Amino Acid Signaling in Saccharomyces cerevisiae: a Permease-Like Sensor of External Amino Acids and F-Box Protein Grr1p Are Required for Transcriptional Induction of the AGP1 Gene, Which Encodes a Broad-Specificity Amino Acid Permease

Abstract

The SSY1 gene of Saccharomyces cerevisiaeencodes a member of a large family of amino acid permeases. Compared to the 17 other proteins of this family, however, Ssy1p displays unusual structural features reminiscent of those distinguishing the Snf3p and Rgt2p glucose sensors from the other proteins of the sugar transporter family. We show here that SSY1 is required for transcriptional induction, in response to multiple amino acids, of the AGP1 gene encoding a low-affinity, broad-specificity amino acid permease. Total noninduction of the AGP1 gene in the ssy1Δ mutant is not due to impaired incorporation of inducing amino acids. Conversely, AGP1 is strongly induced by tryptophan in a mutant strain largely deficient in tryptophan uptake, but it remains unexpressed in a mutant that accumulates high levels of tryptophan endogenously. Induction of AGP1requires Uga35p(Dal81p/DurLp), a transcription factor of the Cys₆-Zn₂ family previously shown to participate in several nitrogen induction pathways. Induction of AGP1by amino acids also requires Grr1p, the F-box protein of the SCF^Grr1 ubiquitin-protein ligase complex also required for transduction of the glucose signal generated by the Snf3p and Rgt2p glucose sensors. Systematic analysis of amino acid permease genes showed that Ssy1p is involved in transcriptional induction of at least five genes in addition to AGP1. Our results show that the amino acid permease homologue Ssy1p is a sensor of external amino acids, coupling availability of amino acids to transcriptional events. The essential role of Grr1p in this amino acid signaling pathway lends further support to the hypothesis that this protein participates in integrating nutrient availability with the cell cycle.

ACKNOWLEDGMENTS

We gratefully acknowledge the excellent technical assistance of Catherine Jauniaux. We are also grateful to Michel Hanocq, Jacques Dubois, Bart Scherens, and Mohamed El Bakkoury for their help in measuring intracellular tryptophan pools. We also thank Anne-Marie Marini for fruitful discussions and for critical reading of the manuscript.

This work was supported by the following contracts: The Commission of the European Communities (EUROFAN, BIO4-CT95-0080), The Fund for Medical Scientific Research (Belgium, FRSM 3.4602.94), The International Brachet Stiftung (grant GR97/9-02), and the Communauté Française de Belgique, Direction de la Recherche Scientifique, Actions de Recherche Concertées. I.I. is a recipient of a predoctoral fellowship from the CommunautéFrançaise de Belgique and from the Fondation Universitaire David et Alice Van Buuren (Belgium). J.-O.D. and F.B. are recipients of FRIA (Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture) predoctoral fellowships.