ISOLATION AND BIOCHEMICAL-CHARACTERIZATION OF STALLION SEMINAL-PLASMA PROTEINS

Seminal plasma proteins exert multiple effects on sperm function and play a pivotal role in sperm capacitation, a complex process which enables spermatozoa to undergo the zona pellucida-induced acrosome reaction. The role of most of the seminal plasma proteins on sperm physiology is however, not known. This may, in part, reflect the fact that the protein composition of seminal plasma has been investigated in only a few mammalian species. The aim of this study was the characterization of stallion seminal plasma proteins. Partial amino-acid sequences from eight different components were obtained. Among these, a protein with high sequence similarity to rat, mouse, and human epididymal sperm-coating protein, thought to play a role in the fusion of the sperm and egg plasma membranes, a calcitonin precursor gene-related protein, and a prostatic androgen-dependent kallikrein-like product were found. The amino-acid sequences of the other protein fractions failed to show similarity to any known protein structure. Interestingly, as with bovine and pig protein, the major proteins of stallion seminal plasma also constitute the low molecular weight heparin-binding components. However, the major heparin-binding proteins of different species seem to be structurally unrelated to each other, indicating the possibility that heparin-binding proteins could represent species-specific capacitation factors. On the other hand, stallion seminal plasma also contains a minor protein which cross-reacts with antibodies against the boar zona pellucida-binding protein AWN-1. Immunofluorescence analysis shows that the topology of this stallion AWN-like protein is restricted to the equatorial segment. Therefore, the data in this study indicate that proteins participating in sperm-egg interactions are structurally conserved among mammalian species.