Syringomycin E Inhibition of Saccharomyces cerevisiae : Requirement for Biosynthesis of Sphingolipids with Very-Long-Chain Fatty Acids and Mannose- and Phosphoinositol-Containing Head Groups

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Abstract
Syringomycin E is an antifungal cyclic lipodepsinonapeptide that inhibits the growth of Saccharomyces cerevisiae by interaction with the plasma membrane. A screen conducted to find the yeast genes necessary for its fungicidal action identified two novel syringomycin E response genes, SYR3 and SYR4 . A syr3 mutant allele was complemented by ELO2 and ELO3 . These genes encode enzymes that catalyze the elongation of sphingolipid very long chain fatty acids. Tetrad analysis showed that SYR3 was ELO2 . Strains with deletions of SYR3/ELO2 and ELO3 were resistant to syringomycin E, and lipid analyses of both mutants revealed shortened fatty acid chains and lower levels of sphingolipids. SYR4 was identified by Tn 5 inactivation of genomic library plasmids that complemented a syr4 mutant allele. SYR4 was found to be identical to IPT1 , which encodes the terminal sphingolipid biosynthetic enzyme, mannosyl-diinositolphosphoryl-ceramide synthase. Deletion Δ syr4/ipt1 strains were viable, were resistant to syringomycin E, did not produce mannosyl-diinositolphosphoryl-ceramide, and accumulated mannosyl-inositolphosphoryl-ceramide. Accumulation of mannosyl-inositolphosphoryl-ceramide was not responsible for resistance since a temperature-sensitive secretory pathway mutant ( sec14-3 ts ) accumulated this sphingolipid and was sensitive to syringomycin E. Finally, Δ csg1/sur1 and Δ csg2 strains defective in the transfer of mannose to inositolphosphoryl-ceramide were resistant to syringomycin E. These findings show that syringomycin E growth inhibition of yeast is promoted by the production of sphingolipids with fully elongated fatty acid chains and the mannosyl and terminal phosphorylinositol moieties of the polar head group.