Evidence for an intrinsic toxicity of phosphatidylcholine to Sec14p-dependent protein transport from the yeast Golgi complex

Yeast phosphatidylinositol-transfer protein (Sec14p) is essential for Golgi secretory function and cell viability. This requirement of Sec14p is relie ved by genetic inactivation of the cytidine diphosphate-choline pathway for phosphatidycholine (PtdCho) biosynthesis. Standard phenotypic analyses ind icate that inactivation of the phosphatidylethanolamine (PtdEtn) pathway fo r PtdCho biosynthesis, however, does not rescue the growth and secretory de fects associated with Sec14p deficiency. We now report inhibition of cholin e uptake from the media reveals an efficient "bypass Sec14p" phenotype asso ciated with PtdEtn-methylation pathway defects. We further show that the by pass Sec14p phenotype associated with PtdEtn-methylation pathway defects re sembles other bypass Sec14p mutations in its dependence on phospholipase D activity. Finally, we find that increased dosage of enzymes that catalyze p hospholipase D-independent turnover of PtdCho, via mechanisms that do not r esult in a direct production of phosphatidic acid or diacylglycerol, effect a partial rescue of sec14-1(ts)-associated growth defects. Taken together, these data support the idea that PtdCho is intrinsically toxic to yeast Go lgi secretory function.