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Immunohistochemical localization in the stallion genital tract, and topography on spermatozoa of seminal plasma protein SSP‐7, a member of the spermadhesin protein family

Summary. SSP‐7 is a protein originally isolated from stallion seminal plasma. It has extensive amino acid sequence homology with boar spermadhesin AWN, and, like its porcine counterpart, SSP‐7 displays zona pellucida‐binding activity. Strikingly, however, immunohistochemical studies presented here show that the stallion and the boar spermadhesin homologues are secreted at different places of the male genital tract. Furthermore, indirect immunofluorescence shows that the topography of SSP‐7 on the surface of stallion spermatozoa is restricted to the equatorial segment, whereas boar AWN epitopes cover the entire acrosomal cap membrane. The different cellular origin and compartimentalization of spermadhesin molecules in different species suggest that structurally related proteins could be involved in species‐specific aspects of mammalian fertilization.