S. cerevisiae 26S protease mutants arrest cell division in G2/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 proposed to be 26S protease subunits. We show here that CIM5 is the functional yeast homologue of the human MSS1 protein and that homologues of CIM3 and CIM5 are present in a highly purified preparation of the Drosophila 26S protease. The short-lived ubiquitin-proline-beta-galactosidase fusion protein is stabilized in cim mutants, but Leu-beta-galactosidase is not. The CLB2 and CLB3 cyclins also accumulate in the cim mutants. Thus the 26S protease is required in vivo for the degradation of ubiquitinated substrates and for anaphase chromosome separation.