Glucose-regulated interaction of a regulatory subunit of protein phosphatase 1 with the Snf1 protein kinase in Saccharomyces cerevisiae

The Snf1 protein kinase family has been conserved in eukaryotes. In the yeast Saccharomyces cerevisiae, Snf1 is essential for transcription of glucose-repressed genes in response to glucose starvation. The direct interaction between Snf1 and its activating subunit, Snf4, within the kinase complex is regulated by the glucose signal. Glucose inhibition of the Snf1-Snf4 interaction depends on protein phosphatase 1 and its targeting subunit, Reg1. Here we show that Reg1 interacts with the Snf1 catalytic domain in the two-hybrid system. This interaction increases in response to glucose limitation and requires the conserved threonine in the activation loop of the kinase, a putative phosphorylation site. The inhibitory effect of Reg1 appears to require the Snf1 regulatory domain because a reg1 Delta mutation no longer relieves glucose repression of transcription when Snf1 function is provided by the isolated catalytic domain. Finally, we show that abolishing the Snf1 catalytic activity by mutation of the ATP-binding site causes elevated, constitutive interaction with Reg1, indicating that Snf1 negatively regulates its own interaction with Reg1. We propose a model in which protein phosphatase 1, targeted by Reg1, facilitates the conformational change of the kinase complex from its active state to the autoinhibited state.