Self-compatible B mutants in Coprinus with altered pheromone-receptor specificities

A successful mating in the mushroom Coprinus cinereus brings together a com patible complement of pheromones and G-protein-coupled receptors encoded by multiallelic genes at the B mating-type locus. Rare B gene mutations lead to constitutive activation of B-regulated development without the need for mating. Here we characterize a mutation that arose in the BG locus and show that it generates a mutant receptor with a Single amino acid substitution (R96H) at the intracellular end of transmembrane domain III. Using a hetero logous yeast assay and synthetic pheromones we show that the mutation does not make the receptor constitutively active but permits it to respond inapp ropriately to a normally incompatible pheromone encoded within the same B6 locus. Parallel experiments carried out in Coprinus showed that a F67W subs titution in this same pheromone enabled it to activate the normally incompa tible wild-type receptor. Together, our experiments show that a single amin o acid replacement in either pheromone or receptor can deregulate the speci ficity of ligand-receptor recognition and confer a self-compatible B phenot ype. In addition, we use the yeast assay to demonstrate that different rece ptors and pheromones found at a single B locus belong to discrete subfamili es within which receptor activation cannot normally occur.