Roles of Phosphatidylethanolamine and of Its Several Biosynthetic Pathways inSaccharomyces cerevisiae

Abstract

Three different pathways lead to the synthesis of phosphatidylethanolamine (PtdEtn) in yeast, one of which is localized to the inner mitochondrial membrane. To study the contribution of each of these pathways, we constructed a series of deletion mutants in which different combinations of the pathways 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 catabolism is sufficient to allow synthesis of the essential amount of PtdEtn through 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 sufficient 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 phosphatidylserine decarboxylase 1 becomes essential. Inpsd1Δ strains or cho1Δ strains (defective in phosphatidylserine synthesis), which contain decreased amounts of PtdEtn, the growth rate on nonfermentable carbon sources correlates with the content of PtdEtn in mitochondria, suggesting that import of PtdEtn into this organelle becomes growth limiting. Although morphological and biochemical analysis revealed no obvious defects of PtdEtn-depleted mitochondria, the mutants exhibited an enhanced formation of respiration-deficient cells. Synthesis of glycosylphosphatidylinositol-anchored proteins is also impaired in PtdEtn-depleted cells, as demonstrated by delayed maturation of Gas1p. Carboxypeptidase Y and invertase, on the other hand, were processed with wild-type kinetics. Thus, PtdEtn depletion does not affect protein secretion in general, suggesting that high levels of nonbilayer-forming lipids such as PtdEtn are not essential for membrane vesicle fusion processes in vivo.

The Kennedy pathway—De novo synthesis of phosphatidylethanolamine and phosphatidylcholine

“…This ability increases with increasing acyl chain length and unsaturation and is required to form essential intermediate structures in membrane fusion and fission events (15,16). Together with other ''nonbilayer'' lipids, PE is believed to cause lateral pressure and introduce curvature stress in membranes; this influences structure, folding, and activity of integral and peripheral membrane proteins.…”

“…Together with other ''nonbilayer'' lipids, PE is believed to cause lateral pressure and introduce curvature stress in membranes; this influences structure, folding, and activity of integral and peripheral membrane proteins. The curvature stress can stabilize membrane proteins in their optimal conformation, provide energy for conformational changes, and generate bilayer ''packing defects'' that influence the binding of peripheral membrane proteins (16,17).…”

“…tion inhibits the localization of negative amino acids on the cytoplasmic side of the membrane and is an essential determinant of the topology of transmembrane domains of membrane proteins (16)(17)(18)(19). Because of its ability to create a structureforming environment, PE is often being referred to as a nonprotein molecular chaperone.…”

“…Yeast cells defective in PS biosynthesis, and consequently containing decreased amounts of PE, show slower growth rates under nonfermenting condition, when mitochondria are fully developed. This suggests that import of PE into the organelle becomes growth limiting and results in the enhanced formation of respiration-deficient cells (16). The mitochondrial membranes of the parasite Trypanosoma brucei are also highly sensitive to perturbations in the phospholipid metabolism that result in a direct or indirect decrease of PE (21,28).…”

The role of lipids in the biogenesis of integral membrane proteins

“…In yeast, PE constitutes approximately 20% of the total phospholipids and a minimal level of PE is required for viability because depletion of PE below 4% affects the growth of the organism on nonfermentable carbon sources or at elevated temperatures (Birner et al 2001;Storey et al 2001). Depletion of PE in yeast affects the uptake of nutrients such as arginine, mediated by Can1; uracil, mediated by Fur4; proline, mediated by Put4; general amino acid uptake, mediated by Gap1; and uptake of maltose, mediated by Mal6.…”