A ROLE OF SEP1 (=KEM1, XRN1) AS A MICROTUBULE-ASSOCIATED PROTEIN IN SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae cells lacking the SEP1 (also known as XRN1, K EM1, DST2, RAR5) gene function exhibit a number of phenotypes in cellu lar processes related to microtubule function. Mutant cells show incre ased sensitivity to the microtubule-destabilizing drug benomyl, increa sed chromosome loss, a karyogamy defect, impaired spindle pole body se paration, and defective nuclear migration towards the bud neck. Analys is of the arrest morphology and of the survival during arrest strongly suggests a structural defect accounting for the benomyl hypersensitiv ity, rather than a regulatory defect in a checkpoint. Biochemical anal ysis of the purified Sep1 protein demonstrates its ability to promote the polymerization of porcine brain and authentic S.cerevisiae tubulin into flexible microtubules in vitro. Furthermore, Sep1 co-sediments w ith these microtubules in sucrose cushion centrifugation. Genetic anal ysis of double mutant strains containing a mutation in SEP1 and in one of the genes coding for alpha- or beta-tubulin further suggests inter action between Sep1 and microtubules. Taken together these three lines of evidence constitute compelling evidence for a role of Sep1 as an a ccessory protein in microtubule function in the yeast S.cerevisiae.