Fusion activity of an amphiphilic polypeptide having acidic amino acid residues: generation of fusion activity by α-helix formation and charge neutralization

Abstract

A sequential polypeptide, poly(Glu-Aib-Leu-Aib) (Aib represents 2-aminoisobutyric acid), was synthesized and the pH-dependence of fusogenic activity of the polypeptide was studied. The polypeptide was designed to take amphiphilic structure upon the formation of α-helix. Circular dichroism spectra of the polypeptide showed a negative Cotton effect with double minima, indicative of an a-helical conformation. The α-helix content was increased with lowering pH and/or increasing the ionic strength. It was found that the polypeptide induces remarkable leakage of calcein from egg-yolk phosphatidylcholine (EYPC) vesicles loaded in the inner aqueous phase with lowering pH and/or increasing ionic strength. The polypeptide caused fusion of EYPC liposomes and dipalmitoylphosphatidylcholine liposomes more strongly with decreasing pH. Moreover, two distinct increases of fusogenic activity of the polypeptide were observed near pH 6.0 and below pH 4.5. The former corresponds to the midpoint of pH-dependent change in helical content of the polypeptide and the latter the p K a of the γ-carboxyl group of glutamic acid. These results indicate that elevation of the fusogenic activity of the polypeptide is related to the increase in two factors, a-helix content and hydrophobicity.