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

Abstract

ABSTRACT Heterotrimeric 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 (Af ricA ) restores growth and conidiation in the ΔAn ricA 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 Gα subunit), but not sfgA , flbA , rgsB , or rgsC , restored vegetative growth and conidiation in ΔAn ricA . 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 ΔAn ricA , 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 Δ ricA mutant. In addition, mutants overexpressing fluG or brlA (OE fluG or OE brlA ) failed to restore development in the ΔAn ricA mutant. These findings suggest that the commencement of asexual development requires unknown RicA-mediated signaling input in A. nidulans .