Genetic dissection of the yeast 26S proteasome: Cell cycle defects caused by the Delta rpn9 mutation

Rpn9 is one of the subunits of the regulatory particle of the yeast 26S pro teasome and is needed for stability or efficient assembly of the 26S protea some. As anticipated from the fact that the rpn9 disruptant grew at 25 degr eesC but arrested in G2/M phase at 37 degreesC, the CDK inhibitor Sic1p was found to be degraded at the G1/S boundary in the Delta rpn9 cells. The deg radation of the anaphase inhibitor Pds1p was delayed in the Delta rpn9 call s. Clb2p in M phase, as well as that ectopically expressed in G1 and S phas es, was degraded more slowly in the Delta rpn9 cells than in the wild type cells, indicating that the 26S proteasome lacking Rpn9 uses Sic1p as a bett er substrate than Pds1p and Clb2p. These results, in addition to the fact t hat multiubiquitinated proteins were accumulated in the Delta rpn9 cells in cubated at 37 degreesC, strongly suggest that Rpn9 is involved in the prote olysis of a subset of the substrates degraded by the 26S proteasome. The De lta rpn9 Delta pds1 double mutant was unable to elongate spindle at a restr ictive temperature, suggesting that some protein(s) other than Scc1 (cohesi n) should be degraded during progression of anaphase. (C) 2001 Societe fran caise de biochimie et biologie moleculaire / Editions scientifiques et medi cales Elsevier SAS. All rights reserved.