Effects of aromatic residues at the ends of transmembrane alpha-helices onhelix interactions with lipid bilayers

We have studied the effects of aromatic residues at the ends of peptides of the type Ac-KKGL(n)WL(m)KKA-amide on their interactions with lipid bilayer s as a function of lipid fatty acyl chain length, physical phase, and charg e. Peptide Ac-KKGFL(6)WL(8)FKKA-amide (F2L14) incorporated into bilayers of phosphatidylcholines containing monounsaturated fatty acyl chains of lengt hs C14-C24 at a peptide: lipid molar ratio of 1:100 in contrast to Ac-KKGL( 7)WL(9)KKA-amide (L-16) which did not incorporate at all into dierucoylphos phatidylcholine [di(C24:1)PC]; Ac-KKGYL(6)WL(8)YKKA-amide (Y2L14) incorpora ted partly into di(C24:1)PC. Lipid-binding constants relative to that for d ioleoylphosphatidylcholine (C18: 1)PC were obtained using a fluorescence qu enching method. For Y2L14 and F2L14, relative lipid-binding constants incre ased with increasing fatty acyl chain length from C14 to C24; strongest bin ding did not occur at the point where the hydrophobic length of the peptide equalled the hydrophobic thickness of the bilayer. For Ac-KKGYL(9)WL(11)YK KA-amide (Y2L20), increasing chain length from C18 to C24 had little effect on relative binding constants. Anionic phospholipids bound more strongly t han zwitterionic phospholipids to Y2L14 and Y2L20 but effects of charge wer e relatively small. In two phase (gel and liquid crystalline) mixtures, all the peptides partitioned more strongly into liquid crystalline than gel ph ase; effects were independent of the structure of the peptide or of the lip id (dipalmitoylphosphatidylcholine or bovine brain sphingomyelin). Addition of cholesterol had little effect on incorporation of the peptides into lip id bilayers. It is concluded that the presence of aromatic residues at the ends of transmembrane ct-helices effectively buffers them against changes i n bilayer thickness caused either by an increase in the chain length of the phospholipid or by the presence of cholesterol.