The carboxyl termini of alpha subunits of mammalian G proteins have been implicated in receptor interactions. We have used a genetic analysis to test such a role for the carboxyl terminus of Scg1, the alpha subunit involved in the yeast pheromone response pathway. A 22-amino-acid truncation (scg1Amb451) resulted in defects in growth and cellular morphology. This phenotype is similar to the null phenotype and represents constitutive activation of the pheromone response pathway; it could result from various effects, including protein instability or constitutive guanine nucleotide exchange, as reported for some altered mammalian G alpha s constructs. A 5-amino-acid truncation (SCG1Och468) resulted in pheromone response and mating defects in both a and alpha cells, which is consistent with defects in interactions with the pheromone receptors. Lysine-to-proline mutations near the carboxyl terminus (SCG1Pro467 and SCG1Pro468) resulted in pheromone response and mating defects, the severity of which differed in a and alpha cells. This differential effect in the two mating types suggests that the specificity for the interactions with the two pheromone receptors may involve different residues of the Scg1 carboxyl terminus. Mutations leading to constitutive activation of the pathway were recessive, whereas mutations that result in decreased pheromone response and mating were partially dominant. These relationships are consistent with the model for the mechanism of action of the G protein subunits in the pheromone response pathway and indicate the importance of the stoichiometry of components of this system.