Saccharomyces cerevisiae Mob1p Is Required for Cytokinesis and Mitotic Exit

Abstract

The Saccharomyces cerevisiae mitotic exit network (MEN) is a conserved set of genes that mediate the transition from mitosis to G₁ by regulating mitotic cyclin degradation and the inactivation of cyclin-dependent kinase (CDK). Here, we demonstrate that, in addition to mitotic exit, S. cerevisiae MEN gene MOB1 is required for cytokinesis and cell separation. The cytokinesis defect was evident in mob1mutants under conditions in which there was no mitotic-exit defect. Observation of live cells showed that yeast myosin II, Myo1p, was present in the contractile ring at the bud neck but that the ring failed to contract and disassemble. The cytokinesis defect persisted for several mitotic cycles, resulting in chains of cells with correctly segregated nuclei but with uncontracted actomyosin rings. The cytokinesis proteins Cdc3p (a septin), actin, and Iqg1p/ Cyk1p (an IQGAP-like protein) appeared to correctly localize in mob1 mutants, suggesting that MOB1functions subsequent to actomyosin ring assembly. We also examined the subcellular distribution of Mob1p during the cell cycle and found that Mob1p first localized to the spindle pole bodies during mid-anaphase and then localized to a ring at the bud neck just before and during cytokinesis. Localization of Mob1p to the bud neck requiredCDC3, MEN genes CDC5, CDC14, CDC15, and DBF2, and spindle pole body gene NUD1 but was independent ofMYO1. The localization of Mob1p to both spindle poles was abolished in cdc15 and nud1 mutants and was perturbed in cdc5 and cdc14mutants. These results suggest that the MEN functions during the mitosis-to-G₁ transition to control cyclin-CDK inactivation and cytokinesis.

ACKNOWLEDGMENTS

We thank members of the Winey laboratory and Erfei Bi for many helpful discussions and Shelly Q. Jones, Michael Atchison, and Erika Holzbaur for critical reading of the manuscript. We also thank Jason Kahana and Pamela Silver for providing anti-GFP antibodies and Erfei Bi, Clyde Denis, Clive Price, and John Kilmartin for yeast strains and reagents.

This work was supported by grants from the Leukemia Society of America (F.C.L.), American Cancer Society IRG-78-002-22 (F.C.L.), and National Institutes of Health R01 GM60575 (F.C.L.) and R01 GM51312 (M.W.).