Lipid homeostasis in yeast

Aim of the research project entitled "lipid homeostasis in yeast" is to improve our understanding of the molecular processes that are operating to determine and maintain the lipid composition of subcellular membranes. While there have been various approaches to this question in the past, most of these where aimed at a description of the homeostatic adaptation itself. This project, however, is aimed at the direct identification of components that regulate the lipid composition of membranes. Three complementary approaches will be followed to uncover the molecular mechanisms of this regulation at different levels of complexity. First, at a global level, genetics and genome-wide transcriptional profiling will be employed to test two basic models of how membrane homeostasis could be regulated: through the action of supervisory components (sensors) or through local membrane-dependent alterations of the activity of individual lipid biosynthetic enzymes. Second, at the level of activity of one key enzyme of membrane homeostasis, the acyl chain desaturase, the role of cellular levels of unsaturated fatty acids in: (i) regulating the post-translational stability of the enzyme and (ii) inducing ER exit and subcellular relocalization of the enzyme, will be examined. Third, at the level of compensatory changes in membrane lipid compositions, the immediate early consequences of a specific defect in the synthesis of the fungal sterol, ergosterol, on the formation of membrane microdomains, rafts, and the sorting of proteins from the ER will be examined. This three-pronged approach to the problem of lipid homeostasis is expected to ensure an overall success of this research project. The results of this project are expected to greatly contribute to an improved understanding of membrane associated biological processes.

The project "Lipid homeostasis in yeast" was aimed at uncovering regulatory mechanism that control lipid and membrane stasis in nucleated cells. This is a very ambitious aim projected on a broad topic. After completion of the project we do have a better understanding of how the cell coordinates lipid metabolism with membrane proliferation. (i) Characterization of a very specific protein-lipid interaction has revealed that proteins that are destined to the cell surface associate with cell surface destined lipids already in the ER and these protein lipid complexes are then co- transported to the cell surface. Non-productive interaction results in mistargeting and destruction of the cell surface destined protein in the vacuole. (ii) Genetic screens for mutants that are deficient in uptake and transport of sterols and fatty acids have revealed candidate genes that may directly regulate the transport of these two lipids, most notable among these are a class of proteins that have been proposed to move lipids from one side of the membrane to the other. (iii) To understand how membrane growth is coordinated with energy levels, we have identified and characterized a novel class of membrane-anchored lipases that cleave steryl esters to liberate a free sterol and free fatty acids, which in turn can be utilized for membrane biogenesis and energy production. Molecular characterization of the regulation of these lipases is expected to reveal pathways that coordinate these two processes. Taken together, this project has considerably broadened our understanding of lipid homeostasis at three different levels of resolution: lipid-protein interactions, lipid transport, and the coordination of membrane biogenesis with cellular energy levels.