Evidence for distinct serine protease activities with a potential role in processing the sperm protein fertilin

The guinea pig sperm protein fertilin (previously termed PH-30) plays an important role in sperm-egg fusion, and was the first recognized membrane-anchored metalloprotease/disintegrin protein. Fertilin is a heterodimeric glycoprotein which undergoes at least two distinct proteolytic processing steps. Fertilin alpha is processed first, in the testis, whereas fertilin beta is processed separately during sperm maturation in the epididymis. The final processing of fertilin beta occurs immediately adjacent to its predicted integrin ligand domain, and exposes an epitope recognized by a fusion blocking monoclonal antibody. Here, we demonstrate that one or more serine protease activities associated with testicular sperm can process fertilin beta in vitro in a fashion that closely mimics the processing pattern observed in vivo during epididymal sperm maturation. In contrast, several proteases that were added to testicular sperm did not mimic the pattern observed in vivo. These findings raise the intriguing possibility that a fertilin beta converting protease(s) active in vivo may originate from sperm, instead of from the epididymal epithelium. Further, we show that fertilin alpha is most likely processed intracellularly in the secretory pathway based on three observations: (i) only processed fertilin alpha, but not the precursor pro-alpha can be cell-surface biotinylated; (ii) some processed fertilin alpha is sensitive to endoglycosidase H, suggesting cleavage occurs prior to the medial Golgi apparatus; (iii) a reanalysis of the N-terminus of processed fertilin alpha showed that the proteolytic cleavage site is next to four arginine residues, a consensus sequence for intracellular subtilysin type pro-protein convertases. The N-terminal sequence analysis further showed that processed fertilin alpha contains an intact membrane anchored disintegrin domain, and not a truncated disintegrin domain as reported previously (Blobel, C. P., Wolfsberg, T. G., Turck, C. W., Myles, D. G., Primakoff, P., and White, J. M., Nature 356, 248-252, 1992). Proteolytic processing is thought to play an important role in regulating the function of fertilin, and the present study represents a first step toward a better understanding of protease activities involved in the maturation of fertilin, and potentially other sperm surface proteins.