Regulatory interactions between the Reg1-Glc7 protein phosphatase and the Snf1 protein kinase

Protein phosphatase 1, comprising the regulatory subunit Reg1 and the catal ytic subunit Glc7, has a role in glucose repression in Saccharomyces cerevi siae. Previous studies showed that Reg1 regulates the Snf1 protein kinase i n response to glucose. Here, we explore the functional relationships betwee n Reg1, Glc7, and Snf1. We show that different sequences of Reg1 interact w ith Glc7 and Snf1. We use a mutant Reg1 altered in the Glc7-binding motif t o demonstrate that Reg1 facilitates the return of the activated Snf1 kinase complex to the autoinhibited state by targeting Glc7 to the complex. Genet ic evidence indicated that the catalytic activity of Snf1 negatively regula tes its interaction with Reg1. We show that Reg1 is phosphorylated in respo nse to glucose limitation and that this phosphorylation requires Snf1; more over, Reg1 is dephosphorylated by Glc7 when glucose is added. Finally, we s how that hexokinase PII. (Hxk2) has a role in regulating the phosphorylatio n state of Reg1, which may account for the effect of Hxk2 on Snf1 function. These findings suggest that the phosphorylation of Reg1 by Snf1 is require d for the release of Reg1-Glc7 from the kinase complex and also stimulates the activity of Glc7 in promoting closure of the complex.