2 TYPES OF ALPHA-FACTOR RECEPTOR DETERMINANTS FOR PHEROMONE SPECIFICITY IN THE MATING-INCOMPATIBLE YEASTS SACCHAROMYCES-CEREVISIAE AND S-KLUYVERI

The specificity of mating-pheromone systems helps prevent mating betwe en different species of yeast. The mechanism of specificity of G prote in-coupled receptors in other organisms may be similar. To determine t he structural basis of species discrimination between alpha-factor var iants, small extracellular segments of the Saccharomyces cerevisiae al pha-factor receptor were substituted with residues from the Saccharomy ces kluyveri alpha-factor receptor. Three substitutions altered specif icity towards S. cerevisiae and S. kluyveri alpha-factor pheromone pep tides. A substitution at residues 47-49 affected specificity for phero mone binding but not for pheromone activation of response. Substitutio n of residues 267-269 affected pheromone specificity for activation of cellular responses, but not for pheromone binding (under the assay co nditions used). Substitution of residues 104-123 modestly affected bot h types of specificity. These results suggest two distinct types of re ceptor/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 a s species.