Liposome fusion induced by a M(r) 18,000 protein localized to the acrosomal region of acrosome-reacted abalone spermatozoa.

A M(r) 18,000 protein is secreted by abalone spermatozoa during the acrosome reaction. Immunofluorescence of acrosome-reacted sperm localizes the protein as a coating on the spent acrosomal granule hull and on the surface of the acrosomal process. The membrane of the acrosomal process fuses with the egg plasma membrane at fertilization. The M(r) 18,000 acrosomal protein aggregates negatively charged (but not neutral) large unilamellar liposomes and renders them permeable to internal probe. The M(r) 18,000 proteins from two abalone species are potent inducers of intervesicular lipid mixing in the resonance energy transfer assay, suggesting that they mediate the fusion of lipid bilayers. Predicted secondary structures of these proteins show the presence of strongly amphipathic alpha-helices that may be active in the perturbation of phospholipid bilayers. The M(r) 18,000 protein may mediate sperm-egg fusion during fertilization.