Abstract
Despite the importance of triacylglycerols (TAG) and steryl esters (SE) in phospholipid synthesis in cells transitioning from stationary-phase into active growth, there is no direct evidence for their requirement in synthesis of phosphatidylinositol (PI) or other membrane phospholipids in logarithmically growing yeast cells. We report that the dga1Δlro1Δare1Δare2Δ strain, which lacks the ability to synthesize both TAG and SE, is not able to sustain normal growth in the absence of inositol (Ino(-) phenotype) at 37 °C especially when choline is present. Unlike many other strains exhibiting an Ino(-) phenotype, the dga1Δlro1Δare1Δare2Δ strain does not display a defect in INO1 expression. However, the mutant exhibits slow recovery of PI content compared with wild type cells upon reintroduction of inositol into logarithmically growing cultures. The tgl3Δtgl4Δtgl5Δ strain, which is able to synthesize TAG but unable to mobilize it, also exhibits attenuated PI formation under these conditions. However, unlike dga1Δlro1Δare1Δare2Δ, the tgl3Δtgl4Δtgl5Δ strain does not display an Ino(-) phenotype, indicating that failure to mobilize TAG is not fully responsible for the growth defect of the dga1Δlro1Δare1Δare2Δ strain in the absence of inositol. Moreover, synthesis of phospholipids, especially PI, is dramatically reduced in the dga1Δlro1Δare1Δare2Δ strain even when it is grown continuously in the presence of inositol. The mutant also utilizes a greater proportion of newly synthesized PI than wild type for the synthesis of inositol-containing sphingolipids, especially in the absence of inositol. Thus, we conclude that storage lipid synthesis actively influences membrane phospholipid metabolism in logarithmically growing cells.
Highlights
Despite the importance of triacylglycerols (TAG) and steryl esters (SE) in phospholipid synthesis in cells transitioning from stationary-phase into active growth, there is no direct evidence for their requirement in synthesis of phosphatidylinositol (PI) or other membrane phospholipids in logarithmically growing yeast cells
The diacylglycerol acyltransferases encoded by the DGA1 and LRO1 genes (Fig. 1) are the main enzymes involved in the biosynthesis of TAG (4 – 6), whereas ARE1 and ARE2 (Fig. 1) encode the enzymes that primarily mediate the esterification of ergosterol and its precursors leading to SE (7, 8)
We tested the growth of the tgl3⌬tgl4⌬tgl5⌬ mutant, in which TAG lipolysis is blocked (10, 32, 33), to determine whether the failure of the dga1⌬lro1⌬are1⌬are2⌬ mutant to grow in the absence of inositol was due to inability of mobilize TAG
Summary
EVIDENCE FOR CROSS-TALK BETWEEN TRIACYLGLYCEROL METABOLISM AND PHOSPHATIDYLINOSITOL SYNTHESIS*. We report that the dga1⌬lro1⌬are1⌬are2⌬ strain, which lacks the ability to synthesize both TAG and SE, is not able to sustain normal growth in the absence of inositol (Ino؊ phenotype) at 37 °C especially when choline is present. The products of TAG degradation, diacylglycerols (DAG) and free fatty acids, serve as precursors for membrane lipid synthesis (13) as well as for energy production when free fatty acids are the only carbon source available in the growth medium (14). We report that upon inositol reintroduction, the dga1⌬lro1⌬are1⌬are2⌬ strain, unable to synthesize TAG or SE, and the tgl3⌬tgl4⌬tgl5⌬ strain, lacking the TAG lipases, both exhibit slow recovery of PI content in comparison to wild type cells. The mutant devotes a larger percentage of newly synthesized PI to the synthesis of inositol-containing sphingolipids for which PI serves as a precursor
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