DEFECTIVE MITOSIS DUE TO A MUTATION IN THE GENE FOR A FISSION YEAST 26S PROTEASE SUBUNIT

WE have isolated a mutant, mts2, in the fission yeast Schizosaccharomy ces pombe which is defective in chromosome segregation. The predicted amino-acid sequence of the cloned mts2+ gene product is 75% identical to the S4 subunit of the human 26S ATP/ubiquitin-dependent protease1. The human S4 subunit complementary DNA expressed from an S. pombe expr ession plasmid can rescue an S. pombe mts2 gene disruption. Both obser vations demonstrate that the mts2+ gene is the S. pombe homologue of t he human S4 subunit. In addition, we provide genetic evidence for a ph ysical interaction between the S4 and the related S7 subunit in the 26 S multiprotein protease. We show that polyubiquitin-conjugated protein s accumulate in the mts2 mutant at the restrictive temperature, demons trating that the mutant has an in vivo defect in the ubiquitin-depende nt proteolysis pathway. Finally, the phenotype for the mts2 mutant ind icates that protein degradation by the 26S protease is essential not f or entry into but for the completion of mitosis.