The Gal4 activation domain binds Sug2 protein, a proteasome component, in vivo and in vitro

An in vivo protein interaction assay was used to search a yeast cDNA librar y for proteins that bind to the acidic activation domain (AD) of the yeast Ga14 protein. Sug2 protein, a component of the 19 S regulatory particle of the 26 S proteasome, was one of seven proteins identified in this screen. I n vitro binding assays confirm a direct interaction between these proteins. SUG2 and SUG1, another 19 S component, were originally discovered as a mut ation able to suppress the phenotype of a Gal4 truncation mutant (Gal4(D)p) lacking much of its AD. Sug1p has previously been shown to bind the Gal4 A D in vitro. Taken together, these genetic and biochemical data suggest a bi ologically significant interaction between the Gal4 protein and the 19 S re gulatory particle of the proteasome. Indeed, it is demonstrated here that t he Gal4 AD interacts specifically with immunopurified 19 S complex. The pro teasome regulatory particle has been shown recently to play a direct role i n RNA polymerase II transcription and the activator-19 S interaction could be important in recruiting this large complex to transcriptionally active G AL genes.