Heterozygosity in MAT locus affects stability and function of microtubules in yeast

The dynamic nature of microtubules is important for their cellular function and is tightly regulated by the cell cycle machinery and through other pathways that act on microtubule-associated proteins. Recently, it was reported that simultaneous expression of MATa and MATa genes in haploid cells of Saccharomyces cerevisiae increases microtubule stability [Mol. Gen. Genet. 264 (2000) 300]. In order to investigate the effect of zygosity in the MAT loci on microtubules independent of the effect of the actual ploidy, we compared microtubule stability and function in homozygous (MATa/MATa) and heterozygous (MATa/MATa) wild-type diploid cells. It was found that homozygosity in the MAT locus decreases stability of both cytoplasmic and nuclear microtubules. This was expressed in a more symmetrical distribution in the placement of the spindle relative to the neck, a delay in cytokinesis and reduced fidelity of chromosome segregation in these cells compared to the heterozygotes. Our results suggest that expression of both MAT loci initiates a pathway that results in an increase in microtubule stability and the fidelity of chromosome segregation in diploids. This pathway is independent of the pathway that determines the budding pattern in haploids and diploids, which is also initiated by simultaneous expression of MATa and MATa genes.