Lipid particles of yeast consist of a core of neutral lipids (triacylglycerols and steryl esters) which is surrounded by a phospholipid monolayer with embedded proteins. In the past, these particles were only regarded as a depot of triacylglycerols and steryl esters. We were able to show that lipid particles may have additional functions. Some lipid particle proteins were identified as sterol-synthesizing enzymes, some others as acyltransferases catalyzing formation of phosphatidic acid. Since the protein pattern of lipid particles is relatively simple we identified the major individual proteins by amino acid sequence analysis, determined the corresponding genes were, and used deletion mutants to study the phenotypic function and the physiological role of the respective gene products. Moreover, we study transport of triacylglycerols and steryl esters from their site of synthesis, most likely the endoplasmic reticulum, to their site of storage. The second major aim of this project is to study the contribution of lipid particles to cellular lipid synthesis. These investigations are focused on further identification and characterization of acyltransferases involved in the biosynthesis of phosphatidic acid and characterization of sterol-synthesizing enzymes. Finally, targeting of proteins to the surface of lipid particles is investigated. Cell biological and molecular biological methods will be applied to obtain insight into this process at the molecular level.