Phosphatidylethanolamine Has an Essential Role inSaccharomyces cerevisiae That Is Independent of Its Ability to Form Hexagonal Phase Structures*

Two yeast enzymes, Psd1p and Psd2p, catalyze the decarboxylation of phosphatidylserine to produce phosphatidylethanolamine (PtdEtn). Mitochondrial Psd1p provides ∼90% of total cellular phosphatidylserine decarboxylase activity. When thePSD1 gene is deleted, the resultant strain(psd1Δ) grows normally at 30 °C in glucose and in the absence of exogenous choline or ethanolamine. However, at elevated temperature (37 °C) or on the nonfermentable carbon source lactate, the growth of psd1Δ strains is minimal without ethanolamine supplementation. The reduced growth and viability correlate with a PtdEtn content below 4% of total phospholipid. These results suggest that there is a critical level of PtdEtn required to support growth. This theory is supported by growth data revealing that a psd1Δ psd2Δ dpl1Δ strain can only grow in the presence of ethanolamine. In contrast, a psd1Δ psd2Δstrain, which makes low levels of PtdEtn from sphingolipid breakdown, can be rescued by ethanolamine, choline, or the ethanolamine analogue propanolamine. psd1Δ psd2Δ cells grown in 2 mm propanolamine accumulate a novel lipid, which was determined by mass spectrometry to be phosphatidylpropanolamine (PtdPrn). PtdPrn can comprise up to 40% of the total phospholipid content in supplemented cells at the expense of phosphatidylcholine and PtdEtn. The absolute level of PtdEtn required for growth when PtdPrn is present appears to be 1% of the total phospholipid content. The essential function of the PtdEtn in the presence of propanolamine does not appear to be the formation of hexagonal phase lipid, insofar as PtdPrn readily forms hexagonal phase structures detectable by31P NMR.