A SCREEN FOR GENES INVOLVED IN THE ANAPHASE PROTEOLYTIC PATHWAY IDENTIFIES TSM1(-POMBE GENE IMPORTANT FOR MICROTUBULE INTEGRITY(), A NOVEL SCHIZOSACCHAROMYCES)

The growth of several mitotic mutants of Schizosaccharomyces pombe, in cluding nuc2-663, is inhibited by the protease inhibitor N-Tosyl-L-Phe nylalanine Chloromethyl Ketone (TPCK). Because nuc2(+) encodes a presu mptive component of the Anaphase Promoting Complex, which is required for the ubiquitin-dependent proteolysis of certain proteins during exi t from mitosis, we have used sensitivity to TPCK as a criterion by whi ch to search for novel S. pombe mutants defective in the anaphase-prom oting pathway. In a genetic screen for temperature-sensitive mitotic m utants that were also sensitive to TPCK at a permissive temperature, w e isolated three tsm (TPCK-sensitive mitotic) strains. Two of these ar e alleles of cut1(+), but tsm1-512 maps to a novel genetic location. T he tsm1-512 mutation leads to delayed nuclear division at restrictive temperatures, apparently as a result of an impaired ability to form a metaphase spindle. After shift of early G(2), cells to 36 degrees, tsm 1-512 arrests transiently in the second mitotic division and then exit s mitosis, as judged by spindle elongation and septation. The chromoso mes, however, often fail to segregate properly. Genetic interactions b etween tsm1-512 and components of the anaphase proteolytic pathway sug gest a functional involvement of the Tsm1 protein in this pathway.