The genome of the filamentous fungus Neurospora crassa contains a single gene encoding a heterotrimeric G-protein  subunit, gnb-1. The predicted GNB-1 protein sequence is most identical to G proteins from the filamentous fungi Cryphonectria parasitica and Aspergillus nidulans. N. crassa GNB-1 is also 65% identical to the human GNB-1 protein but only 38 and 45% identical to G proteins from budding and fission yeasts. Previous studies in animal and fungal systems have elucidated phenotypes of G null mutants, but little is known about the effects of G loss on G␣ levels. In this study, we analyzed a gnb-1 deletion mutant for cellular phenotypes and levels of the three G␣ proteins. ⌬gnb-1 strains are femalesterile, with production of aberrant fertilized reproductive structures. ⌬gnb-1 strains conidiate more profusely and have altered mass on solid medium. Loss of gnb-1 leads to inappropriate conidiation and expression of a conidiation-specific gene during growth in submerged culture. Intracellular cyclic AMP levels are reduced by 60% in vegetative plate cultures of ⌬gnb-1 mutants. Loss of gnb-1 leads to lower levels of the three G␣ proteins under a variety of conditions. Analysis of transcript levels for the gna-1 and gna-2 G␣ genes in submerged cultures indicates that regulation of G␣ protein levels by gnb-1 is posttranscriptional. The results suggest that GNB-1 directly regulates apical extension rate and mass accumulation. In contrast, many other ⌬gnb-1 phenotypes, including female sterility and defective conidiation, can be explained by altered levels of the three N. crassa G␣ proteins.Heterotrimeric G proteins (G␣␥) transmit external signals sensed by seven-helix transmembrane receptors, leading to a variety of physiological responses (reviewed in references 12, 17, and 38). In the inactive state, G␣, G, and G␥ subunits are in association, with GDP bound to G␣. Ligand-induced conformational changes in its coupled receptor cause the G protein to dissociate into a GTP-bound G␣ and the G␥ heterodimer. Both of these complexes can activate or inhibit downstream effectors, thus triggering an array of cellular responses (reviewed in reference 17). Characterized G␥ effectors include adenylyl cyclases, phospholipase A2, phospholipase C, Na ϩ , Ca 2ϩ , and K ϩ channels, and tyrosine and serine/threonine protein kinases (reviewed in references 8 and 17). Hydrolysis of GTP by the G␣ subunit leads to reformation of the inactive heterotrimeric form.G proteins are important for environmental and cell-type signaling in yeasts and filamentous fungi. In the budding yeast Saccharomyces cerevisiae, the G subunit Ste4p functions as a positive regulator of the pheromone response in haploid cells by recruiting the Ste20p mitogen-activated protein kinase kinase kinase kinase (MAPKKKK) to the Ste11p MAPKKK on the Ste5p scaffold (reviewed in reference 14). The Git5 G protein from Schizosaccharomyces pombe was originally thought to participate in the mating pathway through its association with the G␣ Gpa1 (25). However, accum...
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