To better understand the dynamic regulation of microtubule structures in yeast, we studied a conditional-lethal ,B-tubulin mutation tub2-150. This mutation is unique among the hundreds of tubulin mutations isolated in Saccharomyces cerevisiae in that it appears to cause an increase in the stability of microtubules. We report here that this allele is a mutation of threonine 238 to alanine, and that tub2-150 prevents the spindle from elongating during anaphase, suggesting a nuclear microtubule defect. To identify regulators of microtubule stability and/or anaphase, yeast genes were selected that, when overexpressed, could suppress the tub2-150 temperature-sensitive phenotype. One of these genes, JSNI, encodes a protein of 125 kDa that has limited similarity to a number of proteins of unknown function. Overexpression of the JSNI gene in a TUB2 strain causes that strain to become more sensitive to benomyl, a microtubule-destabilizing drug. Of a representative group of microtubule mutants, only one other mutation, tub2-404, could be suppressed by JSNI overexpression, showing that JSNI is an allelespecific suppressor. As tub2-404 mutants are also defective for spindle elongation, this provides additional support for a role for JSNI during anaphase.

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