INTERACTION OF MODEL CLASS-A(1), CLASS-A(2) AND CLASS-Y AMPHIPATHIC HELICAL PEPTIDES WITH MEMBRANES

To test the hypothesis that differences in the lipid affinity of excha ngeable apolipoproteins are due to the presence of different classes o f amphipathic alpha-helical motifs which differ primarily in the distr ibution of charged amino acid residues, we designed and synthesized mo del peptides mimicking class A(1), class A(2), and class Y amphipathic helices present in these apolipoproteins. Both class A(1) and class A (2) helices have positive residues at the polar-nonpolar interface and negative residues at the center of the polar face. However, clusterin g of positive and negative residues is less exact in class A(1) compar ed to class A(2) helices, The class Y helices have two negative residu e clusters on the polar face separating the two arms and the base of t he Y motif formed by three positive residue clusters, The lipid affini ties of three 18 residue model peptides representing these; classes, A c-18A(1)-NH2 (Ac-ELLEKWKEALAALKEALK-NH2), Ac-18A(2)-NH2 (Ac-ELLEKWKEAL AALAEKLK-NH2), and Ac-18(1)-NH2 (Ac-ELLEKWKEALAALAEKLK-NH2), were dete rmined by right-angle light scattering, circular dichroism spectroscop y, differential scanning calorimetry, and fluorescence spectroscopy, T he observed rank order of lipid affinity of these three peptides is: A c-18A(2)-NH2 > Ac-18Y-NH2 > Ac-18A(1)-NH2. This order is consistent wi th the known lipid affinity of exchangeable apolipoproteins containing class A(1), class A(2), and class Y helices (class A(2) > class Y > c lass A(1)), Results of this study illustrate the important role of int erfacial lysine residues in modulating the lipid affinity of amphipath ic helices and suggest that the effect of interfacial lysine residues in increasing lipid affinity is additive. We propose that interfacial lysine residues, in addition to widening the hydrophobic face because of snorkeling, also help anchor the amphipathic helix in the lipid bil ayer.