TESTICULAR BIOSYNTHESIS AND EPIDIDYMAL ENDOPROTEOLYTIC PROCESSING OF RAT SPERM SURFACE-ANTIGEN 2B1

Binding of mammalian spermatozoa to the zona pellucida of homologous e ggs is mediated by specific molecules on their surface membranes, In t he present investigation we describe the biogenesis, epididymal proces sing and cellular distribution of a plasma membrane antigen (2B1) on r at spermatozoa that has a potential role in mediating zona binding. 2B 1 is expressed postmeiotically in the testis as a precursor glycoprote in (similar to 60 kDa) that first appears on the plasma membrane of st age 6 to 8 round spermatids, Northern and western blot analyses show t hat there is a close correlation between the timing of transcription a nd expression of the glycoprotein on the cell surface, During spermati d elongation 2B1 is excluded from the head domain and is sequestered o nto the sperm tail. As spermatozoa pass through the caput epididymidis 2B1 is endoproteolytically cleaved at a specific arginine residue (Ar g 312) to produce a heterodimeric glycoprotein (similar to 40 kDa and similar to 19 kDa) containing intramolecular disulphide bridges, Endop roteolysis at Arg 312 also takes place during culture of washed testic ular or caput spermatozoa in vitro and can be prevented by serine prot einase inhibitors or enhanced by trypsinisation. However, neither proc essing in vivo or in vitro has any effect on the domain organisation o f 2B1 antigen i.e. it remains localised to the tail, These results sup port the hypothesis that sperm antigens that are important for fertili zation are synthesized as precursor molecules in the testis and are th en 'activated' during epididymal maturation and capacitation, thereby ensuring that they only become fully functional at the site of fertili zation.