Purpose of Gpt2 on lipid droplets

A glycerol-3-phosphate acyltransferase (GPAT) catalyzes the first and rate determining reaction of phosphatidic acid biosynthesis. This reaction is essential, because it initiates de novo formation of all glycerophospholipids (membrane lipids) and triacylglycerols (storage lipids). In addition, phosphatidic acid serves as a signaling molecule, contributes in membrane fission and fusion, and functions as a pH- sensor. Thus, to avoid negative consequences on cell metabolism it is of utmost importance that the activity of a GPAT is precisely adapted to the cellular requirements. Beside regulation at the transcriptional level and via post-translational modifications, the subcellular localization plays a similar important role in determining the contribution of an enzyme to cell metabolism, because it affects characteristics such as enzyme activity, protein stability, and/or access to substrates. In virtually all eukaryotic organisms at least two different GPATs with overlapping function exist. Most interestingly, in humans as well as in yeast and several other organisms, one or more of these GPAT isoforms is/are dually distributed to lipid droplets, which are storage compartments for TAG and other nonpolar lipids, and the ER. However, the purpose of this dual distribution is currently not clear. Thus, the present project is aimed at elucidating the purpose of associating a portion of an ER localized GPAT additionally to lipid droplets. For these studies we will use the budding yeast Saccharomyces cerevisiae as our model organism. In this yeast, two GPATs named Sct1 and Gpt2 are expressed. Sct1 exclusively localizes to the ER. However, Gpt2 is dually distributed to lipid droplets and the ER. By generating mutants expressing Gpt2-mutant forms, which are restricted to one organelle, we will investigate the purpose of the dual distribution of Gpt2 and analyze the impact of this manipulation on cellular level.