Ubiquitin-dependent Proteolytic Control of

Posttranslational protein modification with small ubiquitin-relatedmodifier (SUMO) is an important regulatorymechanism implicated in many cellular processes, including several of bio-medical relevance. We report that inhibition of the proteasome leads to accumulation of proteins that are simultaneously con-jugated to both SUMO and ubiquitin in yeast and in human cells. A similar accumulation of such conjugates was detected in Saccharomyces cerevisiae ubc4 ubc5 cells as well as in mutants lacking twoRING finger proteins, Ris1 andHex3/Slx5-Slx8, that bind to SUMO as well as to the ubiquitin-conjugating enzyme Ubc4. In vitro, Hex3-Slx8 complexes promote Ubc4-dependent ubiquitylation. Together these data identify a previously unrec-ognized pathway thatmediates the proteolytic down-regulation of sumoylated proteins. Formation of substrate-linked SUMO chains promotes targeting of SUMO-modified substrates fo