REGULATION OF THE CYCLIN-B DEGRADATION SYSTEM BY AN INHIBITOR OF MITOTIC PROTEOLYSIS

The initiation Of anaphase and exit from mitosis depend on the anaphas e-promoting complex (APC), which mediates the ubiquitin-dependent prot eolysis of anaphase-inhibiting proteins and mitotic cyclins. We have a nalyzed whether protein phosphatases are required for mitotic APC acti vation. In Xenopus egg extracts APC activation occurs normally in the presence of protein phosphatase 1 inhibitors, suggesting that the anap hase defects caused by protein phosphatase 1 mutation in several organ isms are not due to a failure to activate the APC. Contrary to this, t he initiation of mitotic cyclin B proteolysis is prevented by inhibito rs of protein phosphatase 2A such as okadaic acid. Okadaic acid induce s an activity that inhibits cyclin B ubiquitination. We refer to this activity as inhibitor of mitotic proteolysis because it also prevents the degradation of other APC substrates. A similar activity exists in extracts of Xenopus eggs that are arrested at the second meiotic metap hase by the cytostatic factor activity of the protein kinase mos. In X enopus eggs, the initiation of anaphase II may therefore be prevented by an inhibitor of APC-dependent ubiquitination.