Mutations in the Saccharomyces cerevisiae vacuolar fusion proteins Ccz1, Mon1 and Ypt7 cause defects in cell cycle progression in a num1Δ background

The proteins Ccz1 and Mon1 are known to function together with the Rab-GTPase Ypt7 in membrane fusion reactions at the yeast vacuole. In a genome-wide analysis they have also been found to interact genetically with the nuclear-migration protein Num1. In this study we analyze these synthetic effects and we show that the mutants ccz1Δ num1Δ, mon1Δ num1Δ and ypt7Δ num1Δ exhibit severe defects in cell cycle progression. A large fraction of the mutant cells enter a new cell division cycle without having completed mitotic exit, leading to the accumulation of multinuclear, anuclear and multibudded cells. The double deletion strains display also increased sensitivity to calcium ions. The cell-cycle defects are only weakly observed if deletions of other vacuolar protein sorting genes are combined with num1Δ or if other nuclear-migration genes are deleted together with CCZ1, whereas the calcium sensitivity is characteristic for a large subset of the tested double mutants. Further, the cell-cycle defects of the ccz1Δ num1Δ strain can be partially rescued by overproduction of either the calcium pump Pmc1 or the nuclear-migration factors Kar9 and Bim1. Together, these results indicate that deregulation of the cell cycle in these mutants results from two separate mechanisms, one of which is related to calcium homeostasis.