Noncontiguous domains of the alpha-factor receptor of yeasts confer ligand specificity

The Saccharomyces cerevisiae alpha-factor receptor has a 3400-fold higher affinity for the S. cerevisiae alpha-factor peptide (c-alpha-f) than for the Saccharomyces kluyveri alpha-factor peptide (k-alpha-f) as determined by competition for [3H] c-alpha-f binding. The S. kluyveri alpha-factor receptor has an approximately 2-fold higher affinity for k-alpha-f than for c-alpha-f. The S. kluyveri receptor gene (k-STE2) is incompletely regulated by S. cerevisiae mating type and poorly expressed on the surface of an S. cerevisiae mating type a strain. A chimeric receptor (c/k1) with amino acid residues 1-45 derived from S. cerevisiae and amino acid residues 46-427 from S. kluyveri exhibits the binding specificity of the S. kluyveri receptor. However, chimeric receptors containing residues 1-168 (c/k2) or 1-250 (c/k3) from S. cerevisiae and the remainder from the S. kluyveri receptor exhibit specificities similar to one another, but intermediate between the parent S. cerevisiae and S. kluyveri receptors. The relative ability of c-alpha-f and k-alpha-f to induce growth arrest in strains expressing chimeric receptors parallels relative affinity. Thus, two noncontiguous domains that include putative extracellular loops 1 and 3 and associated transmembrane segments, but exclude the extracellular NH2 terminus and loop 2, appear to contribute to alpha-factor receptor ligand specificity. COOH-terminal regions of the S. kluyveri receptor appear to confer a desensitization defect when expressed in S. cerevisiae. The S. cerevisiae receptor truncated at residue 296 retains ligand specificity for growth arrest.