title: The epitopes of apolipoprotein A-I define distinct structural domains including a mobile middle region.
abstract: The definition of epitopes on apoA-I provides evidence for a very dense packing of the peptide chain and supports the proposed supersecondary structure, made of repetitive antiparallel helices. Screening with overlapping synthetic hexapeptides shows that only a few epitopes are continuous and that all continuous contact sequences identified within the epitopes coincide or are contiguous to a putative beta-turn, such as residues 1-8, 48-55, 98-104, 118-123, and 135-140. On the N-terminal half of apoA-I we identified 6 overlapping tertiary discontinuous epitopes, uniquely constituted by amino acids and discontinuous sequences on helical segments that are far apart and which define a particular region with a complex tertiary structure. Among these are the epitopes for antibody 5G6, which reacts with residues 45-51, 83-92, 119-126, and 136-143; for A16, which reacts with 14-19, 23-28, and 60-82; and for r-FC1, which reacts with 1-8, 29-35, 78-83, and 98-121. The very far apart discontinuous sequences included in these epitopes can be explained by the predicted turns and coiled domains, and thus provide evidence for such a tertiary structure. Alternatively, these results could also be explained by intermolecular epitopes involving the N-terminal region. In contrast, in the middle of apoA-I, all identified epitopes are shorter and discontinuous within the secondary structure and are constituted by residues forming a beta-turn with all or part of an adjacent alpha-helix. We hypothesize that these multiple epitopes, that are mostly limited to a single helix, reflect the existence of a very mobile domain, possibly with hinged pairs of adjacent alpha-helices. On the C-terminal half of apoA-I, several monoclonal antibodies react with overlapping epitopes located between residues 149 and 186, which probably reflects 2 antiparallel alpha-helices interrupted by a beta-turn.
authors.affiliation: Laboratory of Lipoprotein Metabolism, Clinical Research Institute of Montreal, Québec, Canada.
authors.forename: Y L
authors.initials: YL
authors.lastname: Marcel
authors.forename: P R
authors.initials: PR
authors.lastname: Provost
authors.forename: H
authors.initials: H
authors.lastname: Koa
authors.forename: E
authors.initials: E
authors.lastname: Raffai
authors.forename: N V
authors.initials: NV
authors.lastname: Dac
authors.forename: J C
authors.initials: JC
authors.lastname: Fruchart
authors.forename: E
authors.initials: E
authors.lastname: Rassart
mesh_terms: D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000276:immunology*; D000939:Epitopes / Q000235:genetics*; D011487:Protein Conformation; D011863:Radioimmunoassay
chemical_list: D000911:Antibodies, Monoclonal; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D000939:Epitopes