In many organisms, polo kinases appear to play multiple roles during M-phase progression. Studies with budding yeast polo kinase cdc5 mutants (cdc5-1 and msd2-1) have revealed a role for Cdc5 in mitotic exit. In addition, overexpression of the C-terminal domain of CDC5 resulted in a dominant-negative cytokinetic defect. To provide new insights into the function of Cdc5, we generated novel temperature-sensitive cdc5 mutants (cdc5-2, cdc5-4, cdc5-7, and cdc5-10) by mutagenizing the C-terminal domain. When the mitotic exit defect was alleviated, the cdc5-2 mutant exhibited an elongated bud phenotype, whereas the cdc5-4, cdc5-7, and cdc5-10 mutants were largely defective in cytokinesis. The cdc5-2 hsl1 deletion mutant exhibited a synergistic bud elongation defect, which was abolished by the introduction of a swe1 deletion, suggesting that Cdc5 functions at a point upstream of Swe1. In addition, the cdc5-4, cdc5-7, and cdc5-10 mutants exhibited a chained cell morphology with shared cytoplasms between the connected cell bodies. Close examination of the cdc5-7 mutant revealed delayed septin disassembly and reassembly in the chained cells. Components in the mitotic exit network (MEN) have also been shown to be important in normal cytokinesis. Loss of BFA1 and BUB2, negative regulators of the MEN, failed to remedy the cytokinetic defect of the cdc5-7 mutant. These data indicate that Cdc5 promotes cytokinesis by acting either downstream of or in parallel to Bfa1 and Bub2.
