Sto1p, a fission yeast protein similar to tubulin folding Cofactor E, plays an essential role in mitotic microtubule assembly

The proper functioning of microtubules depends crucially on the availabilit y of polymerizable alpha/beta tubulin dimers, Their production occurs conco mitant with the folding of the tubulin polypeptides and is accomplished in part by proteins known as Cofactors A through E. In the fission yeast, Schi zosaccharomyces pombe, this tubulin folding pathway is essential, We have t aken advantage of the excellent cytology available in S. pombe to examine t he phenotypic consequences of a deletion of sto1(+), a gene that encodes a protein similar to Cofactor E, which is required for the folding of alpha-t ubulin, The interphase microtubule cytoskeleton in sto1-Delta cells is seve rely disrupted, and as cells enter mitosis their spindles fail to form. Aft er a transient arrest with condensed chromosomes, the cells exit mitosis an d resume DNA synthesis, whereupon they septate abnormally and die. Overexpr ession of Spo1p is toxic to cells carrying a cold-sensitive allele of the a lpha- but not the beta-tubulin gene, consistent with the suggestion that th is protein plays a role like that of Cofactor E, Unlike its presumptive par tner Cofactor D (Alp1p), however, Sto1p does not localize to microtubules b ut is found throughout the cell. Overexpression of Sto1p has no toxic effec ts in wild-type cells, suggesting that it is unable to disrupt alpha/beta t ubulin dimers in vivo.