We have identified a Saccharomyces cerevisiae gene necessary for the step in sphingolipid synthesis in which inositol phosphate is added to ceramide to form inositol-P-ceramide, a reaction catalyzed by phosphatidylinositol:ceramide phosphoinositol transferase (IPC synthase). This step should be an effective target for antifungal drugs. A key element in our experiments was the development of a procedure for isolating mutants defective in steps in sphingolipid synthesis downstream from the first step including a mutant defective in IPC synthase. An IPC synthase defect is supported by data showing a failure of the mutant strain to incorporate radioactive inositol or N-acetylsphinganine into sphingolipids and, by using an improved assay, a demonstration that the mutant strain lacks enzyme activity. Furthermore, the mutant accumulates ceramide when fed exogenous phytosphingosine as expected for a strain lacking IPC synthase activity. Ceramide accumulation is accompanied by cell death, suggesting the presence of a ceramide-activated death response in yeast. A gene, AUR1 (YKL004w), that complements the IPC synthase defect and restores enzyme activity and sphingolipid synthesis was isolated. Mutations in AUR1 had been shown previously to give resistance to the antifungal drug aureobasidin A, leading us to predict that the drug should inhibit IPC synthase activity. Our data show that the drug is a potent inhibitor of IPC synthase with an IC 50 of about 0.2 nM. Fungal pathogens are an increasing threat to human health. Now that IPC synthase has been shown to be the target for aureobasidin A, it should be possible to develop high throughput screens to identify new inhibitors of IPC synthase to combat fungal diseases.

The first and committed step in synthesis of the ceramide moiety of sphingolipids is catalyzed by serine palmitoyltransferase (EC 2.3.1.50), which condenses palmitoyl-CoA and serine to form 3-ketosphinganine. This step is thought to be tightly regulated to control the synthesis of sphingolipids, but data supporting this hypothesis are lacking mainly because the enzyme has resisted purification and consequent characterization. Rather than attempting to purify the enzyme from normal cells, we have taken a different tack and opted to try and overproduce the enzyme to facilitate its purification. Here we demonstrate that overproduction in Saccharomyces cerevisiae requires expression of LCBI, a previously isolated yeast gene, and LCB2, the isolation and characterization of which we describe. Several liles of evidence argue that both genes encode subunits of the enzyme; however, biochemical evidence will be needed to substantiate this hypothesis. Although overproduction was modest, 2-to 4-fold, it should now be possible to devise improved overproduction vectors for yeast or other host organisms. (10). Further support for this hypothesis is lacking because essential experimental reagents-purified SPT, antibodies against the enzyme, and the SPT gene-are not available. SPI activity could be controlled by transcriptional or by posttranscriptional processes or by both in neuronal cells.To begin to prepare the reagents necessary for understanding how de novo sphingolipid synthesis is regulated, we have utilized the genetically tractable organism Saccharomyces cerevisiae. Our goal was to isolate the SPT gene(s) to overproduce the enzyme, which, to our knowledge, has never been purified from any organism. To isolate the gene, we began with a -mutant strain lacking SPT activity and requiring exogenous long-chain base for growth (Lcb-) (11).Complementation of the Lcb-phenotype using a genomic DNA library resulted in the isolation of a gene, LCBJ (12), whose predicted protein showed amino acid similarity to enzymes known to catalyze a reaction chemically similar to that catalyzed by SPT and to use the same cofactor (pyridoxal P) as SPT. These data suggested that the Lcbl protein was the SPT enzyme or a subunit of the enzyme. During the course of these experiments a second complementation group, lcb2, with an Lcb-phenotype, was isolated and shown to be necessary for SPT activity (13). In this report we describe the isolation and characterization of the LCB2 gene* and demonstrate that increased SPT activity is obtained only if both LCB genes are overexpressed.

To identify genes necessary for sphingolipid synthesis in Saccharomyces cerevisiae we developed a procedure to enrich for mutants unable to incorporate exogenous long chain base into sphingolipids. We show here that a mutant strain, AG84-3, isolated by using the enrichment procedure, makes sphingolipids from endogenously synthesized but not from exogenously supplied long chain base. A gene termed LCB3 (YJL134W, GenBank designation X87371x21), which complements the long chain base utilization defect of strain AG84-3, was isolated from a genomic DNA library. The gene is predicted to encode a protein with multiple membrane-spanning domains and a COOH-terminal glycosylphosphatidylinositiol cleavage/attachment site. Deletion of the lcb3 gene in a wild type genetic background reduces the rate of exogenous long chain base incorporation into sphingolipids and makes the host strain more resistant to