Methionine Sulfoximine Treatment and Carbon Starvation Elicit Snf1-independent Phosphorylation of the Transcription Activator Gln3 in Saccharomyces cerevisiae

Abstract

Tor proteins are global regulators situated at the top of a signal transduction pathway conserved from yeast to humans. Specific inhibition of the two Saccharomyces cerevisiae Tor proteins by rapamycin alters many cellular processes and the expression of hundreds of genes. Among the regulated genes are those whose expression is activated by the GATA family transcription activator, Gln3. The extent of Gln3 phosphorylation has been thought to determine its intracellular localization, with phosphorylated and dephosphorylated forms accumulating in the cytoplasm and nucleus, respectively. Data presented here demonstrate that rapamycin and the glutamine synthetase inhibitor, methionine sulfoximine (MSX), although eliciting the same outcomes with respect to Gln3-Myc13 nuclear accumulation and nitrogen catabolite repression-sensitive transcription, generate diametrically opposite effects on Gln3-Myc13 phosphorylation. MSX increases Gln3-Myc13 phosphorylation and rapamycin decreases it. Gln3-Myc13 phosphorylation levels are regulated by at least three mechanisms as follows: (i) depends on Snf1 kinase as observed during carbon starvation, (ii) is Snf1-independent as observed during both carbon starvation and MSX treatment, and (iii) is rapamycin-induced dephosphorylation. MSX and rapamycin act additively on Gln3-Myc13 phosphorylation, but MSX clearly predominates. These results suggest that MSX- and rapamycin-inhibited proteins are more likely to function in separate regulatory pathways than they are to function tandemly in a single pathway as thought previously. Furthermore, as we and others have detected thus far, Gln3 phosphorylation/dephosphorylation is not a demonstrably required step in achieving Gln3 nuclear localization and nitrogen catabolite repression-sensitive transcription in response to MSX or rapamycin treatment.