Genetic interactions between Hsp90 and the Cdc2 mitotic machinery in the fission yeast Schizosaccharomyces pombe

In Schizosaccharomyces ces pombe, wee1 encodes a tyrosine kinase that inhib its entry into mitosis by phophorylating Cdc2, the universal cyclin-depende nt kinase (Cdk) that regulates the G2/M transition in all eukaryotic cells. A search for suppressors of the G2 arrest caused by overexpression of wee1 led to the isolation of a new allele of swo1 (named swo1-w1), the gene cod ing for chaperone Hsp90, which is required to stabilise Wee1. The swo1-w1 a llele carries a glycine to aspartic acid substitution at amino acid 155 tha t results in a partial loss of Hsp90 function. Cells bearing the swo1-w1 mu tation in combination with the point mutation cdc2-33 or cdc2-M26 showed se vere mitotic defects. Genetic interactions were not observed in combination with point mutations in other cdc genes, suggesting that Cdc2 specifically interacts with Hsp90. This synthetic lethal swo1-w1 cdc2-33 (or cdc2-M26) strain had normal levels of Cdc2 protein and histone H1 phosphorylation act ivity, indicating that Hsp90 is required to enable Cdc2 to interact with it s mitotic substrates or regulators, rather than for its proper folding or s tabilisation. In a wild-type background, swo1-w1 mutant cells were sensitiv e to temperature as well as to other stress agents, such as KCl, ethanol an d formamide. Under these stressful growth conditions, the swo1-w1 cells dis played anapbase B arrest and aberrant septation patterns, indicating that a subset of proteins involved in mitosis and cytokinesis is highly dependent on chaperone Hsp90 for function.