Negative control of the Mig1p repressor by Snf1p-dependent phosphorylation in the absence of glucose

Mig1p, a zinc-finger protein that is related to the Krox/Egr, Wilms' tumor and Sp1 proteins, mediates glucose repression in the yeast Saccharomyces cerevisiae. Mig1p is inactive in the absence of glucose, and this inhibition is dependent on the Snf1p (Cat1p) protein kinase. The regulation is mediated by an internal part of Mig1p, and it can be transferred to a Mig1-viral protein 16 (VP16) fusion protein that functions as an activator [Ostling, J., Carlberg, M. & Ronne, H. (1996) Mol. Cell. Biol. 16, 753-761]. We have used Mig1-VP16 to identify three target sites for phosphorylation that mediate Snf1p-dependent inhibition of its activity in the absence of glucose. Two of the sites, Ser278 and Ser311, fit the consensus sequence for phosphorylation by the kinase Snf1p, as determined in vitro. However, a third phosphorylated site, Ser108, does not resemble a Snf1p site. We tested the effect of deleting residues 181-245, which contain two conserved alanine-leucine-serine motifs. We found that the deletion produces a partially constitutive activator, indicating that this region plays a general negative role in regulating Mig1p.