Abstract
Phosphatidylcholine (PtdCho) is the most abundant phospholipid in numerous eukaryotes and is generally thought to be essential for membrane structure and cellular function. We designed a specific test of this idea by using genetic and biochemical manipulation of yeast. Yeast mutants (pem1 pem2Delta) lacking the phosphatidylethanolamine (PtdEtn) methyltransferase enzymes require choline for growth and cannot make N-methylated phospholipids. When these strains are grown on a glucose carbon source supplemented with 20 mm propanolamine (Prn), the PtdCho level declines precipitously to the limits of detection (<0.6%), and the hexagonal phase-forming, primary amine-containing lipids, PtdEtn and PtdPrn, constitute approximately 60% of the total phospholipid content of the cell. When the lipids were analyzed by mass spectrometry, there was no compensatory shift in unsaturation of the PtdEtn and PtdPrn toward more bilayer-forming species. Thus the majority of the cellular amino phospholipids remained hexagonal phase-forming. The pem1 pem2Delta cells will also grow without choline, in the presence of Prn, on nonfermentable carbon sources (requiring functional mitochondria) and accumulate nearly 70% of their phospholipid as hexagonal phase-forming types. These data provide compelling evidence that the functions of PtdCho and N-methylated lipids in membranes are nonessential in Saccharomyces cerevisiae.
Highlights
Phosphatidylcholine (PtdCho)1 is the most abundant phospholipid in many eukaryotic cells and has generally been assumed to be essential for cell viability [1,2,3]
To test if PtdCho and N-methylated lipids are critical for cell viability, we examined whether propanolamine (Prn), an ethanolamine analogue, could rescue the growth defect of methylation-defective mutant strains
In experiments described in this report we tested the essentiality of PtdCho and other methylated forms of PtdEtn for the growth and mitochondrial function of S. cerevisiae
Summary
Phosphatidylcholine (PtdCho) is the most abundant phospholipid in many eukaryotic cells and has generally been assumed to be essential for cell viability [1,2,3]. Etn and PtdCho are synthesized via the Kennedy pathways that use CDP-ethanolamine and CDP-choline intermediates. The growth defect of these mutants can be rescued by choline, Etn(Me), or Etn(Me) but not by Etn [4] These findings have led to the conclusions that some form of methylated PtdEtn is essential for yeast growth [1, 4]. To test if PtdCho and N-methylated lipids are critical for cell viability, we examined whether propanolamine (Prn), an ethanolamine analogue, could rescue the growth defect of methylation-defective mutant strains. Previous research showed that yeast mutants (psd1⌬ psd2⌬) lacking PtdSer decarboxylases require Cho or Etn for growth but can grow on Prn [8]. The results demonstrate that PtdCho and Nmethylated phospholipids are nonessential in yeast and are not required for mitochondrial function
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