Ceramide/long-chain base phosphate rheostat in Saccharomyces cerevisiae: regulation of ceramide synthesis by Elo3p and Cka2p

Sphingolipid precursors, namely, ceramide and long-chain base phosphates (LCBPs), are important growth regulators with often opposite effects on mammalian cells. A set of enzymes that regulate the levels of these precursors, referred to as a ceramide/LCBP rheostat, is conserved in all eukaryotes. In order to gain further insight into the function of the rheostat in Saccharomyces cerevisiae, we searched for mutants that are synthetically lethal with a deletion of the LCB3 gene encoding LCBP phosphatase. In addition to acquiring expected mutants lacking the LCBP lyase, the screen revealed elo3 (sur4) mutants that were defective in fatty acid elongation and cka2 mutants lacking the α′ subunit of the protein kinase CK2 (casein kinase). Both mutations affected the in vivo activity of the acyl coenzyme A (acyl-CoA)-dependent and fumonisin B₁-sensitive ceramide synthase (CS). The Elo3 protein is necessary for synthesis of C₂₆-CoA, which in wild-type yeast is a source of C₂₆ fatty acyls found in the ceramide moieties of all sphingolipids. In the in vitro assay, CS had a strong preference for acyl-CoAs containing longer acyl chains. This finding suggests that a block in the formation of C₂₆-CoA in yeast may cause a reduction in the conversion of LCBs into ceramides and lead to an overaccumulation of LCBPs that is lethal in strains lacking the Lcb3 phosphatase. In fact, elo3 mutants were found to accumulate high levels of LCBs and LCBPs. The cka2 mutants, on the other hand, exhibited only 25 to 30% of the in vitro CS activity found in wild-type membranes, indicating that the α′ subunit of CK2 kinase is necessary for full activation of CS. The cka2 mutants also accumulated high levels of LCBs and had elevated levels of LCBPs. In addition, both the elo3 and cka2 mutants showed increased sensitivity to the CS inhibitors australifungin and fumonisin B₁. Together, our data demonstrate that the levels of LCBPs in yeast are regulated by the rate of ceramide synthesis, which depends on CK2 kinase activity and is also strongly affected by the supply of C₂₆-CoA. This is the first evidence indicating the involvement of protein kinase in the regulation of de novo sphingolipid synthesis in any organism.