Testase 1 (ADAM 24) a plasma membrane-anchored sperm protease implicated in sperm function during epididymal maturation or fertilization

Plasma membrane-anchored proteases have key roles in cell signaling, migrat ion and refashioning the cell surface and its surroundings. We report the f irst example of a plasma membrane-anchored protease on mature sperm, testas e 1 (ADAM 24), Unlike other studied sperm ADAMs (fertilin alpha and beta, c yritestin) whose metalloproterase domains are removed during sperm developm ent, we found testase 1 retains an active metalloprotease domain, suggestin g it acts as a protease on mature sperm. Testase 1 is a glycoprotein (molec ular mass 88 kDa), localized to the equatorial region of the plasma membran e of cauda epididymal sperm. Typically, proteolytic removal of the pro-doma in is an initial activation step for ADAM proteases, The pro-domain of the testase 1 precursor (108 kDa) is proteolytically removed as sperm transit t he caput epididymis to produce processed (mature) testase 1 (88 kDa), Testa se 1 is unique among all studied ADAMs in that its proteolytic processing o ccurs on the sperm plasma membrane instead of at an intracellular site (the Golgi), Using GST-fusion proteins and a synthetic testase 1 C-terminal pep tide, we found that the cytoplasmic tail of testase 1 could be phosphorylat ed in vitro by protein kinase C (PKC), Thus testase 1 apparently has a cyto plasmic PKC phosphorylation site(s), Protein kinase C is known to stimulate other ADAMs' protease activity. Because events of the acrosome reaction in clude PKC activation, we speculate that testase 1 protease function could b e important in sperm penetration of the zona pellucida after sperm PKC is a ctivated during the acrosome reaction.