Inactivation of Pmc1 vacuolar Ca²⁺ ATPase causes G₂ cell cycle delay in Hansenula polymorpha

Abstract

The vacuolar Ca²⁺ ATPase Pmc1 is involved in maintenance of a low Ca²⁺ concentration in cytosol in yeast cells. Here we observed that increase of Ca²⁺ cytosolic concentration in yeast Hansenula polymorpha due to inactivation of Pmc1 resulted in sensitivity to sodium dodecyl sulfate (SDS). To elucidate the mechanisms of the observed effect, a screening for mutations suppressing SDS sensitivity of the H. polymorpha pmc1 mutant was performed. As a result, three genes were identified. Two of them, designated as their Saccharomyces cerevisiae orthologs CCH1 and HOG1 encoded the plasma membrane voltage-gated high-affinity calcium channel and the MAP kinase involved in osmoregulation, respectively. The third gene, designated as WEE1, coded for the ortholog of Wee1/Swe1 kinase involved in cell cycle regulation by inhibiting of the G₂/M transition. Detailed analysis of this mutant demonstrated that suppression of pmc1 SDS sensitivity by the wee1 mutation depended on an accompanying chromosomal rearrangement, whereas inactivation of WEE1 in the absence of this rearrangement caused SDS sensitivity. Expression of a chimeric protein containing an N-terminal portion of Wee1 in the pmc1 mutant led to abnormal morphology characteristic of G₂ delay. Our data indicate that cytosolic Ca²⁺ rise causes SDS sensitivity in H. polymorpha through the activation of the Wee1 kinase, which is mediated by the Hog1 kinase. Wee1 has a dual role in the manifestation of SDS sensitivity in the H. polymorpha pmc1 mutant. Mechanisms of influence of the obtained mutations on the G₂/M transition are discussed.