IMMUNOHISTOCHEMICAL LOCALIZATION IN THE STALLION GENITAL-TRACT, AND TOPOGRAPHY ON SPERMATOZOA OF SEMINAL PLASMA-PROTEIN SSP-7, A MEMBER OF THE SPERMADHESIN PROTEIN FAMILY

SSP-7 is a protein originally isolated from stallion seminal plasma. I t has extensive amino acid sequence homology with boar spermadhesin AW N, and, like its porcine counterpart, SSP-7 displays zona pellucida-bi nding activity. Strikingly, however, immunohistochemical studies prese nted here show that the stallion and the boar spermadhesin homologues are secreted at different places of the male genital tract. Furthermor e, indirect immunofluorescence shows that the topography of SSP-7 on t he surface of stallion spermatozoa is restricted to the equatorial seg ment, whereas boar AWN epitopes cover the entire acrosomal cap membran e. The different cellular origin and compartimentalization of spermadh esin molecules in different species suggest that structurally related proteins could be involved in species-specific aspects of mammalian fe rtilization.