Binding of α-factor pheromone to yeast a cells: Chemical and genetic evidence for an α-factor receptor

The division cycle of yeast a cells is inhibited by α-factor. Haploid a cells were found to bind ³⁵S-labeled α-factor, whereas haploid α cells and diploid a/α cells showed little binding. The association of α-factor with a cells was reversible upon dilution. Unlabeled α-factor competed for binding of ³⁵S-α-factor; the concentration dependence for competition indicated 9 × 10⁵ binding sites per cell with a dissociation constant (K_D) of 3 × 10^-7 M. The rates of association (k_on = 3 × 10³ M^-1 sec^-1) and dissociation (k_off = 9 × 10^-4 sec^-1) were consistent with the equilibrium constant. The α-factor binding activity associated with five temperature-sensitive ste2 mutants was thermolabile, suggesting that the STE2 gene encodes the receptor for α-factor. In contrast, the binding activity of other temperature-sensitive mutants (ste4, ste5, ste7, ste11, and ste12) showed no thermolability.