Regulation of Cytokinesis

Cytokinesis is the process at the end of mitosis by which mother and daughter cells are separated from one another. It involves the coordination of the cell cycle with multiple cellular processes such as cytoskeleton organisation and membrane fusion machinery. Failure in cytokinesis is linked to aneuploidy and a predisposition to a variety of cancers.

The molecular mechanisms that control cytokinesis are still poorly understood. In budding yeast, the site of cell division is established at early stages of the cell cycle by the deposition of scaffold proteins, called septins, at the mother-daughter boundary (bud neck). The septin ring provides the seeding point for the formation of the acto-myosin ring complex, which drives the separation of the two cell bodies in late anaphase. Animal cells also use a contractile ring system composed of actin and myosin filaments, for division. Although the acto-myosin ring complex is formed at variable stages of the cell cycle depending on the organism, it will not contract until late anaphase. This coordination is important to allow cytokinesis to occur only after full partitioning of the chromosomes between mother and daughter cells.

At the end of mitosis, after the full elongation of the mitotic spindle and separation of the sister chromatids, cyclin-dependent kinase (CDK) activity is down-regulated to allow transition out of mitosis (mitotic exit) and cytokinesis. The protein phosphatase Cdc14 plays a pivotal role during mitotic exit in budding yeast. Through dephosphorylation of key targets, Cdc14 promotes the degradation of the mitotic cyclin Clb2 and accumulation of the CDK inhibitor Sic1. Full activation of Cdc14 requires an active MEN, including the protein kinases Cdc15, Dbf2-Mob1 and Dfb20. Dbf2 and Dbf20 function directly upstream of Cdc14 in the MEN pathway. As a consequence of MEN activation, Cdc14 becomes fully active. Although down-regulation of mitotic CDK activity by the MEN is a pre-requisite for cytokinesis, several lines of evidence suggest that MEN components and Cdc14 have a more direct role in cytokinesis. MEN components such as Mob1 and the kinases Cdc15, Dbf2 and Dbf20 localise at the yeast centrosome in a cell cycle-dependent manner, and in addition appear at the plane of division in late mitosis. MEN deficient mutants fail to complete cytokinesis, even if mitotic CDK activity is down-regulated.

Little is known about how Cdc14 and MEN components regulate cytokinesis. We are systematically studying the function of Cdc14 and MEN components in cell division. In addition, proteomics approaches are employed to identify new components involved in cell division in both budding yeast and mammalian cells.