Binding characteristics and immunolocalization of porcine seminal protein, PSP-I

PSP-I, a 13 kDa protein purified from boar seminal plasma, was found to have about 50% amino acid sequence homology with a family of zona pellucida-binding proteins known as spermadhesins. These proteins are produced by the accessory gland(s) of the male reproductive tract and coat the spermatozoa during ejaculation. In this study, we have investigated the possible biological functions of PSP-I using a solid-phase protein binding assay and its site of synthesis using both Western blot and immunocytochemical analyses. PSP-I was found to bind a number of proteins including endo-beta-galactosidase digested ZP3, soybean trypsin inhibitor, IgA, IgG and alpha-casein, indicating that it may have multiple functions. The protein or carbohydrate structures were not critical in the binding, since polyvinyl sulfate could effectively inhibit the binding of PSP-I to these proteins. Western blot analysis using specific antiserum to PSP-I showed that the protein was present in the seminal vesicle but not in the testes, epididymis or prostate. The protein was revealed by immunocytochemical analysis in the epithelium of seminal vesicles but not in the testes or the epididymis. It is concluded that PSP-I is synthesized by the epithelium of the seminal vesicles, secreted into the semen during ejaculation, and may be involved in various reproductive functions, such as preventing premature acrosome reaction and immunosuppression.