The project is aimed at characterizing routes of lipid migration into yeast mitochondria. The experimental approach is based on the fact that phosphatidylethanolamine (PtdEtn) is an essential component of mitochondrial membranes. PtdEtn is formed by the mitochondrial Pds1p, the major phosphatidylserine decarboxylase, Psd2p, an extra-mitochondrial isoenzyme of Psd1p, or the extramito-chondrial CDP-Etn pathway. To study the contribution of the different pathways to the supply of mitochondria with PtdEtn, the mitochondrial lipid pattern of deletion mutants is analyzed and labeling experiments with radioactive precursors using strains lacking Psd1p, Psd2p or enzymes of the CDP-Etn pathway are performed. Species analysis of mitochondrial PtdSer, PtdEtn and phosphatidylcholine (PtdCho) provides evidence for the specificity of phospholipid species utilized for the formation of mitochondrial membranes. To understand the specific requirement of mitochondria for PtdEtn, psd1, psd2 and psd1psd2 mutant strains are tested for mitochondrial membrane properties, assembly of proteins and lipids and functional properties. Lack of mitochondrial PtdEtn formation by deletion of PSD1 leads to characteristic phenotypical defects. Similar defects can be expected when the import of PtdSer, the substrate of Psd1p, is impaired. Based on this prediction we propose genetic screenings to detect components governing the import of PtdSer into mitochondria.