The amphipathic helix concept: length effects on ideally amphipathic LiKj(i=2j) peptides to acquire optimal hemolytic activity

In a minimalist approach to modeling lytic toxins, amphipathic peptides of LiKj with i=2j composition and whose length varies from 5 to 22 residues we re studied for their ability to induce hemolysis and lipid vesicle leakage. Their sequences were designed to generate ideally amphipathic a helices wi th a single K residue per putative turn. All the peptides were lytic, their activities varying by more than a factor of 10(3) from the shortest 5-resi due-long peptide (5-mer) to the longest 22-mer. However, there was no monot onous increase versus length. The 15-mer was as active as the 22-mer and ev en more than melittin which is used as standard. Partition coefficients fro m the buffer to the membrane increased in relation to length up to 12 resid ues, then weakly decreased to reach a plateau, while they were expected to increase monotonously with peptide length and hydrophobicity as revealed fr om HPLC retention times. Fluorescence labeling by a dansyl group at the N-t erminus, or by a W near the CO-terminus, show that up to 12 residues, the p eptides were essentially monomeric while longer peptides strongly aggregate d in the solution. Lipid affinity was then controlled by peptide length and was found to be limited by folding and self-association in buffer. The lyt ic activity resulted both from lipid affinity, which varied by a factor of 20-fold, and from efficiency in disturbing the membrane when bound, the lat ter steeply and monotonously increasing with length. The 15-residue-long pe ptide, KLLKLLLKLLLKLLK, had the optimal size for highest lytic activity. Th e shallow location of the fluorescent labels in the lipids is further evide nce for a model of peptides remaining flat at the interface. (C) 1999 Elsev ier Science B.V. All rights reserved.