Sphingolipids Influence the Sensitivity of Lipid Bilayers to Fungicide,Syringomycin E

Sphingolipids with long chain bases hydroxylated at the C4 position are a requisite for the yeast, Saccharomyces cerevisia, to be sensitive to the ion channel forming antifungal agent, syringomycin E (SRE). A mutant S. cerevisiae strain, Δsyr2, having sphingolipids with a sphingoid base devoid of C4-hydroxylation, is resistant to SRE. To explore the mechanism of this resistance, we investigated the channel forming activity of SRE in lipid bilayers of varying composition. We found that the addition of sphingolipid-rich fraction from Δsyr2 to the membrane-forming solution (DOPS/DOPE/ergosterol) resulted in lipid bilayers with lower sensitivity to SRE compared with those containing sphingolipid fraction from wild-type S. cerevisiae. Other conditions being equal, the rate of increase of bilayer conductance was about 40 times slower, and the number of SRE channels was about 40 times less, with membranes containing Δsyr2 versus wild-type sphingolipids. Δsyr2 sphingolipids altered neither SRE single channel conductance nor the gating charge but the ability of SRE channels to open synchronously was diminished. The results suggest that the resistance of the Δsyr2 mutant to SRE may be partly due to the ability of sphingolipids without the C4 hydroxyl group to decrease the channel forming activity of SRE.