Structure-function analysis of the Saccharomyces cerevisiae tridecapeptide pheromone using alanine-scanned analogs

Twenty-six peptide analogs of the Saccharomyces cerevisiae alpha-factor, a tridecapeptide mating pheromone (W1H2W3L4Q5L6K7p8G9Ql0P11M12Y13) with either L- or D-alanine replacement of each amino acid residue (Ala-scanned) and with the isosteric replacement of methionine at position 12 by norleucine, were synthesized, purified to homogeneity and assayed for biological activity and receptor binding. Two new and effective antagonists, [D-Ala3,Nle12]alpha-factor and [D-Ala4,Nle12]alpha-factor, were found among the series, and the [D-Ala10,Nle12]alpha-factor demonstrated a marked ability to increase the biological activity of [Nle12]alpha-factor without having any effect by itself. One analog, the [L-Ala1 alpha-factor, showed a 3-fold increase in bioactivity over the [Nle12]alpha-factor, although its binding to the alpha-factor receptor was about 70-fold less than [Nle12]alpha-factor. Residues near the carboxyl terminus contributed more strongly to receptor binding than other residues, whereas those near the amine terminus of the alpha-factor played an important role in signal transduction. The effect of insertion of D-Ala residues at positions 7, 8, 9 and 10 on bioactivity and receptor binding of the peptide suggested a specific positioning role of the central loop in establishing optimal contacts between the receptor and the ends of the pheromone. We conclude that the alpha-factor may be divided into segments with dominant roles in forming the biologically active pheromone conformation, in receptor binding and in initiating signal transduction. The discovery of such relationships was made possible by the systematic variation of each residue in the peptide and by the testing of each analog in highly defined biological and binding assays.