YEAST ALPHA-MATING-FACTOR STRUCTURE-ACTIVITY RELATIONSHIP DERIVED FROM GENETICALLY SELECTED PEPTIDE AGONISTS AND ANTAGONISTS OF STE2P

alpha-Factor, a 13-amino-acid pheromone secreted by haploid alpha cell s of Saccharomyces cerevisiae, binds to Ste2p, a seven-transmembrane, G-protein-coupled receptor present on haploid a cells, to activate a s ignal transduction pathway required for conjugation and mating? To det ermine the structural requirements for alpha-factor activity, we devel oped a genetic screen to identify from random and semirandom libraries novel peptides that function as agonists or antagonists of Ste2p, The selection scheme was based on autocrine strains constructed to secret e random peptides and respond by growth to those that were either agon ists or antagonists of Ste2p, Analysis of a number of peptides obtaine d by this selection procedure indicates that Trp1, Trp3, Pro8, and Gly 9 are important for agonist activity specifically, His2, Leu4, Leu6, P ro10, a hydrophobic residue 12, and an aromatic residue 13 are importa nt for both agonist and antagonist activity, Our results also show tha t activation of Ste2p can be achieved with novel, unanticipated combin ations of amino acids, Finally, the results suggest the utility of thi s selection scheme for identifying novel ligands for mammalian G-prote in-coupled receptors heterologously expressed in S. cerevisiae.