Localization of a heparin-binding protein to distinct regions of bovine sperm

Heparin-binding proteins (HBP) in bull seminal fluid bind to epididymal sperm membranes at ejaculation. Peptides recognized by a monoclonal antibody (M1) correspond to proteins identified in complexes that have the greatest affinity for heparin and are present on sperm from bulls with higher fertility. Presence of specific HBP on sperm regulates the ability of sperm to bind heparin, and heparin binding to sperm correlates with the fertility potential of a bull. In these studies, the interaction of HBP with sperm from 10 bulls of proven fertility was analyzed by immunofluorescence of M1 to determine the localization of heparin-binding proteins during capacitation, and the fluorescent binding patterns were compared to bull nonreturn rates. Immunofluorescent localization of M1 binding sites revealed the existence of specific membrane domains containing HBP in acrosomal and postacrosomal regions of ejaculated but not in epididymal sperm. Monoclonal antibody recognition of HBP localized on membranes of sperm revealed variable binding patterns of M1 to the acrosomal region in sperm from bulls of known fertility. Regression analysis indicated a negative relationship between sperm displaying exclusively acrosomal fluorescence and bull nonreturn rate. These data indicate that HBP bind to sperm in distinct patterns, one of which differed among bulls of varying fertility, and indicate no apparent relocalization of these sites during cellular changes that occur in preparation for fertilization.