Design and Characterization of Endosomal‐pH‐Responsive Coiled Coils for Constructing an Artificial Membrane Fusion System

Abstract A weakly acidic pH‐responsive polypeptide is believed to have the potential for an endosome escape function in a polypeptide‐triggered delivery system. For constructing a membrane fusion device with pH‐responsiveness, we have designed novel polypeptides that are capable of forming an α2 coiled coil structure. Circular dichroism spectroscopy reveals that a polypeptide, AP‐LZ(EH5), with a Glu and His salt‐bridge pair at a staggered position in the hydrophobic core forms a stable coiled coil structure only at endosomal pH values (pH 5.0 to 5.5). On the basis of their endosomal‐pH responsiveness, a boronic acid/polypeptide conjugate (BA‐H5‐St) was also designed as a pilot molecule to construct a pH‐responsive, one‐way membrane fusion system with a sugarlike compound (phosphatidylinositol: PI)‐containing liposome as a target. Membrane fusion behavior was characterized by lipid‐mixing, inner‐leaflet lipid‐mixing, and contents‐mixing assays. These studies reveal that membrane fusion is clearly observed when the pH of the experimental system is changed from 7.4 (physiological condition) to 5.0 (endosomal condition).