A kinked antimicrobial peptide from Bombina maxima. II. Behavior in phospholipid bilayers

The preceding contribution by Toke et al. has studied the structure of the cationic antimicrobial peptide maximin-4 in detergent micelles and in organic solvent, revealing a different kink angle and side-chain interactions in the two different environments. Here, we have examined the same peptide in lipid bilayers using oriented circular dichroism (OCD) and solid-state (15)N nuclear magnetic resonance (NMR) in aligned samples. OCD showed that maximin-4 is helical and adopts an oblique alignment in the membrane, and lacks the characteristic realignment response that is often observed for amphipathic α-helical peptides at a peptide:lipid ratio between 1:100 and 1:20. Solid-state (15)N-NMR experiments suggest that maximin-4 also remains unaffected by lipid charge and temperature. Analyzing (15)N labels in positions Ala12, Ala13, and Leu14, an oblique tilt angle of the N-terminal helix of ~130° relative to the membrane normal was found, in good agreement with the amphiphilic profile of this segment. An additional constraint at Ala22 in the C-terminal segment is found to be compatible with a continuous α-helix, but unfavorable side-chain interactions make this solution unlikely. Instead, a kink at Gly16 seems fully compatible with all known constraints and with the biophysical expectations in the membrane-bound state, given the liquid-state NMR structures. It thus seems that the flexible kink in maximin-4 allows the two helical segments to adjust to the local environment. The irregular amphiphilic profile and the resulting versatility in shape might explain why maximin-4 lacks the realignment response that has been characteristically observed for many related frog peptides forming straight amphipathic α-helices.