Isolation and characterization of PTR3 : a novel gene required for amino acid-responsive signal transduction in Saccharomyces cerevisiae

Specific proteins present in virtually all cells examined to date facilitate the transport of small peptides across the plasma membrane in an energy- dependent fashion. The physiology of peptide transport systems has been studied extensively in procaryotic cells and, in many cases, the genes responsible for this phenomenon have been isolated. However, only recently have the genes responsible for peptide transport in eukaryotic cells begun to be identified. Genetic analyses of the model eukaryote Saccharomyces cerevisiae have indicated that at least three genes are required for amino acid-inducible Peptide TRansport: PTR1, PTR2, and PTR3. The major goal of my project was to isolate and characterize one of these genes required for peptide transport in yeast, namely PTR3. Once identified, PTR3 was found to encode a novel gene that exhibited no similarity to any other protein in the database. Deletion of the PTR3 open reading frame had a pleiotropic phenotype; it simultaneously reduced the sensitivity to toxic peptides and amino acid analogues. Initial rates of radiolabeled dipeptide uptake demonstrated that elimination of PTR3 resulted in the loss of amino acid-inducible peptide transport activity. Northern analyses revealed that PTR3 was required for amino acid-induced expression of the PTRl, the gene encoding the di- tripeptide permease of S. cerevisiae. Further studies indicated that PTR3 also regulated the expression of at least two amino acid permease genes. These discoveries provided a molecular basis for the observed pleiotropic phenotype associated with ptr3 alleles, and indicated that the cellular role of PTR3 was the regulation of gene expression in response to small amounts of extracellular amino acids. The completion of additional genetic studies provided evidence that PTR3 functions within a novel regulatory pathway that coordinates amino acid-responsive regulation of a number of genes. Finally, it is hypothesized that PTR3 functions as a component of a novel amino acid-sensing signal transduction pathway.