Defective mitosis due to a mutation in the gene for a fission yeast 26S protease subunit

Abstract

WE have isolated a mutant, mts2, in the fission yeastSchizosaccharomyces 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 expression plasmid can rescue an S. pombe mts2 gene disruption. Both observations demonstrate that the mts2⁺ gene is the S. pombehomologue of the human S4 subunit. In addition, we provide genetic evidence for a physical interaction between the S4 and the related S7 subunit in the 26S multiprotein protease. We show that polyubiquitin-conjugated proteins accumulate in themts2mutant at the restrictive temperature, demonstrating that the mutant has anin vivodefect in the ubiquitin-dependent proteolysis pathway. Finally, the phenotype for themts2 mutant indicates that protein degradation by the 26S protease is essential not for entry into but for the completion of mitosis.