The ␣-factor receptor (Ste2p) that promotes mating in Saccharomyces cerevisiae is similar to other G protein-coupled receptors (GPCRs) in that it contains seven transmembrane domains. Previous studies suggested that the extracellular ends of the transmembrane domains are important for Ste2p function, so a systematic scanning mutagenesis was carried out in which 46 residues near the ends of transmembrane domains 1, 2, 3, 4, and 7 were replaced with cysteine. These mutants complement mutations constructed previously near the ends of transmembrane domains 5 and 6 to analyze all the extracellular ends. Eight new mutants created in this study were partially defective in signaling (V45C, N46C, T50C, A52C, L102C, N105C, L277C, and A281C). Treatment with 2-([biotinoyl] amino) ethyl methanethiosulfonate, a thiol-specific reagent that reacts with accessible cysteine residues but not membrane-embedded cysteines, identified a drop in the level of reactivity over a consecutive series of residues that was inferred to be the membrane boundary. An unusual prolonged zone of intermediate reactivity near the extracellular end of transmembrane domain 2 suggests that this region may adopt a special structure. Interestingly, residues implicated in ligand binding were mainly accessible, whereas residues involved in the subsequent step of promoting receptor activation were mainly inaccessible. These results define a receptor microdomain that provides an important framework for interpreting the mechanisms by which functionally important residues contribute to ligand binding and activation of Ste2p and other GPCRs.
The alpha-factor receptor (Ste2p) stimulates mating of the yeast Saccharomyces cerevisiae. Ste2p belongs to the large family of G protein-coupled receptors that are characterized by seven transmembrane alpha-helices. Receptor activation is thought to involve changes in the packing of the transmembrane helix bundle. To identify residues that contribute to Ste2p activation, second-site suppressor mutations were isolated that restored function to defective receptors carrying either an F204S or Y266C substitution which affect residues at the extracellular ends of transmembrane domains 5 and 6, respectively. Thirty-five different suppressor mutations were identified. On their own, these mutations caused a range of phenotypes, including hypersensitivity, constitutive activity, altered ligand binding, and loss of function. The majority of the mutations affected residues in the transmembrane segments that are predicted to face the helix bundle. Many of the suppressor mutations caused constitutive receptor activity, suggesting they improved receptor function by partially restoring the balance between the active and inactive states. Analysis of mutations in transmembrane domain 7 implicated residues Ala281 and Thr282 in receptor activation. The A281T and T282A mutants were supersensitive to S. cerevisiae alpha-factor, but were defective in responding to a variant of alpha-factor produced by another species, Saccharomyces kluyveri. The A281T mutant also displayed 8.7-fold enhanced basal signaling. Interestingly, Ala281 and Thr282 are situated in approximately the same position as Lys296 in rhodopsin, which is covalently linked to retinal. These results suggest that transmembrane domain 7 plays a role in receptor activation in a wide range of G protein-coupled receptors from yeast to humans.
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