Identification of the gal4 suppressor Sug1 as a subunit of the yeast 26S proteasome

THE SUG1 gene of Saccharomyces cerevisiae encodes a putative ATPase. Mutations in SUG1 were isolated 1 as suppressors of a mutation in the transcriptional activation domain of GAL4 . Sugl was recently proposed to be a subunit of the RNA polymerase II holoenzyme and to mediate the association of transcriptional activators with holoenzyme 2 . We show here that Sugl is not a subunit of the holoenzyme, at least in its purified form, but of the 26S proteasome 3,4 , a large complex of relative molecular-mass 2,OOOK that catalyses the ATP-dependent degradation of ubiqui-tin–protein conjugates. Sugl co-purifies with the proteasome in both conventional and nickel-chelate affinity chromatography. Our observations account for the reduced ubiquitin-dependent proteolysis in sug1 mutants 5 and suggest that the effects of sug1 mutations on transcription are indirect results of defective proteolysis.