Amino acid sensing by Ssy1

Saccharomyces cerevisiae senses extracellular amino acids using two members of the family of amino acid transporters, Gap1 or Ssy1; aspects of the latter are reviewed here. Despite resemblance with bona fide transporters, Ssy1 appears unable to facilitate transport. Exposure of yeast to amino acids results in Ssy1-dependent transcriptional induction of several genes, in particular some encoding amino acid transporters. Amino acids differ strongly in their potency, leucine being the most potent one known. Using a selection system in which potassium uptake was made dependent on amino acid signalling, our laboratory has obtained and described gain-of-function mutations in SSY1. Some alleles conferred inducer-independent signalling; others increased apparent affinity for inducers. These results revealed that amino acid transport is not required for signalling and support the notion that sensing by Ssy1 occurs via its direct interaction with extracellular amino acids. Current work includes development of quantitative assays of sensing. We use the finding by Per Ljungdahl's laboratory that the signal transduction from Ssy1 involves proteolytic removal of an inhibitory part of the transcriptional activator Stp1. Protein-A Z-domain fused to the C-terminus of Stp1 and Western analysis using antibody against horseradish peroxidase allow quantification of sensing.