Two types of alpha-factor receptor determinants for pheromone specificity in the mating-incompatible yeasts S. cerevisiae and S. kluyveri.

The specificity of mating-pheromone systems helps prevent mating between different species of yeast. The mechanism of specificity of G protein-coupled receptors in other organisms may be similar. To determine the structural basis of species discrimination between alpha-factor variants, small extracellular segments of the Saccharomyces cerevisiae alpha-factor receptor were substituted with residues from the Saccharomyces kluyveri alpha-factor receptor. Three substitutions altered specificity towards S. cerevisiae and S. kluyveri alpha-factor pheromone peptides. A substitution at residues 47-49 affected specificity for pheromone binding but not for pheromone activation of response. Substitution of residues 267-269 affected pheromone specificity for activation of cellular responses, but not for pheromone binding (under the assay conditions used). Substitution of residues 104-123 modestly affected both types of specificity. These results suggest two distinct types of receptor/ligand specificity (perhaps corresponding to ligand binding to G protein-coupled or uncoupled conformational states of the receptor) that evolved independently as S. cerevisiae and S. kluyveri diverged as species.