Selective degradation of ubiquitinated Sic1 by purified 26S proteasome yields active S phase cyclin-Cdk

Selective degradation of single subunits of multimeric complexes by the ubi quitin pathway underlies multiple regulatory switches, including those invo lving cyclins and Cdk inhibitors. The machinery that segregates ubiquitinat ed proteins from unmodified partners prior to degradation remains undefined . We report that ubiquitinated Sic1 (Ub-Sic1) embedded within inactive S ph ase cyclin-Cdk (S-Cdk) complexes was rapidly degraded by purified 26S prote asomes, yielding active S-Cdk. Mutant proteasomes that failed to degrade Ub -Sic1 activated S-Cdk only partially in an ATP dependent manner. Whereas Ub -Sic1 was degraded within similar to2 min, spontaneous dissociation of Ub-S ic1 from S-Cdk was similar to 200-fold slower. We propose that the 26S prot easome has the intrinsic capability to extract, unfold, and degrade ubiquit inated proteins while releasing bound partners untouched. Activation of S-C dk reported herein represents a complete reconstitution of the regulatory s witch underlying the G1/S transition in budding yeast.