LIPOSOME FUSION INDUCED BY A M(R)-18000 PROTEIN LOCALIZED TO THE ACROSOMAL REGION OF ACROSOME-REACTED ABALONE SPERMATOZOA

A M(r) 18000 protein is secreted by abalone spermatozoa during the acr osome reaction. Immunofluorescence of acrosome-reacted sperm localizes the protein as a coating on the spent acrosomal granule hull and on t he surface of the acrosomal process. The membrane of the acrosomal pro cess fuses with the egg plasma membrane at fertilization. The M(r) 180 00 acrosomal protein aggregates negatively charged (but not neutral) l arge unilamellar liposomes and renders them permeable to internal prob e. The M(r) 18000 proteins from two abalone species are potent inducer s of intervesicular lipid mixing in the resonance energy transfer assa y, suggesting that they mediate the fusion of Lipid bilayers. Predicte d secondary structures of these proteins show the presence of strongly amphipathic alpha-helices that may be active in the perturbation of p hospholipid bilayers. The M(r) 18000 protein may mediate sperm-egg fus ion during fertilization.