Distinct specificity of two pheromone G-protein coupled receptors, Map3 and Mam2, in fission yeast species.

Abstract

Most sexually reproducing organisms have the ability to recognize individuals of the same species. In ascomycetes including yeasts, potential mating between cells of opposite mating-type depends on the molecular recognition of two peptidyl mating pheromones by their corresponding G-protein coupled receptors (GPCRs). Although such pheromone/receptor systems are likely to function in both mate choice and prezygotic isolation, very few studies have focus on the differences in pheromone/receptor system between mating types. The fission yeast Schizosaccharomyces pombe has two mating types (sexes), Plus (P) and Minus (M). Here we investigated the specificity of the two GPCRs, Mam2 and Map3, for their respective pheromones, P-factor and M-factor, in fission yeast. First, we switched GPCRs between S. pombe and the closely related species Schizosaccharomyces octosporus, which showed that SoMam2 (Mam2 of S. octosporus) is partially functional in S. pombe, whereas SoMap3 (Map3 of S. octosporus) is not interchangeable. Next, we swapped individual domains of Mam2 and Map3 with the respective domains in SoMam2 and SoMap3, which revealed differences between the receptors both in the intracellular regions that regulate the downstream signaling of pheromones and in the molecular recognition for pheromone binding. In particular, we demonstrated that two amino acid residues of Map3, F214 and F215, are essential for the specificity of M-factor recognition. Thus, the differences in these two GPCRs are likely to reflect the significantly distinct specificities of their respective pheromone/receptor systems; that is, the specificity of Map3 is more stringent than that of Mam2. We speculate that this sexual asymmetry might allow ascomycete fungi to generate novel prezygotic barriers within a population, while maintaining strong mate choice. Our genetic analyses also contribute to our understanding of the receptors that comprise the Class D GPCRs belonging to the fungal pheromone receptor family. Author summaryCourtship signals play a key role in the fertilization processes of living beings from animals to microorganisms and, in particular, sex pheromones are involved in differentiating among species. On the other hand, changes in a pheromone/receptor system might also alter species-specificity during the selection of a mating partner, which is likely to facilitate prezygotic isolation. Here we demonstrate a distinct difference in the specificity of GPCRs for pheromones between cells of opposite mating-type in the fission yeast Schizosaccharomyces pombe, by conducting a comprehensive genetic analysis and comparison of these proteins between closely related fission yeast species. Our finding of sexual asymmetry due to the relative strictness of receptors in S. pombe suggests that it might be one of the driving forces behind the formation of new species. Such differences in pheromone/receptor interactions between the sexes may occur in a variety of life-forms such as insects and amphibians; hence, the findings of this study might be extended to other organisms. In addition, this study of S. pombe pheromone receptors sheds light on the functions of the unique Class D GPCRs.