STRUCTURE-FUNCTION-RELATIONSHIPS IN SIDE-CHAIN LACTAM CROSS-LINKED PEPTIDE MODELS OF A CONSERVED N-TERMINAL DOMAIN OF APOLIPOPROTEIN-E

Bioactive peptides have multiple conformations in solution but adopt w ell-defined conformations at lipid surfaces and in interactions with r eceptors. We have used side chain lactam cross-links to stabilize seco ndary structures in the following peptide models of a conserved N-term inal domain of apolipoprotein E (cross-link periodicity in parentheses ): I, H2N-GQTLSEQVQEELLSSQVTQELRAG-COOH (none); III, H2N-GDTLKEQVQEELL SEQVKDELKAG-COOH (i to i + 3); IV, H2N-GQDLSEKVDLSEKVQEELLESQVKDELLKAG -COOH (i to i + 4); IVa, H2N-GQDLSEKVQEELLSEQVKDELLKAG-COOH (i to i 4) (lactams above the sequence, potential salt bridges below the seque nce). We previously demonstrated [Luo et al. (1994) Biochemistry 33, 1 2367-12377; Braddock et al. (1996) Biochemistry, 35, 13975-13984] that peptide III, containing lactam cross-links between the i and i + 3 si de chains, enhances specific binding of LDL via a receptor other than the LDL-receptor. Peptide III in solution consists of two short alpha helices connected by a non alpha helical segment. Here we examine the hypothesis that the domain modeled by peptide III is one antipode of a conformational switch. To model another antipode of the switch, we in troduced two strategic modifications into peptide III to examine struc ture-function relationships in this domain: (1) the spacing of the lac tam cross-links was changed (i to i + 4 in peptides IV and IVa) and (2 ) peptides IV and Na contain the two alternative sequences at a site o f a possible end-capping interaction in peptide III. The structure of peptide IV, determined by 2D-NMR, is alpha helical across its entire l ength. Despite the remarkable degree of structural order, peptide IV i s biologically inactive. In contrast, peptides III and possibly Na con tain a central interruption of the alpha helix, which appears necessar y for biological activity. These and other studies support the hypothe sis that this domain is a conformational switch which, to the extent t hat it models apolipoprotein E itself, may modulate interactions betwe en apo E and its various receptors.