Apolipoprotein A-I: structure-function relationships Philippe Frank, Yves Marcel To cite this version: Philippe Frank, Yves Marcel. Apolipoprotein A-I: structure-function relationships. Journal of Lipid Research, American Society for Biochemistry and Molecular Biology, 2000, 41. hal-02460786 HAL Id: hal-02460786 https://hal.archives-ouvertes.fr/hal-02460786 Submitted on 30 Jan 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. review Apolipoprotein A-I: structure–function relationships Philippe G. Frank1 and Yves L. Marcel2 Lipoprotein & Atherosclerosis Group, University of Ottawa Heart Institute, 40 Ruskin Street, Ottawa, Ontario, Canada K1Y 4W7 Abstract The inverse relationship between high density lipo- Supplementary key words reverse cholesterol transport • apolipopro- protein (HDL) plasma levels and coronary heart disease has tein A-I mutants • HDL • cholesterol efflux • LCAT been attributed to the role that HDL and its major constitu- ent, apolipoprotein A-I (apoA-I), play in reverse cholesterol Downloaded from transport (RCT). The efficiency of RCT depends on the spe- cific ability of apoA-I to promote cellular cholesterol efflux, This review focuses on the identification of apoA-I bind lipids, activate lecithin:cholesterol acyltransferase structural domains and their participation to HDL RCT. (LCAT), and form mature HDL that interact with specific We will summarize the current knowledge on apoA-I receptors and lipid transfer proteins. From the intensive structural domains and more particularly their function in analysis of apoA-I secondary structure has emerged our cur- lipid binding and formation of HDL, their contributions www.jlr.org rent understanding of its different classes of amphipathic to cellular cholesterol efflux, and their role in LCAT acti- ␣-helices, which control lipid-binding specificity. The main challenge now is to define apoA-I tertiary structure in its vation. In each case, a particular emphasis will be placed lipid-free and lipid-bound forms. Two models are consid- on the study of apoA-I mutants and the analysis of struc- at Thomas Jefferson Univ on January 24, 2008 ered for discoidal lipoproteins formed by association of tural and functional domains. two apoA-I with phospholipids. In the first or picket fence model, each apoA-I wraps around the disc with antiparallel adjacent ␣-helices and with little intermolecular interac- 1. APOLIPOPROTEIN A-I STRUCTURE tions. In the second or belt model, two antiparallel apoA-I are paired by their C-terminal ␣-helices, wrap around the AND HDL FORMATION lipoprotein, and are stabilized by multiple intermolecular in- Apolipoprotein A-I was one of the first apolipoproteins teractions. While recent evidence supports the belt model, other models, including hybrid models, cannot be ex- to be identified and characterized. Yet its structural char- cluded. ApoA-I ␣-helices control lipid binding and associa- acterization, notably in the lipid-associated form, has not tion with varying levels of lipids. The N-terminal helix 44– been achieved and the domains involved in its multiple 65 and the C-terminal helix 210–241 are recognized as im- functions are not yet fully identified. portant for the initial association with lipids. In the central domain, helix 100–121 and, to a lesser extent, helix 122– A. Primary structure and physicochemical 143, are also very important for lipid binding and the for- properties of apoA-I mation of mature HDL, whereas helices between residues The sequence of apoA-I was determined by Brewer et 144 and 186 contribute little. The LCAT activation domain al. (1), and followed by cloning and characterization of its has now been clearly assigned to helix 144–165 with second- cDNA (2–5) and genomic DNA (6, 7). The gene encod- ary contribution by helix 166–186. The lower lipid binding affinity of the region 144–186 may be important to the acti- ing apoA-I is a member of the apolipoprotein multigene vation mechanism allowing displacement of these apoA-I superfamily, which includes genes encoding exchange- helices by LCAT and presentation of the lipid substrates. No specific sequence has been found that affects diffu- sional efflux to lipid-bound apoA-I. In contrast, the C-terminal helices, known to be important for lipid binding and main- Abbreviations: ABC1, ATP-binding cassette transporter 1; apoA-I, tenance of HDL in circulation, are also involved in the inter- apolipoprotein A-I; CE, cholesteryl ester; CETP, cholesterol ester trans- action of lipid-free apoA-I with macrophages and specific fer protein; DMPC, dimyristoyl phosphatidylcholine; FC, free choles- lipid efflux. While much progress has been made, other as- terol; GdnHCl, guanidine hydrochloride; HDL, high density lipopro- pects of apoA-I structure–function relationships still need tein; LCAT, lecithin:cholesterol acyltransferase; LpA-I, apoA-I-containing to be studied, particularly its lipoprotein topology and its in- lipoproteins; POPC, 1-palmitoyl 2-oleylphosphatidylcholine, RCT, re- teraction with other enzymes, lipid transfer proteins and re- verse cholesterol transport; SR-BI, scavenger receptor class B type I. 1 Present address: Albert Einstein College of Medicine, Department ceptors important for HDL metabolism.—Frank, P. G., and of Molecular Pharmacology, 1300 Morris Park Avenue, The Bronx, NY, Y. L. Marcel. Apolipoprotein A-I: structure–function rela- 10461. tionships. J. Lipid Res. 2000. 41: 853–872. 2 To whom correspondence should be addressed. Journal of Lipid Research Volume 41, 2000 853 Downloaded from www.jlr.org at Thomas Jefferson Univ on January 24, 2008 Fig. 1. 854 Journal of Lipid Research Volume 41, 2000 able apolipoproteins (apoA-I, A-II, Cs, and E) (8). They HCl, urea, or calorimetric denaturation experiments, a are thought to have evolved from a common ancestor by low free energy of denaturation about 2.2–2.7 kcal/mol duplication/deletion of a 33-mer motif. has been determined (22, 23). This value is much lower Analysis of the secondary structure of mature apoA-I than the value determined for globular proteins (in the has allowed the identification of 11- and 22-mer repeats range of 5–10 kcal/mol) (24, 25). This observation is in (9, 10). The 22-mer repeats, usually separated by Pro resi- agreement with a loosely folded and relatively flexible dues, have been associated with the formation of amphi- structure of the protein in the lipid-free form. This loosely pathic ␣-helices (11), which allow the interaction of the folded conformation may allow rapid lipid-interaction of protein with phospholipids through its hydrophobic face, exposed hydrophobic portions of the protein. In support while the hydrophilic face of the helices interacts with the of this view, thermal denaturation experiments by Gursky aqueous phase. and Atkinson have suggested a molten globular-like state ApoA-I sequences from a number of species have been for lipid-free apoA-I that may explain its lipid-binding determined (Fig. 1), with sizes ranging between 258 and properties in vivo (26). Sedimentation velocity experi- 267 amino acids (mature protein plus pre-pro segment). ments on human apoA-I indicate significant conforma- Comparison of sequences amongst mammals indicates tional heterogeneity, which also support a flexible struc- that the N-terminal domain of apoA-I is highly conserved ture (27). This observation may be relevant in the case of while the central and C-terminal domains show conserva- pre␤1-HDL, which has been shown to be the earliest ac- tive substitutions between species. This result is consistent ceptor of cellular cholesterol (28) and appears to contain with the study of Collet et al. (12), who showed, using only 1–2 molecules of partially lipidated apoA-I (29). The Downloaded from mAbs to human apoA-I, that the antigenicity of this pro- N-terminal sequence (residues 1–43) of apoA-I, appar- tein is better conserved between species in the N-terminal ently not required for lipid binding, may be important for domain (residues 1 to 98 of the mature protein). These the stabilization of lipid-free apoA-I in solution (30, 31). observations are in accord with the major role this do- Site directed mutagenesis of tryptophans and fluores- main plays in the structure and/or function of human cence studies show that in the lipid-free state the apoA-I apoA-I. As described in the latter sections of this review, monomer is a prolate ellipsoid and its tryptophans exist in www.jlr.org the N- and C-terminal domains of apoA-I have been in- nonpolar environments compatible with an N-terminal volved in the binding of lipids, an essential function of half that is organized into a bundle of helices (32). this apolipoprotein (13–15). The central domain of the Two cysteine mutants (D9C and A232C) have been at Thomas Jefferson Univ on January 24, 2008 protein, which has been involved in LCAT activation, may studied by circular dichroism and fluorescence spectro- have evolved in parallel with the LCAT sequence in the photometry. The adduct formed with the thiol reactive corresponding species. However, in species in which this probe acrylodan affects the structure and stability of the domain has evolved faster than in humans (mouse and lipid-free D9C mutant but not that of the mutant A234C. rat), the ability of mouse apoA-I to activate mouse LCAT is The fluorescence analysis showed rigid N- and C-terminal reduced as compared to that of human apoA-I (16, 17). structures of the lipid-free apoA-I and a fold that brings Despite changes in the primary structure of apoA-I amongst the C-terminus close to the tryptophans of the N-terminal different species, the secondary structure appears unmod- half (33).