PROTEIN INVOLVEMENT IN THE FUSION BETWEEN THE EQUATORIAL SEGMENT OF ACROSOME-REACTED HUMAN SPERMATOZOA AND LIPOSOMES

Artificial membranes (liposomes) can interact with the equatorial segm ent (ES) of human spermatozoa, provided that the acrosome reaction (AR ) has occurred [Arts, Kuiken, Jager and Hoekstra (1993) fur. J. Bioche m. 217, 1001-1009]. Using fluorescently labelled liposomes, this inter action can be seen as either punctate fluorescence in the ES (lip-ESp) , reflecting only bound liposomes, or as diffuse fluorescence in this region (lip-ESd), indicating that the liposomes have fused with the ES membrane. Only equatorial segments that still contain constituents of the acrosomal matrix have the capacity to bind liposomes and eventual ly to fuse with them. Since the exposure of such intact equatorial seg ments is the exclusive result of induction of the AR under physiologic al conditions, these results imply that liposomes can be used for the rapid detection of acrosome-reacted spermatozoa. The lip-ESp and lip-E Sd patterns were shown to be reflections of two distinct properties of the ES. Proteolytic treatment after AR completely inhibited the forma tion of a lip-ESd pattern, whereas formation of the lip-ESp pattern wa s only marginally inhibited by the proteolytic treatment. The same res ults were obtained using antisperm antibodies, which did not react wit h acrosome-intact spermatozoa. Proteolytic treatment of spermatozoa be fore AR induction had no effect on the fusion capacity of the ES after subsequent AR, which implies that the putative fusion protein is not accessible before AR. Thus fusion of liposomes with the ES of human sp ermatozoa is mediated by a sperm protein(s), whereas the lip-ESp patte rn is not likely to represent the liposome-binding stage that precedes the fusion step.