Evidence that proteolysis of Ga14 cannot explain the transcriptional effects of proteasome ATPase mutations

The Gal system of Saccharomyces cerevisiae is a paradigm for eukaryotic gen e regulation. Expression of genes required for growth on galactose is regul ated by the transcriptional activator Ga14. The activation function of Ga14 has been localized to 34 amino acids near the C terminus of the protein. T he gal4D allele of GAL4 encodes a truncated protein in which only 14 amino acids of the activation domain remain. Expression of GAL genes is dramatica lly reduced in gal4D strains and these strains are unable to grow on galact ose as the sole carbon source. Overexpression of gal4D partially relieves t he defect in GAL gene expression and allows growth on galactose. A search f or extragenic suppressors of gal4D identified recessive mutations in the SU G1 and SUG2 genes, which encode ATPases of the 19S regulatory complex of th e proteasome. The proteasome is responsible for the ATP-dependent degradati on of proteins marked for destruction by the ubiquitin system. It has been commonly assumed that effects of SUGI and SUG2 mutations on transcription a re explained by alterations in the proteolysis of gal4D protein. We have in vestigated this assumption. Surprisingly, we find that SUGI and SUG2 allele s that are unable to suppress gal4D cause a larger increase in gal4D protei n levels than do suppressing alleles. in addition, mutations in genes encod ing subunits of the proteolytic 20S sub-complex of the proteasome increase the levels of gal4D protein but do not rescue its transcriptional activity, Therefore, an alteration in the proteolysis of gal4D by the proteasome can not explain the effects of mutations in SUGI and SUG2 on expression of GAL genes. These findings suggest that the 19S regulatory complex may play a mo re direct role in transcription.