1
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First eight residues of apolipoprotein A-I mediate the C-terminus control of helical bundle unfolding and its lipidation. PLoS One 2020; 15:e0221915. [PMID: 31945064 PMCID: PMC6964839 DOI: 10.1371/journal.pone.0221915] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2019] [Accepted: 12/30/2019] [Indexed: 11/23/2022] Open
Abstract
The crystal structure of a C-terminal deletion of apolipoprotein A-I (apoA1) shows a large helical bundle structure in the amino half of the protein, from residues 8 to 115. Using site directed mutagenesis, guanidine or thermal denaturation, cell free liposome clearance, and cellular ABCA1-mediated cholesterol efflux assays, we demonstrate that apoA1 lipidation can occur when the thermodynamic barrier to this bundle unfolding is lowered. The absence of the C-terminus renders the bundle harder to unfold resulting in loss of apoA1 lipidation that can be reversed by point mutations, such as Trp8Ala, and by truncations as short as 8 residues in the amino terminus, both of which facilitate helical bundle unfolding. Locking the bundle via a disulfide bond leads to loss of apoA1 lipidation. We propose a model in which the C-terminus acts on the N-terminus to destabilize this helical bundle. Upon lipid binding to the C-terminus, Trp8 is displaced from its interaction with Phe57, Arg61, Leu64, Val67, Phe71, and Trp72 to destabilize the bundle. However, when the C-terminus is deleted, Trp8 cannot be displaced, the bundle cannot unfold, and apoA1 cannot be lipidated.
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2
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Melchior JT, Walker RG, Cooke AL, Morris J, Castleberry M, Thompson TB, Jones MK, Song HD, Rye KA, Oda MN, Sorci-Thomas MG, Thomas MJ, Heinecke JW, Mei X, Atkinson D, Segrest JP, Lund-Katz S, Phillips MC, Davidson WS. A consensus model of human apolipoprotein A-I in its monomeric and lipid-free state. Nat Struct Mol Biol 2017; 24:1093-1099. [PMID: 29131142 PMCID: PMC5749415 DOI: 10.1038/nsmb.3501] [Citation(s) in RCA: 46] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2017] [Accepted: 10/06/2017] [Indexed: 11/09/2022]
Abstract
Apolipoprotein (apo)A-I is an organizing scaffold protein that is critical to high-density lipoprotein (HDL) structure and metabolism, probably mediating many of its cardioprotective properties. However, HDL biogenesis is poorly understood, as lipid-free apoA-I has been notoriously resistant to high-resolution structural study. Published models from low-resolution techniques share certain features but vary considerably in shape and secondary structure. To tackle this central issue in lipoprotein biology, we assembled a team of structural biologists specializing in apolipoproteins and set out to build a consensus model of monomeric lipid-free human apoA-I. Combining novel and published cross-link constraints, small-angle X-ray scattering (SAXS), hydrogen-deuterium exchange (HDX) and crystallography data, we propose a time-averaged model consistent with much of the experimental data published over the last 40 years. The model provides a long-sought platform for understanding and testing details of HDL biogenesis, structure and function.
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Affiliation(s)
- John T Melchior
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio, USA
| | - Ryan G Walker
- Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati, Cincinnati, Ohio, USA
| | - Allison L Cooke
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio, USA
| | - Jamie Morris
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio, USA
| | - Mark Castleberry
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio, USA
| | - Thomas B Thompson
- Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati, Cincinnati, Ohio, USA
| | - Martin K Jones
- Department of Medicine, Division of Cardiovascular Medicine, Vanderbilt University Medical Center, Nashville, Tennessee, USA
| | - Hyun D Song
- Department of Medicine, Division of Cardiovascular Medicine, Vanderbilt University Medical Center, Nashville, Tennessee, USA
| | - Kerry-Anne Rye
- School of Medical Sciences, Faculty of Medicine, University of New South Wales, Sydney, New South Wales, Australia
| | - Michael N Oda
- Children's Hospital Oakland Research Institute, Oakland, California, USA
| | - Mary G Sorci-Thomas
- Department of Medicine, Section on Endocrinology, Medical College of Wisconsin, Milwaukee, Wisconsin, USA
| | - Michael J Thomas
- Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin, USA
| | - Jay W Heinecke
- Department of Medicine, University of Washington, Seattle, Washington, USA
| | - Xiaohu Mei
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts, USA
| | - David Atkinson
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts, USA
| | - Jere P Segrest
- Department of Medicine, Division of Cardiovascular Medicine, Vanderbilt University Medical Center, Nashville, Tennessee, USA
| | - Sissel Lund-Katz
- Division of Translational Medicine and Human Genetics, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania, USA
| | - Michael C Phillips
- Division of Translational Medicine and Human Genetics, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania, USA
| | - W Sean Davidson
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio, USA
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3
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Horn JVC, Ellena RA, Tran JJ, Beck WHJ, Narayanaswami V, Weers PMM. Transfer of C-terminal residues of human apolipoprotein A-I to insect apolipophorin III creates a two-domain chimeric protein with enhanced lipid binding activity. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1859:1317-1325. [PMID: 28434970 DOI: 10.1016/j.bbamem.2017.04.017] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2017] [Revised: 04/14/2017] [Accepted: 04/19/2017] [Indexed: 01/11/2023]
Abstract
Apolipophorin III (apoLp-III) is an insect apolipoprotein (18kDa) that comprises a single five-helix bundle domain. In contrast, human apolipoprotein A-I (apoA-I) is a 28kDa two-domain protein: an α-helical N-terminal domain (residues 1-189) and a less structured C-terminal domain (residues 190-243). To better understand the apolipoprotein domain organization, a novel chimeric protein was engineered by attaching residues 179 to 243 of apoA-I to the C-terminal end of apoLp-III. The apoLp-III/apoA-I chimera was successfully expressed and purified in E. coli. Western blot analysis and mass spectrometry confirmed the presence of the C-terminal domain of apoA-I within the chimera. While parent apoLp-III did not self-associate, the chimera formed oligomers similar to apoA-I. The chimera displayed a lower α-helical content, but the stability remained similar compared to apoLp-III, consistent with the addition of a less structured domain. The chimera was able to solubilize phospholipid vesicles at a significantly higher rate compared to apoLp-III, approaching that of apoA-I. The chimera was more effective in protecting phospholipase C-treated low density lipoprotein from aggregation compared to apoLp-III. In addition, binding interaction of the chimera with phosphatidylglycerol vesicles and lipopolysaccharides was considerably improved compared to apoLp-III. Thus, addition of the C-terminal domain of apoA-I to apoLp-III created a two-domain protein, with self-association, lipid and lipopolysaccharide binding properties similar to apoA-I. The apoA-I like behavior of the chimera indicate that these properties are independent from residues residing in the N-terminal domain of apoA-I, and that they can be transferred from apoA-I to apoLp-III.
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Affiliation(s)
- James V C Horn
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, CA 90840, United States
| | - Rachel A Ellena
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, CA 90840, United States
| | - Jesse J Tran
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, CA 90840, United States
| | - Wendy H J Beck
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, CA 90840, United States
| | - Vasanthy Narayanaswami
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, CA 90840, United States
| | - Paul M M Weers
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, CA 90840, United States.
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4
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Gorshkova IN, Atkinson D. Increased Binding of Apolipoproteins A-I and E4 to Triglyceride-Rich Lipoproteins is linked to Induction of Hypertriglyceridemia. JSM ATHEROSCLEROSIS 2017; 2:1026. [PMID: 28597004 PMCID: PMC5460632] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Hypertriglyceridemia (HTG) is an independent factor of atherosclerotic cardiovascular disease and a hallmark of many metabolic disorders. However, the molecular etiology of HTG is still largely unknown. In mice, severe HTG may be induced by expression of specific mutants of apolipoprotein (apo) A-I or wild type (WT) apoE4. Expression of a certain apoE4 mutant results in mild HTG, while expression of another apoE4 mutant or WT apoA-I results in normal plasma triglyceride (TG) levels. Biophysical studies of the apoA-I and apoE4 forms associated with HTG help better understand the molecular mechanisms of induction of HTG by these proteins. The studies show that the apoA-I and apoE4 forms that induce HTG have a destabilized and more loosely folded conformation in solution than their counterparts not associated with HTG. Disruption of the protein salt bridge networks by the mutations is likely responsible for the observed structural changes. Each apoA-I and apoE4 form that induced HTG show enhanced binding to model TG-rich particles. HTG appeared to positively correlate with the apolipoprotein ability to bind to TG-rich particles. This implies that in vivo, the conformational changes in the apolipoproteins that induce HTG facilitate their binding to plasma TG-rich lipoproteins. We discuss metabolic pathways leading to the development of HTG that may result from enhanced binding of the apolipoproteins to TG-rich lipoproteins in circulation. While various factors may be involved in the development of HTG in humans, it is possible that structural alterations that increase affinity of apolipoproteins to TG-rich lipoproteins may contribute to some cases of this disorder.
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Affiliation(s)
- Irina N. Gorshkova
- Corresponding author: Irina N. Gorshkova, Department
of Physiology and Biophysics, Boston University School of Medicine, 700 Albany Street,
Boston, Massachusetts 02118, USA, Tel: 1-617-638-4207;
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5
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Oda MN. Lipid-free apoA-I structure - Origins of model diversity. Biochim Biophys Acta Mol Cell Biol Lipids 2016; 1862:221-233. [PMID: 27890580 DOI: 10.1016/j.bbalip.2016.11.010] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/29/2016] [Revised: 10/20/2016] [Accepted: 11/20/2016] [Indexed: 01/22/2023]
Abstract
Apolipoprotein A-I (apoA-I) is a prominent member of the exchangeable apolipoprotein class of proteins, capable of transitioning between lipid-bound and lipid-free states. It is the primary structural and functional protein of high density lipoprotein (HDL). Lipid-free apoA-I is critical to de novo HDL formation as it is the preferred substrate of the lipid transporter, ATP Binding Cassette Transporter A1 (ABCA1) Remaley et al. (2001) [1]. Lipid-free apoA-I is an important element in reverse cholesterol transport and comprehension of its structure is a core issue in our understanding of cholesterol metabolism. However, lipid-free apoA-I is highly conformationally dynamic making it a challenging subject for structural analysis. Over the past 20years there have been significant advances in overcoming the dynamic nature of lipid-free apoA-I, which have resulted in a multitude of proposed conformational models.
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Affiliation(s)
- Michael N Oda
- Children's Hospital Oakland Research Institute, Oakland, CA 94609, United States.
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6
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Chistiakov DA, Orekhov AN, Bobryshev YV. ApoA1 and ApoA1-specific self-antibodies in cardiovascular disease. J Transl Med 2016; 96:708-18. [PMID: 27183204 DOI: 10.1038/labinvest.2016.56] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2016] [Revised: 03/21/2016] [Accepted: 04/03/2016] [Indexed: 12/15/2022] Open
Abstract
Apolipoprotein A1 (ApoA1) is a main protein moiety in high-density lipoprotein (HDL) particles. Generally, ApoA1 and HDL are considered as atheroprotective. In prooxidant and inflammatory microenvironment in the vicinity to the atherosclerotic lesion, ApoA1/HDL are subjected to modification. The chemical modifications such as oxidation, nitration, etc result in altering native architecture of ApoA1 toward dysfunctionality and abnormality. Neutrophil myeloperoxidase has a prominent role in this mechanism. Neo-epitopes could be formed and then exposed that makes them immunogenic. Indeed, these epitopes may be recognized by immune cells and induce production of proatherogenic ApoA1-specific IgG antibodies. These antibodies are biologically relevant because they are able to react with Toll-like receptor (TLR)-2 and TLR4 in target cells and induce a variety of pro-inflammatory responses. Epidemiological and functional studies underline a prognostic value of ApoA1 self-antibodies for several cardiovascular diseases, including myocardial infarction, acute coronary syndrome, and severe carotid stenosis.
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Affiliation(s)
- Dimitry A Chistiakov
- Department of Molecular Genetic Diagnostics and Cell Biology, Division of Laboratory Medicine, Institute of Pediatrics, Research Center for Children's Health, Moscow, Russia
| | - Alexander N Orekhov
- Institute of General Pathology and Pathophysiology, Russian Academy of Sciences, Moscow, Russia.,Faculty of Biology, Department of Biophysics, Lomonosov Moscow State University, Moscow, Russia
| | - Yuri V Bobryshev
- Institute of General Pathology and Pathophysiology, Russian Academy of Sciences, Moscow, Russia.,Faculty of Medicine, School of Medical Sciences, University of New South Wales, Sydney, NSW, Australia.,School of Medicine, University of Western Sydney, Campbelltown, NSW, Australia
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7
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Protein conformational perturbations in hereditary amyloidosis: Differential impact of single point mutations in ApoAI amyloidogenic variants. Biochim Biophys Acta Gen Subj 2016; 1860:434-44. [DOI: 10.1016/j.bbagen.2015.10.019] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2015] [Revised: 10/15/2015] [Accepted: 10/23/2015] [Indexed: 12/31/2022]
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8
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Takase H, Furuchi H, Tanaka M, Yamada T, Matoba K, Iwasaki K, Kawakami T, Mukai T. Characterization of reconstituted high-density lipoprotein particles formed by lipid interactions with human serum amyloid A. Biochim Biophys Acta Mol Cell Biol Lipids 2014; 1842:1467-74. [PMID: 25063355 DOI: 10.1016/j.bbalip.2014.07.012] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2014] [Revised: 07/12/2014] [Accepted: 07/15/2014] [Indexed: 12/21/2022]
Abstract
The acute-phase human protein serum amyloid A (SAA) is enriched in high-density lipoprotein (HDL) in patients with inflammatory diseases. Compared with normal HDL containing apolipoprotein A-I, which is the principal protein component, characteristics of acute-phase HDL containing SAA remain largely undefined. In the present study, we examined the physicochemical properties of reconstituted HDL (rHDL) particles formed by lipid interactions with SAA. Fluorescence and circular dichroism measurements revealed that although SAA was unstructured at physiological temperature, α-helix formation was induced upon binding to phospholipid vesicles. SAA also formed rHDL particles by solubilizing phospholipid vesicles through mechanisms that are common to other exchangeable apolipoproteins. Dynamic light scattering and nondenaturing gradient gel electrophoresis analyses of rHDL after gel filtration revealed particle sizes of approximately 10nm, and a discoidal shape was verified by transmission electron microscopy. Thermal denaturation experiments indicated that SAA molecules in rHDL retained α-helical conformations at 37°C, but were almost completely denatured around 60°C. Furthermore, trypsin digestion experiments showed that lipid binding rendered SAA molecules resistant to protein degradation. In humans, three major SAA1 isoforms (SAA1.1, 1.3, and 1.5) are known. Although these isoforms have different amino acids at residues 52 and 57, no major differences in physicochemical properties between rHDL particles resulting from lipid interactions with SAA isoforms have been found. The present data provide useful insights into the effects of SAA enrichment on the physicochemical properties of HDL.
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Affiliation(s)
- Hiroka Takase
- Department of Biophysical Chemistry, Kobe Pharmaceutical University, Kobe 658-8558, Japan
| | - Hiroki Furuchi
- Department of Biophysical Chemistry, Kobe Pharmaceutical University, Kobe 658-8558, Japan
| | - Masafumi Tanaka
- Department of Biophysical Chemistry, Kobe Pharmaceutical University, Kobe 658-8558, Japan.
| | - Toshiyuki Yamada
- Department of Clinical and Laboratory Medicine, Jichi Medical University, Tochigi 329-0498, Japan
| | - Kyoko Matoba
- Laboratory of Protein Synthesis and Expression, Institute for Protein Research, Osaka University, Suita 565-0871, Japan
| | - Kenji Iwasaki
- Laboratory of Protein Synthesis and Expression, Institute for Protein Research, Osaka University, Suita 565-0871, Japan
| | - Toru Kawakami
- Laboratory of Protein Organic Chemistry, Institute for Protein Research, Osaka University, Suita 565-0871, Japan
| | - Takahiro Mukai
- Department of Biophysical Chemistry, Kobe Pharmaceutical University, Kobe 658-8558, Japan
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9
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Gorshkova IN, Mei X, Atkinson D. Binding of human apoA-I[K107del] variant to TG-rich particles: implications for mechanisms underlying hypertriglyceridemia. J Lipid Res 2014; 55:1876-85. [PMID: 24919401 DOI: 10.1194/jlr.m047241] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
We found earlier that apoA-I variants that induced hypertriglyceridemia (HTG) in mice had increased affinity to TG-rich lipoproteins and thereby impaired their catabolism. Here, we tested whether a naturally occurring human apoA-I mutation, Lys107del, associated with HTG also promotes apoA-I binding to TG-rich particles. We expressed apoA-I[Lys107del] variant in Escherichia coli, studied its binding to TG-rich emulsion particles, and performed a physicochemical characterization of the protein. Compared with WT apoA-I, apoA-I[Lys107del] showed enhanced binding to TG-rich particles, lower stability, and greater exposure of hydrophobic surfaces. The crystal structure of truncated, Δ(185-243), apoA-I suggests that deletion of Lys107 disrupts helix registration and disturbs a stabilizing salt bridge network in the N-terminal helical bundle. To elucidate the structural changes responsible for the altered function of apoA-I[Lys107del], we studied another mutant, apoA-I [Lys107Ala]. Our findings suggest that the registry shift and ensuing disruption of the inter-helical salt bridges in apoA-I[Lys107del] result in destabilization of the helical bundle structure and greater exposure of hydrophobic surfaces. We conclude that the structural changes in the apoA-I[Lys107del] variant facilitate its binding to TG-rich lipoproteins and thus, may reduce their lipolysis and contribute to the development of HTG in carriers of the mutation.
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Affiliation(s)
- Irina N Gorshkova
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118 Department of Medicine, 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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10
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Effect of amino acid distribution of amphipathic helical peptide derived from human apolipoprotein A-I on membrane curvature sensing. FEBS Lett 2013; 587:510-5. [PMID: 23347831 DOI: 10.1016/j.febslet.2013.01.026] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2012] [Revised: 01/08/2013] [Accepted: 01/11/2013] [Indexed: 11/22/2022]
Abstract
Amphipathic helix, which senses membrane curvature, is of growing interest. Here we explore the effect of amino acid distribution of amphipathic helical peptide derived from the C-terminal region (residues 220-241) of human apolipoprotein (apo) A-I on membrane curvature sensing. This peptide preferred a curved membrane in a manner similar to full-length apoA-I, although its model peptide did not sense membrane curvature. Substitution of several residues both on the polar and non-polar faces of the amphipathic helix had no significant effect on sensing, suggestive of the elaborate molecular architecture in the C-terminal helical region of apoA-I to exert lipid efflux function.
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11
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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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12
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Phillips MC. New insights into the determination of HDL structure by apolipoproteins: Thematic review series: high density lipoprotein structure, function, and metabolism. J Lipid Res 2012; 54:2034-2048. [PMID: 23230082 DOI: 10.1194/jlr.r034025] [Citation(s) in RCA: 128] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Apolipoprotein (apo)A-I is the principal protein component of HDL, and because of its conformational adaptability, it can stabilize all HDL subclasses. The amphipathic α-helix is the structural motif that enables apoA-I to achieve this functionality. In the lipid-free state, the helical segments unfold and refold in seconds and are located in the N-terminal two thirds of the molecule where they are loosely packed as a dynamic, four-helix bundle. The C-terminal third of the protein forms an intrinsically disordered domain that mediates initial binding to phospholipid surfaces, which occurs with coupled α-helix formation. The lipid affinity of apoA-I confers detergent-like properties; it can solubilize vesicular phospholipids to create discoidal HDL particles with diameters of approximately 10 nm. Such particles contain a segment of phospholipid bilayer and are stabilized by two apoA-I molecules that are arranged in an anti-parallel, double-belt conformation around the edge of the disc, shielding the hydrophobic phospholipid acyl chains from exposure to water. The apoA-I molecules are in a highly dynamic state, and they stabilize discoidal particles of different sizes by certain segments forming loops that detach reversibly from the particle surface. The flexible apoA-I molecule adapts to the surface of spherical HDL particles by bending and forming a stabilizing trefoil scaffold structure. The above characteristics of apoA-I enable it to partner with ABCA1 in mediating efflux of cellular phospholipid and cholesterol and formation of a heterogeneous population of nascent HDL particles. Novel insights into the structure-function relationships of apoA-I should help reveal mechanisms by which HDL subclass distribution can be manipulated.
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Affiliation(s)
- Michael C Phillips
- Lipid Research Group, Division of Gastroenterology, Hepatology and Nutrition, The Children's Hospital of Philadelphia, University of Pennsylvania Perelman School of Medicine, Philadelphia, PA.
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13
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Folded functional lipid-poor apolipoprotein A-I obtained by heating of high-density lipoproteins: relevance to high-density lipoprotein biogenesis. Biochem J 2012; 442:703-12. [PMID: 22150513 DOI: 10.1042/bj20111831] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
HDL (high-density lipoproteins) remove cell cholesterol and protect from atherosclerosis. The major HDL protein is apoA-I (apolipoprotein A-I). Most plasma apoA-I circulates in lipoproteins, yet ~5% forms monomeric lipid-poor/free species. This metabolically active species is a primary cholesterol acceptor and is central to HDL biogenesis. Structural properties of lipid-poor apoA-I are unclear due to difficulties in isolating this transient species. We used thermal denaturation of human HDL to produce lipid-poor apoA-I. Analysis of the isolated lipid-poor fraction showed a protein/lipid weight ratio of 3:1, with apoA-I, PC (phosphatidylcholine) and CE (cholesterol ester) at approximate molar ratios of 1:8:1. Compared with lipid-free apoA-I, lipid-poor apoA-I showed slightly altered secondary structure and aromatic packing, reduced thermodynamic stability, lower self-associating propensity, increased adsorption to phospholipid surface and comparable ability to remodel phospholipids and form reconstituted HDL. Lipid-poor apoA-I can be formed by heating of either plasma or reconstituted HDL. We propose the first structural model of lipid-poor apoA-I which corroborates its distinct biophysical properties and postulates the lipid-induced ordering of the labile C-terminal region. In summary, HDL heating produces folded functional monomolecular lipid-poor apoA-I that is distinct from lipid-free apoA-I. Increased adsorption to phospholipid surface and reduced C-terminal disorder may help direct lipid-poor apoA-I towards HDL biogenesis.
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14
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Gao X, Yuan S, Jayaraman S, Gursky O. Role of apolipoprotein A-II in the structure and remodeling of human high-density lipoprotein (HDL): protein conformational ensemble on HDL. Biochemistry 2012; 51:4633-41. [PMID: 22631438 DOI: 10.1021/bi300555d] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
High-density lipoproteins (HDL, or "good cholesterol") are heterogeneous nanoparticles that remove excess cell cholesterol and protect against atherosclerosis. The cardioprotective action of HDL and its major protein, apolipoprotein A-I (apoA-I), is well-established, yet the function of the second major protein, apolipoprotein A-II (apoA-II), is less clear. In this review, we postulate an ensemble of apolipoprotein conformations on various HDL. This ensemble is based on the crystal structure of Δ(185-243)apoA-I determined by Mei and Atkinson combined with the "double-hairpin" conformation of apoA-II(dimer) proposed in the cross-linking studies by Silva's team, and is supported by the wide array of low-resolution structural, biophysical, and biochemical data obtained by many teams over decades. The proposed conformational ensemble helps integrate and improve several existing HDL models, including the "buckle-belt" conformation of apoA-I on the midsize disks and the "trefoil/tetrafoil" arrangement on spherical HDL. This ensemble prompts us to hypothesize that endogenous apoA-II (i) helps confer lipid surface curvature during conversion of nascent discoidal HDL(A-I) and HDL(A-II) containing either apoA-I or apoA-II to mature spherical HDL(A-I/A-II) containing both proteins, and (ii) hinders remodeling of HDL(A-I/A-II) by hindering the expansion of the apoA-I conformation. Also, we report that, although endogenous apoA-II circulates mainly on the midsize spherical HDL(A-I/A-II), exogenous apoA-II can bind to HDL of any size, thereby slightly increasing this size and stabilizing the HDL assembly. This suggests distinctly different effects of the endogenous and exogenous apoA-II on HDL. Taken together, the existing results and models prompt us to postulate a new structural and functional role of apoA-II on human HDL.
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Affiliation(s)
- Xuan Gao
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118, USA
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15
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Lagerstedt JO, Budamagunta MS, Liu GS, DeValle NC, Voss JC, Oda MN. The "beta-clasp" model of apolipoprotein A-I--a lipid-free solution structure determined by electron paramagnetic resonance spectroscopy. Biochim Biophys Acta Mol Cell Biol Lipids 2012; 1821:448-55. [PMID: 22245143 DOI: 10.1016/j.bbalip.2011.12.010] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2011] [Revised: 12/06/2011] [Accepted: 12/20/2011] [Indexed: 02/02/2023]
Abstract
Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoproteins (HDL) and plays a central role in cholesterol metabolism. The lipid-free/lipid-poor form of apoA-I is the preferred substrate for the ATP-binding cassette transporter A1 (ABCA1). The interaction of apoA-I with ABCA1 leads to the formation of cholesterol laden high density lipoprotein (HDL) particles, a key step in reverse cholesterol transport and the maintenance of cholesterol homeostasis. Knowledge of the structure of lipid-free apoA-I is essential to understanding its critical interaction with ABCA1 and the molecular mechanisms underlying HDL biogenesis. We therefore examined the structure of lipid-free apoA-I by electron paramagnetic resonance spectroscopy (EPR). Through site directed spin label EPR, we mapped the secondary structure of apoA-I and identified sites of spin coupling as residues 26, 44, 64, 167, 217 and 226. We capitalize on the fact that lipid-free apoA-I self-associates in an anti-parallel manner in solution. We employed these sites of spin coupling to define the central plane in the dimeric apoA-I complex. Applying both the constraints of dipolar coupling with the EPR-derived pattern of solvent accessibility, we assembled the secondary structure into a tertiary context, providing a solution structure for lipid-free apoA-I. 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)
- Jens O Lagerstedt
- Department of Experimental Medical Science, Lund University, S-221 84 Lund, Sweden
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16
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Petrlova J, Duong T, Cochran MC, Axelsson A, Mörgelin M, Roberts LM, Lagerstedt JO. The fibrillogenic L178H variant of apolipoprotein A-I forms helical fibrils. J Lipid Res 2011; 53:390-398. [PMID: 22184756 PMCID: PMC3276462 DOI: 10.1194/jlr.m020883] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
A number of amyloidogenic variants of apoA-I have been discovered but most have not
been analyzed. Previously, we showed that the G26R mutation of apoA-I leads to
increased β-strand structure, increased N-terminal protease susceptibility, and
increased fibril formation after several days of incubation. In vivo, this and other
variants mutated in the N-terminal domain (residues 26 to ∼90) lead to renal and
hepatic accumulation. In contrast, several mutations identified within residues 170
to 178 lead to cardiac, laryngeal, and cutaneous protein deposition. Here, we
describe the structural changes in the fibrillogenic variant L178H. Like G26R, the
initial structure of the protein exhibits altered tertiary conformation relative to
wild-type protein along with decreased stability and an altered lipid binding
profile. However, in contrast to G26R, L178H undergoes an increase in helical
structure upon incubation at 37°C with a half time (t1/2) of about 12
days. Upon prolonged incubation, the L178H mutant forms fibrils of a diameter of 10
nm that ranges in length from 30 to 120 nm. These results show that apoA-I, known for
its dynamic properties, has the ability to form multiple fibrillar conformations,
which may play a role in the tissue-specific deposition of the individual
variants.
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Affiliation(s)
- Jitka Petrlova
- Department of Experimental Medical Sciences, Lund University, S-221 84 Lund, Sweden; and
| | - Trang Duong
- Department of Chemistry, California State University Sacramento, Sacramento, CA 95819
| | - Megan C Cochran
- Department of Chemistry, California State University Sacramento, Sacramento, CA 95819
| | - Annika Axelsson
- Department of Experimental Medical Sciences, Lund University, S-221 84 Lund, Sweden; and
| | - Matthias Mörgelin
- Department of Experimental Medical Sciences, and Department of Infection Medicine, Lund University, S-221 84 Lund, Sweden; and
| | - Linda M Roberts
- Department of Chemistry, California State University Sacramento, Sacramento, CA 95819.
| | - Jens O Lagerstedt
- Department of Experimental Medical Sciences, Lund University, S-221 84 Lund, Sweden; and.
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17
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Jayaraman S, Abe-Dohmae S, Yokoyama S, Cavigiolio G. Impact of self-association on function of apolipoprotein A-I. J Biol Chem 2011; 286:35610-35623. [PMID: 21835924 DOI: 10.1074/jbc.m111.262485] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Self-association is an inherent property of the lipid-free forms of several exchangeable apolipoproteins, including apolipoprotein A-I (apoA-I), the main protein component of high density lipoproteins (HDL) and an established antiatherogenic factor. Monomeric lipid-free apoA-I is believed to be the biologically active species, but abnormal conditions, such as specific natural mutations or oxidation, produce an altered state of self-association that may contribute to apoA-I dysfunction. Replacement of the tryptophans of apoA-I with phenylalanines (ΔW-apoA-I) leads to unusually large and stable self-associated species. We took advantage of this unique solution property of ΔW-apoA-I to analyze the role of self-association in determining the structure and lipid-binding properties of apoA-I as well as ATP-binding cassette A1 (ABCA1)-mediated cellular lipid release, a relevant pathway in atherosclerosis. Monomeric ΔW-apoA-I and wild-type apoA-I activated ABCA1-mediated cellular lipid release with similar efficiencies, whereas the efficiency of high order self-associated species was reduced to less than 50%. Analysis of specific self-associated subclasses revealed that different factors influence the rate of HDL formation in vitro and ABCA1-mediated lipid release efficiency. The α-helix-forming ability of apoA-I is the main determinant of in vitro lipid solubilization rates, whereas loss of cellular lipid release efficiency is mainly caused by reduced structural flexibility by formation of stable quaternary interactions. Thus, stabilization of self-associated species impairs apoA-I biological activity through an ABCA1-mediated mechanism. These results afford mechanistic insights into the ABCA1 reaction and suggest self-association as a functional feature of apoA-I. Physiologic mechanisms may alter the native self-association state and contribute to apoA-I dysfunction.
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Affiliation(s)
- Shobini Jayaraman
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118
| | - Sumiko Abe-Dohmae
- Department of Biochemistry, Nagoya City University Graduate School of Medical Sciences, Nagoya 467-8601, Japan
| | - Shinji Yokoyama
- Department of Biochemistry, Nagoya City University Graduate School of Medical Sciences, Nagoya 467-8601, Japan
| | - Giorgio Cavigiolio
- Children's Hospital Oakland Research Institute, Oakland, California 94609.
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18
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Lyssenko NN, Hata M, Dhanasekaran P, Nickel M, Nguyen D, Chetty PS, Saito H, Lund-Katz S, Phillips MC. Influence of C-terminal α-helix hydrophobicity and aromatic amino acid content on apolipoprotein A-I functionality. Biochim Biophys Acta Mol Cell Biol Lipids 2011; 1821:456-63. [PMID: 21840419 DOI: 10.1016/j.bbalip.2011.07.020] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2011] [Revised: 06/30/2011] [Accepted: 07/29/2011] [Indexed: 11/17/2022]
Abstract
The apoA-I molecule adopts a two-domain tertiary structure and the properties of these domains modulate the ability to form HDL particles. Thus, human apoA-I differs from mouse apoA-I in that it can form smaller HDL particles; the C-terminal α-helix is important in this process and human apoA-I is unusual in containing aromatic amino acids in the non-polar face of this amphipathic α-helix. To understand the influence of these aromatic amino acids and the associated high hydrophobicity, apoA-I variants were engineered in which aliphatic amino acids were substituted with or without causing a decrease in overall hydrophobicity. The variants human apoA-I (F225L/F229A/Y236A) and apoA-I (F225L/F229L/A232L/Y236L) were compared to wild-type (WT) apoA-I for their abilities to (1) solubilize phospholipid vesicles and form HDL particles of different sizes, and (2) mediate cellular cholesterol efflux and create nascent HDL particles via ABCA1. The loss of aromatic residues and concomitant decrease in hydrophobicity in apoA-I (F225L/F229A/Y236A) has no effect on protein stability, but reduces by a factor of about three the catalytic efficiencies (V(max)/K(m)) of vesicle solubilization and cholesterol efflux; also, relatively large HDL particles are formed. With apoA-I (F225L/F229L/A232L/Y236L) where the hydrophobicity is restored by the presence of only leucine residues in the helix non-polar face, the catalytic efficiencies of vesicle solubilization and cholesterol efflux are similar to those of WT apoA-I; this variant forms smaller HDL particles. Overall, the results show that the hydrophobicity of the non-polar face of the C-terminal amphipathic α-helix plays a critical role in determining apoA-I functionality but aromatic amino acids are not required. 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)
- Nicholas N Lyssenko
- Lipid Research Group, Gastroenterology, Hepatology and Nutrition Division, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-4318, USA
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19
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Garai K, Baban B, Frieden C. Self-association and stability of the ApoE isoforms at low pH: implications for ApoE-lipid interactions. Biochemistry 2011; 50:6356-64. [PMID: 21699199 DOI: 10.1021/bi2006702] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Apolipoprotein E (apoE) isoforms are known to differentially accumulate in the lysosomes of neuronal cells, and the deleterious effects of the apoE4 isoform in Alzheimer's disease may relate to its properties at the low lysosomal pH. However, the effect of pH on the molecular properties of full-length apoE is unclear. Here we examine the pH dependence of the monomer-dimer-tetramer reaction, of lipid binding, and of the stability of the three major apoE isoforms. Using FRET measurements, we find that the association-dissociation behavior of apoE proteins changes dramatically with changes in pH. At pH 4.5, approximating the pH of the lysosome, rate constants for association and dissociation are 2-10 times faster than those at pH 7.4. Aggregation beyond the tetrameric form is also more evident at lower pH values. Stability, as measured by urea denaturation at pH 4.5, is found to be considerably greater than that at neutral pH and to be isoform dependent. Lipid binding, as measured by turbidity clearance of unilamellar vesicles of DMPC, is faster at acidic pH values and consistent with our previous hypothesis that it is only the monomeric form of apoE that binds lipid tightly. Since apoE is more stable at pH 4.5 than at neutral pH, the more rapid apoE-lipid interactions at low pH are not correlated with the stability of the apoE isoforms, but rather to the faster association-dissociation behavior. Our results indicate that pathological behavior of apoE4 may arise from altered molecular properties of this protein at the acidic pH of the lysosome.
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Affiliation(s)
- Kanchan Garai
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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20
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Gorshkova IN, Atkinson D. Enhanced binding of apolipoprotein A-I variants associated with hypertriglyceridemia to triglyceride-rich particles. Biochemistry 2011; 50:2040-7. [PMID: 21288012 PMCID: PMC6128146 DOI: 10.1021/bi200158b] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Hypertriglyceridemia (HTG) is a common lipid abnormality in humans. However, its etiology remains largely unknown. It was shown that severe HTG can be induced in mice by overexpression of wild-type (WT) apolipoprotein E (apoE) or specific apoA-I mutants. Certain mutations in apoE4 were found to affect plasma triglyceride (TG) levels in mice overexpressing the protein. HTG appeared to positively correlate with the ability of the apoE4 variants to bind to TG-rich particles, protein destabilization, and the exposure of protein hydrophobic surface in solution. Here, we propose that the apoA-I mutations that cause HTG may also lead to changes in the conformation and stability that promote binding of apoA-I to TG-rich lipoproteins. To test this hypothesis, we studied binding to TG-rich emulsion and biophysical properties of the apoA-I mutants that induce HTG, apoA-I[E110A/E111A] and apoA-I[Δ(61-78)], and compared them to those of WT apoA-I and another apoA-I mutant, apoA-I[Δ(89-99)], that does not induce HTG but causes hypercholesterolemia in mice. We found that the apoA-I[E110A/E111A] and apoA-I[Δ(61-78)] mutations lead to enhanced binding of apoA-I to TG-rich particles, destabilization, and greater exposure of the hydrophobic surface of the protein. The apoA-I[Δ(89-99)] mutant did not show enhanced binding to the emulsion or a more exposed hydrophobic surface. Thus, like apoE4, the apoA-I variants that cause HTG in mice have the altered conformation and stability that facilitate their binding to TG-rich lipoproteins and thereby may lead to the reduced level of lipolysis of these lipoproteins. While many factors may be involved in induction of HTG, we suggest that an increased level of association of destabilized loosely folded apolipoproteins with TG-rich lipoproteins may contribute to some cases of HTG in humans.
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Affiliation(s)
- Irina N Gorshkova
- Department of Physiology and Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, Massachusetts 02118, United States.
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