Résumé
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.