MEMBRANE INTERACTION AND CONFORMATIONAL PROPERTIES OF THE PUTATIVE FUSION PEPTIDE OF PH-30, A PROTEIN ACTIVE IN SPERM-EGG FUSION

A peptide representing the putative fusion domain of PH-30, a sperm su rface protein involved in sperm-egg fusion, was synthesized, and its i nteraction with model lipid membranes was characterized by biophysical methods. While the peptide binds to the vesicles composed of both neu tral and acidic lipids, the apparent affinity is significantly higher for the latter lipid class. The intervesicular lipid mixing assay sugg ests that the synthetic peptide is able to induce fusion of large unil amellar vesicles. Circular dichroism and Fourier-transform infrared sp ectroscopy show that while in an aqueous buffer the peptide exists in an essentially unordered conformation, binding to the membranes result s in a conformational transition to a beta-structure. These data indic ate that the fragment identified on the alpha-subunit of PH-30 as a pu tative fusion peptide is indeed a good candidate for this role. Howeve r, in contrast to what has been proposed for some viral fusion peptide s, the PH-30 fusion domain is highly unlikely to act as an insertional ''sided'' helix.