Mutant strains of Saccharomyces cerevisiae lacking sphingolipids synthesize novel inositol glycerophospholipids that mimic sphingolipid structures

Mutant strains of Saccharomyces cerevisiae, termed SLC, make no detectable sphingolipids when cultured without a sphingolipid long chain base such as phytosphingosine (Dickson, R. C., Wells, G. B., Schmidt, A., and Lester, R. L. (1990) Mol. Cell. Biol. 10, 2176-2181). When grown with phytosphingosine they make sphingolipids in normal amounts. SLC strains carry a secondary suppressor mutation that bypasses the need to synthesize a long chain base. Hypothetically, the suppressor mutation could function by mutating a gene whose protein product required a sphingolipid for function, by increasing the level of one or more endogenous lipids, or by making new lipid(s). Here we demonstrate that SLC strains make novel inositol glycerophospholipids when cultured without a long chain base. The novel lipids are phosphatidylinositol (PI), mannosyl-PI, and inositol-P-(mannosyl-PI), containing 1 mol of C₂₆ fatty acid, ordinarily found in yeast sphingolipids but not usually found in glycerophospholipids; the C₂₆ fatty acid appears to be located at the SN-2 position of the glycerol. In addition, mono-fattyacyl versions of these three compounds were also identified. The polar head groups and hydrophobic portions of these novel lipids are strikingly similar to S. cerevisiae sphingolipids found in wild-type cells. We speculate that the novel lipids structurally mimic sphingolipids and thereby compensate for some sphingolipid function(s) necessary for growth.