The Yeast Glycogen Synthase Kinase Homolog Rim11 Phosphorylates the Phosphatidic Acid Phosphatase Pah1

Abstract

The yeast phosphatidic acid (PA) phosphatase, Pah1, regulates the synthesis of triacylglycerol and phospholipids by controlling the relative amounts of its product diacylglycerol and substrate PA. Pah1 is cytosolic as a phosphoprotein, but translocates to the nuclear/endoplasmic reticulum membrane for catalytic function through its dephosphorylation. The phosphorylation of Pah1, which is mediated by multiple protein kinases, has been shown to protect the protein against proteasomal degradation and regulates its catalytic activity. In this study, we show that Rim11, a yeast homolog of glycogen synthase kinase (GSK) known for its role in meiotic induction, is involved in the phosphorylation of Pah1. Rim11 purified from yeast phosphorylated the Escherichia coli-expressed Pah1, which is phosphorylation-deficient, in a manner that is dependent on reaction time and the amounts of Rim11, Pah1, and ATP. Phosphoamino acid analysis and phosphopeptide mapping showed that Pah1 phosphorylation by Rim11 occurs at multiple serine residues. As GSKs are known to require priming phosphorylation, we are examining the Rim11 phosphorylation of Pah1 that is pre-phosphorylated by other protein kinases.