Genes that mutate to suppress the Ca2+sensitivity of 5. Cerevisiae csg2 mutants encode sphingolipid biosynthetic enzymes

The CSC I and CSG2 genes are required for conversion of inositolphosphorylceramidr (IPC) to mannosylinositolphosphorylceramide (MIPC), and therefore accumulate high levels of IPC that render them Ca2+ sensitive. Mutants that reduce the accumulation of IPC or alter its structure suppress the Ca2+sensitivity of the csg mutants. Since sphingolipids are essential for viability, many genes that mutate to suppress Ca2+- sensitivity (through reduced accumulation of IPC) are essential. Therefore, we screened suppressor mutants (isolated at 26°) for an associated temperature sensitive lethal phenotype. Of 946 suppressors, 59 were found to simultaneously acquire temperature sensitive lethality due to the suppressing mutation. Complementation analysis indicates that 15 genes are represented in the TSC mutant collection, and since many groups are represented by a single isolate, the collection is far from saturated. Genes identified among the tsc suppressors include: l)TSC4/5/FASe required for palmitoyl-CoA synthesis, 1}TSCl/LCBt and TSC2/LCB1 encoding subunits of serine palmitoyltransferase, 3)TSClO encoding 3- ketosphinganine reductase. The tsc7 mutants (three isolates) have a mutation in SURS which encodes the dîhydrosphingosine hydroxylaae as well as in a linked gene. The continued characterization of tsc mutants is expected to identify many novel genes involved in sphingolipid synthesis.