Enzymatic properties of the proteasome purified from starfish oocytes and its catalytic subunits involved in oocyte maturation

The 20S proteasome was purified from oocytes of the starfish Asterina pecti nifera and its enzymatic properties were investigated. The chymotrypsin-lik e activities were potently inhibited by PSI as well as MG115, whereas the t rypsin-like and peptidyl-glutamyl peptide-hydrolyzing (PGPH) activities wer e not or only weakly inhibited by PSI and MG115. The inhibitory ability of MG115 toward germinal vesicle breakdown (GVBD) coincided with those toward the trypsin-like and PGPH activities, and PSI showed no inhibitory effect o n GVBD. We have previously reported that the inhibition pattern toward GVBD of peptidyl-argininals, which potently inhibited the proteasomal trypsin-l ike activity rather than the chymotrypsin-like activity, correlated with th e inhibition pattern toward the chymotrypsin-like activity of the proteasom e. These results; together with the peptidyl-argininals scarcely inhibiting the PGPH activity at concentrations sufficient for the inhibition toward G VBD, indicate that both the chymotrypsin-like and trypsin-like activities, but not the PGPH activity, of the proteasome are responsible for degradatio n of the physiological substrate during starfish oocyte maturation. It was also suggested that the inhibition of a single catalytic site of the protea some is not sufficient for prevention of the proteasomal function. (C) 2000 Elsevier Science Inc. All rights reserved.