SUG1 and SUG2: ATPase family components of the yeast 26 S proteasome

Using a genetic strategy designed to find proteins involved in the function of the Saccharomyces cerevisiue transcriptional activator Gal4, we isolated two complementation groups of mutants which rescue gal4 activation domain mutants. One of these defines Sug1, an ATPase family subunit of the 26S proteasome. SUG2 encodes an essential 19 kD protein 13% identical to Sug1, Sug2 co-immunoprecipitates nearly quantitatively with Sug1 and with 20 S proteasome subunits from yeast extract. Sug1, Sug2 and 20 S proteasome subunits quantitatively co-elute with each other from a gel fltration column at a molecular mass of 2 MDa, consistent with the size of the 26 S proteasome. After depletion of ATP and salt treatment of the 26 S fraction, Sug1, Sug2 and 20 S subunits elute from a gel filtration colurun at molecular masses consistent with dissociation into 20 S and PN700 complexes. Sug2 interacts with Sugl in vitro as demonstrated by two-hybrid assay. We conclude that Sug2 is a novel subunit of the PN700 regulatory complex of the 26S proteasome. With its highly conserved mammalian homologs, Sug2 defines a new, sixth class of proteasoreal ATPase family proteins. Current studies are focused on the mechanism of interaction between sug mutatants and activation defective Gal4 proteins, as well as the roles of Sugl and Sug2 in the assembly, stability and activities of the 26 S proteasome.