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Carlsberg Laboratory, DK-2500 Copenhagen, Denmark

Correspondence to Morten C. Kielland-Brandt: mkb{at}crc.dk

Recent studies of Saccharomyces cerevisiae revealed sensors that detect extracellular amino acids (Ssy1p) or glucose (Snf3p and Rgt2p) and are evolutionarily related to the transporters of these nutrients. An intriguing question is whether the evolutionary transformation of transporters into nontransporting sensors reflects a homeostatic capability of transporter-like sensors that could not be easily attained by other types of sensors. We previously found SSY1 mutants with an increased basal level of signaling and increased apparent affinity to sensed extracellular amino acids. On this basis, we propose and test a general model for transporter- like sensors in which occupation of a single, central ligand binding site increases the activation energy needed for the conformational shift between an outward-facing, signaling conformation and an inward-facing, nonsignaling conformation. As predicted, intracellular leucine accumulation competitively inhibits sensing of extracellular amino acids. Thus, a single sensor allows the cell to respond to changes in nutrient availability through detection of the relative concentrations of intra- and extracellular ligand.

K. Ottow's present address is Technical University of Denmark, DK-2800 Lyngby, Denmark.

P. Poulsen's present address is University of Copenhagen, DK-1017 Copenhagen, Denmark.

R.F. Gaber's present address Northwestern University, Evanston, IL 60208.

E. Albers's present address is Chalmers University of Technology, SE-412 96 Göteborg, Sweden.

Abbreviation used in this paper: DW, dry weight.

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