Identification of ISC1 (YER019w) as inositol phosphosphingolipid phospholipase C in Saccharomyces cerevisiae

Sphingolipids have emerged as novel bioactive mediators in eukaryotic cells including yeast. It has been proposed that sphingomyelin (SM) hydrolysis a nd the concomitant generation of ceramide are involved in various stress re sponses in mammalian cells, The yeast Saccharomyces cerevisiae has inositol phosphosphingolipids (IPS) instead of SM and glycolipids, and synthesis of IFS is indispensable to its growth. Although the genes responsible for the synthesis of IFS have been identified, the gene(s) for the degradation of IFS has not been reported, Here we show that ISC1 (YER019w), which has homo logy to bacterial neutral sphingomyelinase (SMase), encodes IFS phospholipa se C (IPS-PLC). First, we observed that overexpression of ISC1 greatly incr eased neutral SMase activity, and this activity was dependent on the presen ce of phosphatidylserine. Cells deleted in ISC1 demonstrated negligible neu tral SMase activity. Because yeast cells have IFS instead of SM, we investi gated whether IFS are the physiologic substrates of this enzyme. Lysates of ISC1-overexpressing cells demonstrated very high PLC activities on IFS. De letion of ISC1 eliminated endogenous IPS-PLC activities, Labeling yeast cel ls with [H-3]dihydrosphingosine showed that IFS were increased in the delet ion mutant cells, This study identifies the first enzyme involved in catabo lism of complex sphingolipids in S. cerevisiae.