Reg1p targets protein phosphatase 1 to dephosphorylate hexokinase II in Saccharomyces cerevisiae:: characterizing the effects of a phosphatase subunit on the yeast proteome

Protein phosphatase 1 (Glc7p) and its binding protein Reg1p are essential for the regulation of glucose repression pathways in Saccharomyces cerevisiae, In order to identify physiological substrates for the Glc7p-Reg1p complex, we examined the effects of deletion of the REG1 gene on the yeast phosphoproteome, Analysis by two-dimensional phosphoprotein mapping identified two distinct proteins that were greatly increased in phosphate content in reg1 Delta mutants. Mixed peptide sequencing identified these proteins as hexokinase II (Hxk2p) and the Ela subunit of pyruvate dehydrogenase, Consistent with increased phosphorylation of Hxk2p in response to REG1 deletion, fractionation of yeast extracts by anion-exchange chromatography identified Hxk2p phosphatase activity in wild-type strains that was selectively lost in the reg1 Delta mutant. The phosphorylation state of Hxk2p and Hxk2p phosphatase activity was restored to wild-type levels in the reg1 Delta mutant by expression of a LexA-Reg1p fusion protein. In contrast, expression of LexA-Reg1p containing mutations at phenylalanine in the putative PP-1C-binding site motif (WR)(X)(I/V)XF was unable to rescue Hxk2p dephosphorylation in intact yeast or restore Hxk2p phosphatase activity. These results demonstrate that Reg1p targets PP-1C to dephosphorylate Hxk2p in vivo and that the motif (K/R)(X) (I/V)XF is necessary for its PP-1 targeting function.