08/24/2004] [Accepted: 09/10/2004] [Indexed: 10/26/2022]
Abstract
We recently proposed an all-atom model for apolipoprotein (apo) A-I in discoidal high-density lipoprotein in which two monomers form stacked antiparallel helical rings rotationally aligned by interhelical salt-bridges. The model can be derived a priori from the geometry of a planar bilayer disc that constrains the hydrophobic face of a continuous amphipathic alpha helix in lipid-associated apoA-I to a plane inside of an alpha-helical torus. This constrains each apoA-I monomer to a novel conformation, that of a slightly unwound, curved, planar amphipathic alpha 11/3 helix (three turns per 11 residues). Using non-denaturing gradient gel electrophoresis, we show that dimyristoylphosphocholine discs containing two apoA-I form five distinct particles with maximal Stokes diameters of 98 A (R2-1), 106 A (R2-2), 110 A (R2-3), 114 A (R2-4) and 120 A (R2-5). Further, we show that the Stokes diameters of R2-1 and R2-2 are independent of the N-terminal 43 residues (the flexible domain) of apoA-I, while the flexible domain is necessary and sufficient for the formation of the three larger complexes. On the basis of these results, the conformation of apoA-I on the R2-2 disc can be modeled accurately as an amphipathic helical double belt extending the full length of the lipid-associating domain with N and C-terminal ends in direct contact. The smallest of the discs, R2-1, models as the R2-2 conformation with an antiparallel 15-18 residue pairwise segment of helixes hinged off the disc edge. The conformations of full-length apoA-I on the flexible domain-dependent discs (R2-3, R2-4 and R2-5) model as the R2-2 conformation extended on the disc edge by one, two or three of the 11-residue tandem amphipathic helical repeats (termed G1, G2 and G3), respectively, contained within the flexible domain. Although we consider these results to favor the double belt model, the topographically very similar hairpin-belt model cannot be ruled out entirely.
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Affiliation(s)
- Ling Li
- Department of Medicine, UAB Medical Center, Birmingham, AL 35294, USA.
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Toledo JD, Prieto ED, Gonzalez MC, Soulages JL, Garda HA. Functional independence of a peptide with the sequence of human apolipoprotein A-I central region. Arch Biochem Biophys 2004; 428:188-97. [PMID: 15246876 DOI: 10.1016/j.abb.2004.05.009] [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: 04/13/2004] [Revised: 05/10/2004] [Indexed: 11/23/2022]
Abstract
Previous results [J. Biol. Chem. 276 (2001) 16978] indicated that an apolipoprotein A-I (apoAI) central region swings away from lipid contact in discoidal high density lipoproteins (HDL), but it is able to penetrate into the bilayer of lipid vesicles. In this work, we have studied the interaction with lipid membranes of a synthetic peptide with the sequence of apoAI region between residues 77 and 120 (AI 77-120). Like apoAI, AI 77-120 binds to phospholipid vesicles and shows selectivity for cholesterol-containing membranes. Moreover, AI 77-120 promotes cholesterol desorption from membranes in a similar fashion as apoAI and can stimulate cholesterol efflux from Chinese hamster ovary cells. AI 77-120 has a considerable alpha-helical content in water solution, and its secondary structure is not largely modified after binding to membranes. Both apoA-I and AI 77-120 are oligomeric in the lipid-bound state, suggesting that dimerization of the central domain could be required for the membrane binding activity of apoA-I in HDL.
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Affiliation(s)
- Juan Domingo Toledo
- Instituto de Investigaciones Bioquímicas de La Plata (INIBIOLP)-Consejo Nacional de Investigaciones Científicas y Técnicas / Universidad Nacional de La Plata, Facultad de Ciencias Médicas, Calles 60 y 120, 1900 La Plata, Argentina
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Saito H, Lund-Katz S, Phillips MC. Contributions of domain structure and lipid interaction to the functionality of exchangeable human apolipoproteins. Prog Lipid Res 2004; 43:350-80. [PMID: 15234552 DOI: 10.1016/j.plipres.2004.05.002] [Citation(s) in RCA: 165] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Exchangeable apolipoproteins function in lipid transport as structural components of lipoprotein particles, cofactors for enzymes and ligands for cell-surface receptors. Recent findings with apoA-I and apoE suggest that the tertiary structures of these two members of the human exchangeable apolipoprotein gene family are related. Characteristically, these proteins contain a series of proline-punctuated, 11- or 22-amino acid, amphipathic alpha-helical repeats that can adopt a helix bundle conformation in the lipid-free state. The amino- and carboxyl-terminal regions form separate domains with the latter being primarily responsible for lipid binding. Interaction with lipid induces changes in the conformation of the amino-terminal domain leading to alterations in function; for example, opening of the amino-terminal four-helix bundle in apolipoprotein E upon lipid binding is associated with enhanced receptor-binding activity. The concept of a two-domain structure for the larger exchangeable apolipoproteins is providing new molecular insights into how these apolipoproteins interact with lipids and other proteins, such as receptors. The ways in which structural changes induced by lipid interaction modulate the functionality of these apolipoproteins are reviewed.
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Affiliation(s)
- Hiroyuki Saito
- Lipid Research Group, The Children's Hospital of Philadelphia, Abramson Research Center, Suite 1102, 3615 Civic Center Boulevard, University of Pennsylvania School of Medicine, Philadelphia, 19104-4318, USA
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Saito H, Dhanasekaran P, Nguyen D, Deridder E, Holvoet P, Lund-Katz S, Phillips MC. α-Helix Formation Is Required for High Affinity Binding of Human Apolipoprotein A-I to Lipids. J Biol Chem 2004; 279:20974-81. [PMID: 15020600 DOI: 10.1074/jbc.m402043200] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Apolipoprotein (apo) A-I is thought to undergo a conformational change during lipid association that results in the transition of random coil to alpha-helix. Using a series of deletion mutants lacking different regions along the molecule, we examined the contribution of alpha-helix formation in apoA-I to the binding to egg phosphatidylcholine (PC) small unilamellar vesicles (SUV). Binding isotherms determined by gel filtration showed that apoA-I binds to SUV with high affinity and deletions in the C-terminal region markedly decrease the affinity. Circular dichroism measurements demonstrated that binding to SUV led to an increase in alpha-helix content, but the helix content was somewhat less than in reconstituted discoidal PC.apoA-I complexes for all apoA-I variants, suggesting that the helical structure of apoA-I on SUV is different from that in discs. Isothermal titration calorimetry showed that the binding of apoA-I to SUV is accompanied by a large exothermic heat and deletions in the C-terminal regions greatly decrease the heat. Analysis of the rate of release of heat on binding, as well as the kinetics of quenching of tryptophan fluorescence by brominated PC, indicated that the opening of the N-terminal helix bundle is a rate-limiting step in apoA-I binding to the SUV surface. Significantly, the correlation of thermodynamic parameters of binding with the increase in the number of helical residues revealed that the contribution of alpha-helix formation upon lipid binding to the enthalpy and the free energy of the binding of apoA-I is -1.1 and -0.04 kcal/mol per residue, respectively. These results indicate that alpha-helix formation, especially in the C-terminal regions, provides the energetic source for high affinity binding of apoA-I to lipids.
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Affiliation(s)
- Hiroyuki Saito
- Division of Gastroenterology and Nutrition, The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Abramson Research Center, 3625 Civic Center Boulevard, Philadelphia, PA 19104-4318, USA
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Gudheti MV, Gonzalez YI, Lee SP, Wrenn SP. Interaction of apolipoprotein A-I with lecithin-cholesterol vesicles in the presence of phospholipase C. Biochim Biophys Acta Mol Cell Biol Lipids 2003; 1635:127-41. [PMID: 14729075 DOI: 10.1016/j.bbalip.2003.11.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Here we study the anti-nucleating mechanism of apolipoprotein A-I (apo A-I) on model biliary vesicles in the presence of phospholipase C (PLC) utilizing dynamic light scattering (DLS), steady-state fluorescence spectroscopy, cryogenic transmission electron microscopy (cryo-TEM), and UV/Vis spectroscopy. PLC induces aggregation of cholesterol-free lecithin vesicles from an initial, average size of 100 nm to a maximal size of 600 nm. The presence of apo A-I likely inhibits vesicle aggregation by shielding the PLC-generated hydrophobic moieties, which results in vesicles of an average size of 200 nm. A similar phenomenon is observed in cholesterol-enriched lecithin vesicles. Whereas PLC alone produces aggregates of 300 nm, no aggregation is observed when apo A-I is present along with PLC. However, the ability of apo A-I to inhibit aggregation is temporary, and after 8 h, a broad particle size distribution with sizes as high as 800 nm is observed. Apo A-I possibly induces the formation of small apo A-I/lecithin/cholesterol complexes of about 5-20 nm similar to the discoidal pre-HDL complexes found in blood when it can no longer effectively shield all the DAG molecules. Concomitant with formation of complexes, DAG molecules coalesce into large oil droplets, which account for the large particles observed by light scattering. Thus, apo A-I acts as an anti-nucleating agent by two mechanisms, anti-aggregation and microstructural transition. The mode of protection is dependent on the cholesterol content and the relative amounts of DAG and apo A-I present. This study supports the possibility of apo A-I solubilizing lipids in bile in a similar fashion as it does in blood and also delineates the mechanism of formation of the complexes.
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Affiliation(s)
- Manasa V Gudheti
- Department of Chemical Engineering, College of Engineering, Drexel University, Philadelphia, PA 19104, USA
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Marcel YL, Kiss RS. Structure-function relationships of apolipoprotein A-I: a flexible protein with dynamic lipid associations. Curr Opin Lipidol 2003; 14:151-7. [PMID: 12642783 DOI: 10.1097/00041433-200304000-00006] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
PURPOSE OF REVIEW Apolipoprotein A-I is the major structural protein of HDL. Its physicochemical properties maintain a delicate balance between maintenance of stable lipoproteins and the ability to associate with and dissociate from the lipid transported. Here we review the progress made in the last 2-3 years on the structure-function relationships of apolipoprotein A-I, including elements related to the ATP binding cassette transporter A1. RECENT FINDINGS Current evidence now supports the so-called 'belt' or 'hairpin' models for apolipoprotein A-I conformation when bound to discoidal lipoproteins. In-vivo expression of apolipoprotein A-I mutant proteins has shown that both the N- and C-terminal domains are important for lipid association as well as for the esterification reaction, particularly binding of cholesteryl esters and formation of mature alpha-migrating lipoproteins. This property is apparently quite distinct from the activation of the enzyme lecithin cholesterol acyl transferase, which requires interaction with the central helix 6. The interaction of apolipoprotein A-I with the ATP binding cassette transporter A1 has been shown to require the C-terminal domain, which is proposed to mediate the opening of the helix bundle formed by lipid-free or lipid-poor apolipoprotein A-I and allow its association with hydrophobic binding sites. SUMMARY Significant progress has been made in the understanding of the molecular mechanisms controlling the folding of apolipoprotein A-I and its interaction with lipids and various other protein factors involved in HDL metabolism.
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Affiliation(s)
- Yves L Marcel
- Lipoprotein and Atherosclerosis Research Group, University of Ottawa Heart Institute, Room H460, 40 Ruskin Street, Ottawa, Ontario, Canada, K1Y 4W7.
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Dergunov AD, Vorotnikova YY, Visvikis S, Siest G. Homo- and hetero-complexes of exchangeable apolipoproteins in solution and in lipid-bound form. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2003; 59:1127-1137. [PMID: 12633731 DOI: 10.1016/s1386-1425(02)00298-6] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
The self-association state of human plasma apolipoprotein E (apoE) in solution and in complexes with dimyristoylphosphatidylcholine (DMPC) varying in stoichiometry was studied in sub-micromolar concentration range by gel filtration, fluorescence anisotropy, fluorescence quenching and energy transfer measurements with apolipoprotein labeled with lysine-specific fluorescent dyes. Together, these results confirm the equilibrium scheme for various apoE structures in solution: oligomer (in aged preparations) <==> 'closed' tetramer <==> 'open' tetramer ('molten globule' state) <==> native or partially denatured monomer <==> fully denatured monomer. Within DMPC:apoE discoidal complex (125:1) the apolipoprotein association state seems to be intermediate between that in solution and in larger vesicular complex (1000:1); for both complexes, the degree of exposure of fluorescein chromophores into water phase decreased. Hetero-associates of apoA-I and apoC-III-1 in solution and in the complexes with DMPC appear to behave similarly to apoE. When extrapolated to native HDL particles, 'molten globule' state seems to be a structure responsible for the interaction of exchangeable apolipoproteins with phospholipid. For a first time, the location of various apolipoprotein molecules on disc periphery was confirmed. The lysine residue(s) seems to locate closely to reacting residue(s) within apolipoprotein molecules in associates, however, with different package constraints for discoidal versus vesicular complexes with phospholipid.
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Affiliation(s)
- Alexander D Dergunov
- National Research Center for Preventive Medicine, 10, Petroverigsky street, 101953 Moscow, Russia.
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Li HH, Lyles DS, Pan W, Alexander E, Thomas MJ, Sorci-Thomas MG. ApoA-I structure on discs and spheres. Variable helix registry and conformational states. J Biol Chem 2002; 277:39093-101. [PMID: 12167653 DOI: 10.1074/jbc.m206770200] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Apolipoprotein A-I (apoA-I) readily forms discoidal high density lipoprotein (HDL) particles with phospholipids serving as an ideal transporter of plasma cholesterol. In the lipid-bound conformation, apoA-I activates the enzyme lecithin:cholesterol acyltransferase stimulating the formation of cholesterol esters from free cholesterol. As esterification proceeds cholesterol esters accumulate within the hydrophobic core of the discoidal phospholipid bilayer transforming it into a spherical HDL particle. To investigate the change in apoA-I conformation as it adapts to a spherical surface, fluorescence resonance energy transfer studies were performed. Discoidal rHDL particles containing two lipid-bound apoA-I molecules were prepared with acceptor and donor fluorescent probes attached to cysteine residues located at specific positions. Fluorescence quenching was measured for probe combinations located within repeats 5 and 5 (residue 132), repeats 5 and 6 (residues 132 and 154), and repeats 6 and 6 (residue 154). Results from these experiments indicated that each of the 2 molecules of discoidal bound apoA-I exists in multiple conformations and support the concept of a "variable registry" rather than a "fixed helix-helix registry." Additionally, discoidal rHDL were transformed in vitro to core-containing particles by incubation with lecithin:cholesterol acyltransferase. Compositional analysis showed that core-containing particles contained 11% less phospholipid and 633% more cholesterol ester and a total of 3 apoA-I molecules per particle. Spherical particles showed a lowering of acceptor to donor probe quenching when compared with starting rHDL. Therefore, we conclude that as lipid-bound apoA-I adjusts from a discoidal to a spherical surface its intermolecular interactions are significantly reduced presumably to cover the increased surface area of the particle.
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Affiliation(s)
- Hui-Hua Li
- Department of Pathology, The Wake Forest University Baptist Medical Center, Winston-Salem, North Carolina 27157, USA
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