FIGURE 3.

TgPSD1mt can rescue the growth of an S. cerevisiae mutant lacking its endogenous PSD activity. A, scheme of phospholipid synthesis in S. cerevisiae. Yeast harbors two PSD enzymes, ScPSD1 and ScPSD2, to produce PtdEtn from PtdSer in its mitochondria and the Golgi apparatus, respectively. The last reaction of the three-step CDP-ethanolamine pathway to synthesize PtdEtn is located in the ER. Exchange of PtdSer and PtdEtn between the organelles requires membrane contact sites and lipid transfer proteins (PstA/B and PeeA/B; adapted from Ref. 35). B, yeast complementation by heterologous expression of TgPSD1mt in S. cerevisiae psd1Δpsd2Δ mutant (BY23480). TgPSD1mt was expressed under the control of the ScGAL10 promoter. Empty pESC-Ura and ScPSD1 expression vectors were included as negative and positive controls. C, PSD activity in subcellular fractions of the yeast strains expressing TgPSD1mt or endogenous ScPSD1 or harboring empty vector. The indicated organelle-enriched fractions were subjected to PSD enzyme assays as described under “Materials and Methods.” ND, not detectable. Error bars, S.E.