Roles of phosphatidylethanolamine and of its several biosynthetic pathwaysin Saccharomyces cerevisiae

Three different pathways lead to the synthesis of phosphatidylethanolamine (PtdEtn) in yeast, one of which is localized to the inner mitochondrial mem brane. To study the contribution of each of these pathways, we constructed a series of deletion mutants in which different combinations of the pathway s are blocked. Analysis of their growth phenotypes revealed that a minimal level of PtdEtn is essential for growth. On fermentable carbon sources such as glucose, endogenous ethanolaminephosphate provided by sphingolipid cata bolism is sufficient to allow synthesis of the essential amount of PtdEtn t hrough the cytidyldiphosphate (CDP)-ethanolamine pathway. On nonfermentable carbon sources, however, a higher level of PtdEtn is required for growth, and the amounts of PtdEtn produced through the CDP-ethanolamine pathway and by extramitochondrial phosphatidylserine decarboxylase 2 are not sufficien t to maintain growth unless the action of the former pathway is enhanced by supplementing the growth medium with ethanolamine. Thus, in the absence of such supplementation, production of PtdEtn by mitochondrial phosphatidylse rine decarboxylase 1 becomes essential. In psd1 Delta strains or cho1 Delta strains (defective in phosphatidylserine synthesis), which contain decreas ed amounts of PtdEtn, the growth rate on nonfermentable carbon sources corr elates with the content of PtdEtn in mitochondria, suggesting that import o f PtdEtn into this organelle becomes growth limiting. Although morphologica l and biochemical analysis revealed no obvious defects of PtdEtn-depleted m itochondria, the mutants exhibited an enhanced formation of respiration-def icient cells. Synthesis of glycosylphosphatidylinositol-anchored proteins i s also impaired in PtdEtn-depleted cells, as demonstrated by delayed matura tion of Gas1p. Carboxypeptidase Y and invertase, on the other hand, were pr ocessed with wild-type kinetics. Thus, PtdEtn depletion does not affect pro tein secretion in general, suggesting that high levels of nonbilayer-formin g lipids such as PtdEtn are not essential for membrane vesicle fusion proce sses in vivo.