AN ACROSOMAL PROTEIN, SP32, IN MAMMALIAN SPERM IS A BINDING-PROTEIN SPECIFIC FOR 2 PROACROSINS AND AN ACROSIN INTERMEDIATE

An acrosomal protein, sp32, was completely purified from acid extracts of ejaculated porcine sperm. Purified sp32 gave a single 32-kDa prote in band on SDS-polyacrylamide gel electrophoresis and was characterize d as a binding protein specific for 55-, 53-, and 49-kDa forms of (pro )acrosin. This protein was not capable of binding a 43-kDa acrosin int ermediate and 35-kDa mature acrosin. sp32 significantly accelerated au toactivation of proacrosin at a basic pH in vitro and affected the mat uration pathway of proacrosin. In the presence of sp32, the 49-kDa acr osin intermediate from the 55- and 53-kDa proacrosins was accumulated, instead of the 43-kDa acrosin intermediate. These results suggest tha t sp32 interacts with both the amino- and carboxyl-terminal sequences of the 53-kDa proacrosin. The cDNA clones coding for porcine and guine a pig sp32 have been identified from testis cDNA libraries in lambdagt 11. The deduced amino acid sequence indicates that sp32 is initially s ynthesized as a 61-kDa precursor protein with a putative signal peptid e at the amino terminus. The carboxyl-terminal half of the precursor m olecule corresponds to the mature sp32. Thus, sp32 is produced by post -translational modification of the precursor. The binding of sp32 to p roacrosin may be involved in packaging the acrosin zymogen into the ac rosomal matrix.