High-molecular-weight protease complexes (proteasomes) of sperm of the ascidian, Halocynthia roretzi: isolation, characterization, and physiological roles in fertilization.

It has been proposed that a chymotrypsin-like enzyme plays a key role in sperm penetration through the vitelline coat of eggs of the ascidian, Halocynthia roretzi. In this report, two kinds of chymotrypsin-like enzymes were isolated from spermatozoa of H. roretzi. Both enzymes were characterized as high-molecular-weight protease complexes that comprised several different subunits and that showed several different protease activities. Both showed immunoreactivity with anti-proteasome antibody. One showed properties characteristic of the 20 S proteasome and its molecular mass was 630 kDa, whereas the other protease complex of 930 kDa showed inhibitor susceptibility different from that of the 20 S proteasome. Upon the sperm reaction which is induced by treatment with high pH, the proteasomes were found to be released from sperm and they had the activity to digest the vitelline coats. These results suggest that the proteasomes are exposed extracellularly when sperm contact with the egg coat and may function as lysins in the fertilization process.