MEMBRANE-FUSION IS INDUCED BY A DISTINCT PEPTIDE SEQUENCE OF THE SEA-URCHIN FERTILIZATION PROTEIN BINDIN

Fertilization in the sea urchin is mediated by the membrane-associated acrosomal protein bindin, which plays a key role in the adhesion and fusion between sperm and egg. We have investigated the structure/funct ion relationship of an 18-amino acid peptide fragment ''B18,'' which r epresents the minimal membrane binding motif of the protein and resemb les a putative fusion peptide. The peptide was found to mimic the beha vior of its parent protein bindin with respect to (a) its high affinit y for lipid bilayers, (b) the ability to aggregate and fuse vesicles, (c) the binding of Zn2+ by a histidine-rich motif, (d) the tendency to self-assemble, and (e), as indicated earlier, the adhesion to cell su rface polysaccharides. Fluorescence and light scattering assays were u sed here to monitor peptide-induced lipid mixing, leakage, and aggrega tion of large unilamellar sphingomyelin/cholesterol vesicles. For thes e activities, B18 requires the presence of Zn2+ ions, with which it fo rms oligomeric complexes and assumes a partially cc-helical conformati on, as observed by circular dichroism. We conclude that aggregation an d fusion involves a ''transcomplex'' between peptides on apposing vesi cles that are connected by Zn2+ bridges.