Overlapping Functions for Two G Protein α Subunits in Neurospora crassa

Heterotrimeric G proteins, consisting of α, β and γ subunits, mediate a variety of signaling pathways in eukaryotes. We have previously identified two genes, gna-1 and gna-2, that encode G protein α subunits in the filamentous fungus Neurospora crassa. Mutation of gna-1 results in female infertility and sensitivity to hyperosmotic media. In this study, we investigate the expression and functions of gna-2. Results from Western analysis and measurements of gna-2 promoter-lacZ fusion activity indicate that gna-2 is expressed during the vegetative and sexual cycle of N. crassa in both A and a mating types. Activating mutations predicted to abolish the GTPase activity of GNA-2 cause subtle defects in aerial hyphae formation and conidial germination. Extensive phenotypic analysis of Δgna-2 strains did not reveal abnormalities during vegetative or sexual development. In contrast, deletion of gna-2 in a Δgna-1 strain accentuates the Δgna-1 phenotypes. Δgna-1 Δgna-2 strains have a slower rate of hyphal apical extension than Δgna-1 strains on hyperosmotic media. Moreover, Δgna-1 Δgna-2 mutants have more pronounced defects in female fertility than Δgna-1 strains. We propose that gna-1 and gna-2 have overlapping functions and may constitute a gene family. This is the first report of G protein α subunits with overlapping functions in eukaryotic microbes.