The epitopes of apolipoprotein A-I define distinct structural domains including a mobile middle region

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.