EC Accepts, published online ahead of print on 12 June 2009

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Eukaryotic Cell doi:10.1128/EC.00085-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The glycerol 3-phosphate acyltransferases Gat1p and Gat2p are microsomal phosphoproteins with differential contributions to polarized cell growth

Martin W. Bratschi, David P. Burrowes, Adam Kulaga, Jing F. Cheung, Ana L. Alvarez, Jennifer Kearley, and Vanina Zaremberg^*

Department of Biological Sciences, University of Calgary, Alberta, T2N 1N4 Canada

^* To whom correspondence should be addressed. Email: vzarembe{at}ucalgary.ca.

Glycerol-3-phosphate acyltransferase (GPAT) catalyses the initial step in the synthesis of all glycerolipids. It is the committed and rate limiting step and is redundant in yeast, mammals and plants. GPAT controls the formation of lipid intermediates that serve not only as precursors of more complex lipids but also as intracellular signaling molecules. Saccharomyces cerevisiae possesses two GPATs, encoded by GAT1 and GAT2 genes. Metabolic analysis of yeast lacking either GAT1 or GAT2 indicated partitioning of the two main branches of phospholipid synthesis at the initial and rate limiting GPAT step. We are particularly interested in identifying molecular determinants mediating lipid metabolic pathway partitioning, therefore as a start point we have performed a detailed study of Gat1p and Gat2p cellular localization. We have compared Gat1p and Gat2p localization by fluorescence microscopy and subcellular fractionation using equilibrium density gradients. Our results indicate Gat1p and Gat2p mostly overlap in their localization and are in fact microsomal GPATs, localized to both perinuclear and cortical endoplasmic reticulum in cells actively proliferating. A more detailed analysis suggests a differential enrichment of Gat1p and Gat2p in distinct ER fractions. Furthermore, overexpression of these enzymes in the absence of endogenous GPATs, induces proliferation of distinct ER arrays, differentially affecting cortical ER morphology and polarized cell growth. In addition, our studies also uncovered a dynamic post-translational regulation of Gat1p and Gat2p, and a compensation mechanism through phosphorylation that responds to cellular GPAT imbalance.