The Putative Guanine Nucleotide Exchange Factor RicA Mediates Upstream Signaling for Growth and Development in Aspergillus

Abstract

dHeterotrimeric G proteins (G proteins) govern growth, development, and secondary metabolism in various fungi. Here, we characterized ricA, which encodes a putative GDP/GTP exchange factor for G proteins in the model fungus Aspergillus nidulans and the opportunistic human pathogen Aspergillus fumigatus. In both species, ricA mRNA accumulates during vegetative growth and early developmental phases, but it is not present in spores. The deletion of ricA results in severely impaired colony growth and the total (for A. nidulans) or near (for A. fumigatus) absence of asexual sporulation (conidiation). The overexpression (OE) of the A. fumigatus ricA gene (AfricA) restores growth and conidiation in the ⌬AnricA mutant to some extent, indicating partial conservation of RicA function in Aspergillus. A series of double mutant analyses revealed that the removal of RgsA (an RGS protein of the GanB G␣ subunit), but not sfgA, flbA, rgsB, or rgsC, restored vegetative growth and conidiation in ⌬AnricA. Furthermore, we found that RicA can physically interact with GanB in yeast and in vitro. Moreover, the presence of two copies or OE of pkaA suppresses the profound defects caused by ⌬AnricA, indicating that RicA-mediated growth and developmental signaling is primarily through GanB and PkaA in A. nidulans. Despite the lack of conidiation, brlA and vosA mRNAs accumulated to normal levels in the ⌬ricA mutant. In addition, mutants overexpressing fluG or brlA (OEfluG or OEbrlA) failed to restore development in the ⌬AnricA mutant. These findings suggest that the commencement of asexual development requires unknown RicA-mediated signaling input in A. nidulans.

A Ric8/Synembryn Homolog Promotes Gpa1 and Gpa2 Activation To Respectively Regulate Cyclic AMP and Pheromone Signaling in Cryptococcus neoformans

“…In Neurospora crassa, disruption of the RIC8 gene results in a phenotype similar to that of G␣ gna1 and G␣ gna3 mutants (25), and expression of a dominant-active GNA3 in the ric8 deletion mutant leads to a significant increase in conidial germination (26). Deletion of the ricA gene (a RIC8 homolog) leads to impaired colony growth and total or partial loss of asexual sporulation in Aspergillus nidulans and Aspergillus fumigatus, respectively (27). In the rice blast fungus Magnaporthe oryzae, MoRic8, a Ric8 homolog, interacts with the G␣ protein MoMagB to regulate appressorial differentiation (28).…”

Gβ-Like CpcB Plays a Crucial Role for Growth and Development of Aspergillus nidulans and Aspergillus fumigatus

“…nidulans grown on solid medium were similar to those observed for WT strains [11], [13], those of the Δ cpcB mutant were severely impaired resulting in extremely small colonies (Figs. 2A & 3A), which resembles the mutant lacking the putative GEF RicA [56]. In contrast, in A. fumigatus , whereas the cpcB deletion mutant exhibited radial growth that is about 80% WT, the Δ AfusfaD and Δ AfugpgA mutants showed extremely impaired hyphal growth on solid medium (Fig.…”

“…Likewise, CpcB might be necessary for the stabilization of one or more of Gα proteins in Aspergillus , and a prolonged activation of FadA and GanB by the removal of the cognate negative regulator and the absence of CpcB may cause effects similar to a total lack of heterotrimeric G protein signaling. In this regard, it is noteworthy that the deletion of the putative GEF RicA caused severely impaired growth and the total absence of conidiation in A. nidulans [56].…”

Functions, Cooperation, and Interplays of the Vegetative Growth Signaling Pathway in theAspergilli

“…RicA, which is a putative GDP/GTP exchange factor for G-proteins in A. nidulans and A. fumigatus, can physically interact with GanB, mediates growth and developmental signaling primarily through GanB and PkaA in A. nidulans. The deletion of ricA resulted in severely impaired colony growth and decreased conidiation [78]. Loss of function mutations of fluG and flbA resulted in the appearance of undifferentiated mycelia in "fluffy" colonies.…”