Role of Suc1 in the activation of the cyclosome by protein kinase Cdk1 cyclin B

A large complex, called the cyclosome or anaphase-promoting complex, has sp ecific and regulated protein-ubiquitin ligase activity that targets mitotic regulators (such as cyclin B) for degradation at the end of mitosis. In ea rly embryonic cell cycles the cyclosome is inactive in the interphase, but is subsequently converted by protein kinase Cdk1/cyclin B to an active, pho sphorylated form, in a process that includes an initial lag period. This ti me lag may be important to prevent premature self-inactivation of Cdk1/cycl in B before the end of mitosis. We have previously observed that the phosph orylated form of the cyclosome binds to Suc1, a protein that associates wit h Cdk1 and with phosphate-containing compounds. We now report that low, phy siological concentrations of Suc1 stimulate the activation of the interphas e form of the cyclosome by the protein kinase. When Suc1 was present from t he beginning of the incubation together with protein kinase Cdk1/cyclin B, activation of the cyclosome took place with the normal lag kinetics. Howeve r, when interphase cyclosome was first incubated with protein kinase Cdk1/c yclin B without Suc1, the subsequent addition of Suc1 caused a rapid burst of cyclosome activation and the lag was completely abolished. These finding s are consistent with the interpretation that following initial slow phosph orylations of the cyclosome by the protein kinase, Suc1 accelerates multipl e phosphorylations that culminate in the full activation of the cyclosome. In support of this interpretation, we find that Suc1 stimulates the phospho rylation of several proteins in the preparation of interphase cyclosome and that the effect of Suc1 on phosphorylation was augmented by prior incubati on of interphase cyclosome with protein kinase Cdk1/cyclinB. (C) 1999 Acade mic Press.