Conformation and interactions of bombolitin I analogues with SDS micelles and phospholipid vesicles: CD, fluorescence, two-dimensional NMR and computer simulations.

Abstract

Bombolitins are five structurally related heptadecapeptides acting at the membrane level able to lyse erythrocytes and liposomes and to enhance the activity of phospholipase A2 (PLA2). In the presence of SDS or phospholipid vesicles bombolitins are able to form amphiphilic alpha-helical structures and this property seems to be the major determinant of bioactivity. In order to test the model of interaction between bombolitin I and membranes, an analogue was synthesized in which all the lysines were replaced by arginines: Ile-Arg-Ile-Thr-Thr-Met-Leu-Ala-Arg-Ile-Gly-Arg-Val-Leu-Ala-His-Val-NH2 ([Arg2,9,12, Ile10]bombolitin I). The design of this sequence allowed the synthesis of a second analogue through a specific postsynthetic dansylation at the epsilon-amino group of a lysine residue replacing the original leucine residue at position 7. The first analogue was fully characterized by CD and two-dimensional nmr in the presence of SDS or phospholipid vesicles. The peptide folds into an amphiphilic alpha-helical conformation with the helical segment spanning the central part of the sequence from Ile3 to His16. This behavior is identical to that observed for the native sequence. The replacement of lysine residues by arginine has no detectable effect on the conformational preference of the peptide chain. By CD and fluorescence spectroscopy measurements, the fluorophore-containing analogue [Arg2,9,12, Lys7 (epsilon-dansyl)] bombolitin I also folded into the alpha-helical conformation in the presence of SDS micelles or phospholipid vesicles. In particular, the dansyl fluorophore, which is located approximately in the middle of the apolar surface of the amphiphilic helix, is clearly buried in a hydrophobic environment when the peptide is bound to phospholipid vesicles. These findings support the hypothesis that the peptide helices are oriented parallel to the vesicle surface.