Simultaneous expression of both MAT loci in haploid cells suppresses mutations in yeast microtubule motor genes
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ORIGINAL PAPER
O. Steinberg-Neifach á D. Eshel
Simultaneous expression of both MAT loci in haploid cells suppresses mutations in yeast microtubule motor genes
Received: 17 March 2000 / Accepted: 4 July 2000 / Published online: 8 August 2000 Ó Springer-Verlag 2000
Abstract The kinesin-related Cin8p and cytoplasmic dynein are microtubule-associated motor proteins required for anaphase spindle elongation in the yeast Saccharomyces cerevisiae. Cells deleted for DYN1 (the gene encoding the dynein heavy chain) and carrying the temperature-sensitive allele cin8-3 cannot grow at temperatures above 35 °C. Here, we report that the temperature sensitivity of haploid cin8-3 dyn1D cells is suppressed by the simultaneous presence of the loci MATa and MATa, which contain the regulatory genes that determine mating-type and ploidy-dependent phenotypes. The presence of the two MAT loci also rendered haploid cells more resistant to the antimicrotubule drug benomyl. Our results suggest that, in preparation for handling double the amount of DNA in mitosis, properties of microtubules in diploid cells are modi®ed in a pathway controlled by the mating-type regulatory genes. Key words Microtubule á Motor protein á Kinesin á Dynein á Yeast á Mating-type
Introduction The eukaryotic genome is distributed between chromosomes, the numbers of which dier among species. The number of chromosomes can also vary in cells of the same organism during development, sexual reproduction or as a result of disease. To accommodate additional chromosomes (e.g. following karyogamy) and be able to
Communicated by C. P. Hollenberg O. Steinberg-Neifach á D. Eshel (&) Biology Department, The City University of New York, Brooklyn College, 2900 Bedford Avenue, Brooklyn, NY 11210, USA E-mail: [email protected] Tel.: +1-718-951-5717; Fax: +1-718-951-4826
segregate them correctly in mitosis, the cell has to adjust its mitotic apparatus. This adjustment can include an increase in the number of spindle microtubules to reinforce it mechanically, or other modi®cations that will facilitate the action of anaphase motor proteins. Therefore, cells must have a mechanism with which to monitor changes in chromosome number and initiate a process that will augment the mitotic apparatus appropriately. Because very little is known about how the actions and properties of mitotic spindles are modi®ed in response to change in ploidy, mitosis is considered to be similarly regulated in haploid and diploid cells. The yeast Saccharomyces cerevisiae has six kinesinlike motor proteins and one cytoplasmic dynein. Five of the kinesins (Cin8p, Kip1p, Kip2p, Kip3p and Kar3p) and dynein play various roles in mitosis, but none of them is essential for viability (Stearns 1997). Three motor proteins have been directly implicated in anaphase chromosome movement (Saunders et al. 1995). Two kinesin-like proteins of the bimC family, Cin8p and Kip1p, redundantly operate from within the spindle, crosslinking microtubules and pushing them apart. The cytoplasmic dynein complex, best characterized
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