ABSTRACTHeterotrimeric G proteins are critical regulators of growth and asexual and sexual development in the filamentous fungusNeurospora crassa. Three Gα subunits (GNA-1, GNA-2, and GNA-3), one Gβ subunit (GNB-1), and one Gγ subunit (GNG-1) have been functionally characterized, but genetic epistasis relationships between Gβ and Gα subunit genes have not been determined. Physical association between GNB-1 and FLAG-tagged GNG-1 has been previously demonstrated by coimmunoprecipitation, but knowledge of the Gα binding partners for the Gβγ dimer is currently lacking. In this study, the threeN. crassaGα subunits are analyzed for genetic epistasis withgnb-1and for physical interaction with the Gβγ dimer. We created double mutants lacking one Gα gene andgnb-1and introduced constitutively active, GTPase-deficient alleles for each Gα gene into the Δgnb-1background. Genetic analysis revealed thatgna-3is epistatic tognb-1with regard to negative control of submerged conidiation.gnb-1is epistatic togna-2andgna-3for aerial hyphal height, whilegnb-1appears to act upstream ofgna-1andgna-2during aerial conidiation. None of the activated Gα alleles restored female fertility to Δgnb-1mutants, and thegna-3Q208Lallele inhibited formation of female reproductive structures, consistent with a need for Gα proteins to cycle through the inactive GDP-bound form for these processes. Coimmunoprecipitation experiments using extracts from thegng-1-FLAG strain demonstrated that the three Gα proteins interact with the Gβγ dimer. The finding that the Gβγ dimer interacts with all three Gα proteins is supported by epistasis betweengnb-1andgna-1,gna-2, andgna-3for at least one function.