Study of cyclin proteolysis in anaphase-promoting complex (APC) mutant cells reveals the requirement for APC function in the final steps of the fission yeast septation initiation network

Cytokinesis in eukaryotic cells requires the inactivation of mitotic cyclin -dependent kinase complexes. An apparent exception to this relationship is found in Schizosaccharomyces pombe mutants with mutations of the anaphase-p romoting complex (APC). These conditional lethal mutants arrest with unsegr egated chromosomes because they cannot degrade the securin, Cut2p. Although failing at nuclear division, these mutants septate and divide. Since septa tion requires Cdc2p inactivation in wild-type S. pombe, it has been suggest ed that Cdc2p inactivation occurs in these mutants by a mechanism independe nt of cyclin degradation. In contrast to this prediction, we show that Cdc2 p kinase activity fluctuates in APC cut mutants due to Cdc13/cyclin B destr uction. In APC-null mutants, however, septation and cutting do not occur an d Cdc13p is stable. We conclude that APC cut mutants are hypomorphic with r espect to Cdc13p degradation. Indeed, overproduction of nondestructible Cdc 13p prevents septation in APC cut mutants and the normal reorganization of septation initiation network components during anaphase.