Compartmentalization of the functions and regulation of the mitotic cyclin Clb2 in S. cerevisiae.

Orderly progression through the eukaryotic cell cycle is a complex process involving both regulation of cyclin dependent kinase activity and control of specific substrate-Cdk interactions. In Saccharomyces cerevisiae, the mitotic cyclin Clb2 has a central role in regulating the onset of anaphase and in maintaining the cellular shape of the bud by inhibiting growth polarization induced in G1. However, how Clb2 and the partially redundant cyclin Clb1 confer specificity to Cdk1 in these processes still remains unclear. Here, we show that Clb2 mutants impaired in nuclear import or export are differentially affected for subsets of Clb2 functions while remaining fully functional for others. Our data support a direct role of the cytoplasmic pool of Clb1,2-Cdk1 in terminating cytoskeleton and growth polarization, independently of G1 cyclin transcriptional regulation. By contrast, the nuclear form of the cyclin is required for timely initiation of anaphase. Clb2 localization influences its stage-specific degradation as well. We report that Clb2 trapped in the cytoplasm is stabilized during anaphase but not at the time of mitotic exit. Altogether, our results demonstrate that the subcellular localization of the mitotic cyclin Clb2 is one of the key determinants of its biological function.