Aim of the project is to improve our understanding of molecular processes that are operating to determine and maintain the lipid composition of subcellular membranes and to directly identify components that regulate the lipid composition of membranes. Three approaches will be followed to uncover the molecular mechanisms of this regulation at different levels of complexity. (i) Genetics and genome-wide transcriptional profiling will be employed to test two basic models of how membrane homeostasis is regulated: through the action of supervisory components (sensors) or through local membrane-dependent alterations of the activity of individual lipid biosynthetic enzymes. (ii) 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 regulating the post-translational stability of the enzyme and inducing ER exit and subcellular relocalization of the enzyme will be examined. (iii) At the level of compensatory changes in membrane lipid compositions, the immediate early consequences of a defect in the synthesis of ergosterol on the formation of membrane microdomains and the sorting of proteins from the ER will be examined. This three-pronged approach to the problem of lipid homeostasis will contribute to an improved understanding of membrane associated biological processes.