Structural heterogeneity, post-translational modifications, and biological activities of SV-IV, a major protein secreted from the rat seminal vesicle epithelium

The primary structure of purified SV-IV, a major secretory protein synthesized by the rat seminal vesicle (SV) epithelium, was analysed by electrospray mass spectrometry (ES-MS). The protein was found to be highly heterogeneous. The various components were separated and identified by reversed phase high-performance liquid chromatography (HPLC) on line with ES-MS. Structural characterization of the SV-IV cyanogen bromide digests revealed the occurrence of a Val/Met substitution in about 50% of the purified protein molecules. We suggest that this mutation is the expression of a genetic polymorphism. Other minor components, corresponding to structural changes (fragmentation, deletion, and phosphorylation) of SV-IV and probably due to post-translational modifications of the native protein, were also detected. In particular, by using protein tyrosine phosphatase hydrolysis combined with ES-MS, we demonstrated that, in the phosphorylated species of SV-IV, a single phosphate group was covalently bound to the Tyr-36 residue. The significance of these findings in relation to the regulation of important biological processes, such as immune response, blood coagulation, inflammatory reaction, and mammalian reproduction, are discussed.