An in vivo protein interaction assay was used to search a yeast cDNA library for proteins that bind to the acidic activation domain (AD) of the yeast Gal4 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. In vitro binding assays confirm a direct interaction between these proteins. SUG2 and SUG1, another 19 S component, were originally discovered as a mutation 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 AD in vitro. Taken together, these genetic and biochemical data suggest a biologically significant interaction between the Gal4 protein and the 19 S regulatory particle of the proteasome. Indeed, it is demonstrated here that the Gal4 AD interacts specifically with immunopurified 19 S complex. The proteasome regulatory particle has been shown recently to play a direct role in RNA polymerase II transcription and the activator-19 S interaction could be important in recruiting this large complex to transcriptionally active GAL genes.