The cDNA cloning of a 55-kilodalton protein from guinea pig seminal vesicle. Evidence that the protein is the precursor of a 25-kilodalton basic secretory protein.

Guinea pig seminal vesicle epithelium synthesizes and secretes large amounts of four secretory proteins including a basic protein designated SVP-1. The latter migrates as a protein of 25 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. When total cellular RNA from seminal vesicle epithelium was translated in vitro by a rabbit reticulocyte system, SVP-1 was not identifiable as a product. In contrast, a major product of the reticulocyte system was a 55-kDa protein which paradoxically could not be identified as one of the four known secretory proteins or as an abundant tissue protein. We isolated a cDNA clone which corresponded to the mRNA which coded for the 55-kDa protein. The mRNA was 1800 bases and of very high abundance; only the transcripts corresponding to the major secretory proteins SVP-3 and SVP-4 were as conspicuous. From these observations, we decided to test whether the 55-kDa protein was related to SVP-1. The 55-kDa protein was found to share properties of SVP-1, including an alkaline pI and selection by antibody against SVP-1. After proteolytic digestion of the 55-kDa protein, one fragment co-migrated with authentic SVP-1. Furthermore, detectable fragments of the 55-kDa protein smaller than SVP-1 all co-migrated with fragments generated from the proteolytic digestion of SVP-1. Finally, we show that the 55-kDa protein could be processed in vitro by seminal vesicle lumenal extracts to yield a 30-kDa protein and a protein the size of native SVP-1. The best explanation of our results is that the 55-kDa protein represents the primary translation product and precursor of SVP-1. Our proteolytic mapping and in vitro processing studies are consistent with the idea that the 55-kDa protein is a tandem repeat of two SVP-1 molecules. We used the cDNA clone to study the expression of the corresponding gene in different states of androgen depletion and repletion.