GPR-4 Is a Predicted G-Protein-Coupled Receptor Required for Carbon Source-Dependent Asexual Growth and Development in Neurospora crassa

Abstract

ABSTRACT The filamentous fungus Neurospora crassa is able to utilize a wide variety of carbon sources. Here, we examine the involvement of a predicted G-protein-coupled receptor (GPCR), GPR-4, during growth and development in the presence of different carbon sources in N. crassa . Δ gpr-4 mutants have reduced mass accumulation compared to the wild type when cultured on high levels of glycerol, mannitol, or arabinose. The defect is most severe on glycerol and is cell density dependent. The genetic and physical relationship between GPR-4 and the three N. crassa Gα subunits (GNA-1, GNA-2, and GNA-3) was explored. All three Gα mutants are defective in mass accumulation when cultured on glycerol. However, the phenotypes of Δ gna-1 and Δ gpr-4 Δ gna-1 mutants are identical, introduction of a constitutively activated gna-1 allele suppresses the defects of the Δ gpr-4 mutation, and the carboxy terminus of GPR-4 interacts most strongly with GNA-1 in the yeast two-hybrid assay. Although steady-state cyclic AMP (cAMP) levels are normal in Δ gpr-4 strains, exogenous cAMP partially remediates the dry mass defects of Δ gpr-4 mutants on glycerol medium and Δ gpr-4 strains lack the transient increase in cAMP levels observed in the wild type after addition of glucose to glycerol-grown liquid cultures. Our results support the hypothesis that GPR-4 is coupled to GNA-1 in a cAMP signaling pathway that regulates the response to carbon source in N. crassa . GPR-4-related GPCRs are present in the genomes of several filamentous ascomycete fungal pathogens, raising the possibility that a similar pathway regulates carbon sensing in these organisms.