SACCHAROMYCES-CEREVISIAE 26S PROTEASE MUTANTS ARREST CELL-DIVISION ING2 METAPHASE/

WE isolated two mutants from the yeast Saccharomyces cerevisiae, cim3- 1 and cim5-1, that arrest cell division in G2/metaphase at 37-degrees- C. CIM3 (identical to SUG1; ref. 1) and CIM5 are similar to each other and are members of a family of putative ATPases that have been propos ed to be 26S protease subunits2. We show here that CIM5 is the functio nal yeast homologue of the human MSS1 protein3 and that homologues of CIM3 and CIM5 are present in a highly purified preparation of the Dros ophila 26S protease4. The short-lived ubiquitin-proline-beta-galactosi dase fusion protein is stabilized in cim mutants, but Leu-beta-galacto sidase is not. The CLB2 and CLB3 cyclins also accumulate in the cim mu tants. Thus the 26S protease is required in vivo for the degradation o f ubiquitinated substrates and for anaphase chromosome separation.