The saccharomyces cerevisial TSCW gene encodes 3-ketosphinganine reductase

'I'he CSC! and CSC2 genes are required for growth in medium containing high Ca-4+ concentrations. These mutants, being defective in the conversion of iuositolphos- phorylceramide (IPC) to mannosylinositolphosphorylceramide (MIPC). accumulate high levels of IPC. Mutants that suppress the accumulation of IPC or alter its struct u re suppress the Ça24" -sensitivity of csg mutants. The.se suppressor mutants identify genes required for sphingolipid synthesis. Since sphingolipids are essential, many genes that mutate to suppress Ca²⁺-ensitivity (through reduced accumulation of IPC) ;>r<> essential. Therefore, we -croened t fie suppressor mutants (isolated at 26'' ) for an associated temperature .-ensitivo phenotype. Of 9-46 suppressors. 50 were acquire temperature sensitive lethality due to the suppressing mutation. Complémentation analysis indicates tli H t 10 genes are represented in the TSC mutant collection. The tsclO mutants accumulate It-ketosphinganine, and t ho ts phonotype is rescued by exogenous piiytosphingosine suggesting a deficiency in 3-ketosphinganine reductase. The J.STVogene was cloned and found to encode H protein belonging to the short fhaiii dehydrogenase/reductase (SDR) enzyme family, members of which cat.ily/e reduction of ketones to alcohols. In vilro assays of the tsclO mutant membranes confirm that they are deficient in .'t keiosphinganino reductase aclivity. The mutant alleles of this gene provide powerful tools for investigating Ihi.i enzvme and for the identification of other genes involved in sphingolipid synthesis -4nd funrtion.