THE FUNCTIONS IN THE PROGESTERONE-INDUCED OOCYTE MATURATION OF TOAD UBIQUITIN CARBOXYL-TERMINAL HYDROLASE (tUCH) IS INDEPENDENT OF ITS UCH ACTIVITY

p28, a protein derived from toad ( Bufo bufo gargarizans) oocytes, has a high level of sequence homology to mouse UCH L1. We report here that it is a toad ubiquitin carboxyl-terminal hydrolase (UCH), termed tUCH for its ability to hydrolyze the UCH substrate ubiquitin ethyl ester (UboEt). The similarities of secondary structures between tUCH and UCH L1 highlight that they might have common functions. The total extracted proteins both from immature and matured oocytes contain 2% tUCH. The enzyme kinetic constants (Km and Kc) both of the tUCH and UCH L1 reveal that they possess similar catalytic properties on their common substrate Ub-AMC. Anti-tUCH monoclonal antibody (tUCH mAb) can recognize tUCH and dominant-negative tUCH (tUCH C(90)S), but not mouse UCH L1, suggesting that it does not target on the conservative UCH active sites. Furthermore, anti-tUCH mAb when injected into oocytes blocked the progesterone-induced GVBD but anti-tUCH mAb could not inhibit the tUCH catalytic hydrolysis of Ub-AMC, revealing that the implication of tUCH in the oocyte maturation regulation is not dependent on its UCH activity.