Regulation of neutral lipid metabolism through phosphorylation of the yeast acyltransferase Gpt2

Abstract

Glycerol-3-phosphate acyltransferases (GPATs) catalyze the first step of glycerol-3-phosphate acylation at the sn-1 position producing lyso-phosphatidic acid. This is the committed and rate limiting step in de-novo synthesis of phosphatidic acid, the key intermediate in the glycerophospholipids and triacylglycerols (TAG) biosynthetic pathways. Two GPATs have been identified in S cerevisiae, Gpt2p and Sct1p. The role of phosphorylation in the serine-rich C-terminal tail of Gpt2p is unknown. In this work is shown that lack of phosphorylation on three conserved phosphorylation sites (S664, S668, S671) of Gpt2p alters protein stability, activity and neutral lipid metabolism. Specifically, a triple Gpt2p mutant (Gpt2-3A) where all these three residues were converted to alanine to mimic dephosphorylation induce a rise in the protein abundance, displayed more activity and resultedi in higher accumulation of DAG and TAG during exponential phase of growth. Lack of phosphorylation on Gpt2p delayed TAG lipolysis upon growth resumption from stationary phase which might be due to a futile TAG cycle that slows down mobilization of produced acyl-chains channeled for the formation of phospholipids. Unregulated Gpt2p probably remains constitutively active and displaces Sct1p in exponential phase. Notoriously, Sct1p is found associated with lipid droplets in cells carrying Gpt2-3A mutant. Considering Sct1p has never been identified in lipid droplets proteomes this consequence of having a constitutively de-phosphorylated Gpt2p might explain the alterations seen in neutral lipid metabolism of this mutant cells.