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

In response to discrete environmental cues, Saccharomyces cerevisiae cells adjust patterns of gene expression and protein activity to optimize metabolism. Nutrient-sensing systems situated in the plasma membrane (PM) of yeast have only recently been discovered. Ssy1p is one of three identified components of the Ssy1p–Ptr3p–Ssy5 (SPS) sensor of extracellular amino acids. SPS sensor-initiated signals are known to modulate the expression of a number of amino acid and peptide transporter genes (i.e. AGP1, BAP2, BAP3, DIP5, GAP1, GNP1, TAT1, TAT2 and PTR2) and arginase (CAR1). To obtain a better understanding of how cells adjust metabolism in response to extracellular amino acids in the environment and to assess the consequences of loss of amino acid sensor function, we investigated the effects of leucine addition to wild-type and ssy1 null mutant cells using genome-wide transcription profile analysis. 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, including the branched-chain, lysine and arginine, and the sulphur amino acid biosynthetic pathways, are modulated by Ssy1p. Additionally, the proper transcription of several nitrogen-regulated genes, including NIL1 and DAL80, encoding well-studied GATA transcription factors, is dependent upon Ssy1p. Finally, several genes were identified that require Ssy1p for wild-type expression independently of amino acid addition. These findings demonstrate that yeast cells require the SPS amino acid sensor component, Ssy1p, to adjust diverse cellular metabolic processes properly.