Biosynthesis of inositol phosphoceramides and remodeling of glycosylphosphatidylinositol anchors in Saccharomyces cerevisiae are mediated by different enzymes

Metabolic labeling of cells with 3H]dihydrosphingosine (3H]DHS) allows us to follow the incorporation of this tracer into ceramides (Cer), inositol phosphoceramides (IPCs), and mannosylated IPCs and at the same time to assess the remodeling of glycosylphosphatidylinositol proteins during which preexisting anchor lipid moieties are replaced by 3H]Cer-containing anchors. The results indicate that the remodelases in the endoplasmic reticulum and Golgi use as their substrate Cers that are not generated by the breakdown of IPCs but are newly synthesized. Aureobasidin A, an inhibitor of the IPC synthase Aur1p completely blocks IPC biosynthesis at 0.5 micrograms/ml but does not block remodeling of glycosylphosphatidylinositol anchors even at concentrations up to 10 micrograms/ml. In addition, a synthetic Cer analogue, N-hexanoyl-3H]DHS, is used as a substrate by Aur1p but not by the remodelases. Thus, remodeling is not mediated by Aur1p although remodeling presumably proceeds by an analogous reaction. Studies with secretion mutants deficient in COPII or COPI coat proteins show that all COPII mutants are unable to introduce 3H]Cer by the Golgi remodelase at the restrictive temperature. This suggests that Cer has to be transported by a COPII-dependent way from the endoplasmic reticulum to Golgi for Golgi remodeling to occur. Golgi remodeling is also not operating in the erd2 mutant and is significantly reduced in COPI mutants, suggesting a dependence of Golgi remodeling on retrotransport.