The role of the yeast plasma membrane SPS nutrient sensor in the metabolicresponse to extracellular amino acids

In response to discrete environmental cues, Saccharomyces cerevisiae cells adjust patterns of gene expression and protein activity to optimize metabol ism. Nutrient-sensing systems situated in the plasma membrane (PM) of yeast have only recently been discovered. Ssy1p is one of three identified compo nents of the Ssy1p-Ptr3p-Ssy5 (SPS) sensor of extracellular amino acids. SP S sensor-initiated signals are known to modulate the expression of number o f amino acid and peptide transporter gen (i.e. AGP1, BAP2, BAP3, DIP5, GAP1 , GNP1, TAT1, TAT2 and PTR2) and arginase (CAR1). To obtain a better unders tanding of how cells adjust metabolism in response to extracellular amino a cids in the environment and to assess the consequences of loss of amino aci d sensor function, we investigated the effects of leucine addition to wild- type and ssy1 null mutant cells using genome-wide transcription profile ana lysis. Our results indicate that the previously identified genes represent only a subset of the full spectrum of Ssy1p-dependent genes. The expression of several genes encoding enzymes in amino acid biosynthetic pathways, inc luding the branched-chain, lysine and arginine, and the sulphur amino acid biosynthetic pathways, are modulated by Ssy1p. Additionally, the proper tra nscription of several nitrogen-regulated genes, including NIL1 and DAL80, e ncoding well-studied GATA transcription factors, is dependent upon Ssy1p. F inally, several genes were identified that require Ssy1p for wild-type expr ession independently of amino acid addition. These findings demonstrate tha t yeast cells require the SPS amino acid sensor component, Ssy1p, to adjust diverse cellular metabolic processes properly.