PMID: 22383884
Bosch DE, Willard FS, Ramanujam R, Kimple AJ, Willard MD, Naqvi NI, Siderovski DP
A P-loop Mutation in Galpha Subunits Prevents Transition to the Active State: Implications for G-protein Signaling in Fungal Pathogenesis.
PLoS Pathog. 2012 Feb;8(2):e1002553. Epub 2012 Feb 23.
Heterotrimeric G-proteins are molecular switches integral to a panoply of different physiological responses that many organisms make to environmental cues. The switch from inactive to active Galphabetagamma heterotrimer relies on nucleotide cycling by the Galpha subunit: exchange of GTP for GDP activates Galpha, whereas its intrinsic enzymatic activity catalyzes GTP hydrolysis to GDP and inorganic phosphate, thereby reverting Galpha to its inactive state. In several genetic studies of filamentous fungi, such as the rice blast fungus Magnaporthe oryzae, a G42R mutation in the phosphate-binding loop of Galpha subunits is assumed to be GTPase-deficient and thus constitutively active. Here, we demonstrate that Galpha(G42R) mutants are not GTPase deficient, but rather incapable of achieving the activated conformation. Two crystal structure models suggest that Arg-42 prevents a typical switch region conformational change upon Galpha(i1)(G42R) binding to GDP.AlF(4) (-) or GTP, but rotameric flexibility at this locus allows for unperturbed GTP hydrolysis. Galpha(G42R) mutants do not engage the active state-selective peptide KB-1753 nor RGS domains with high affinity, but instead favor interaction with Gbetagamma and GoLoco motifs in any nucleotide state. The corresponding Galpha(q)(G48R) mutant is not constitutively active in cells and responds poorly to aluminum tetrafluoride activation. Comparative analyses of M. oryzae strains harboring either G42R or GTPase-deficient Q/L mutations in the Galpha subunits MagA or MagB illustrate functional differences in environmental cue processing and intracellular signaling outcomes between these two Galpha mutants, thus demonstrating the in vivo functional divergence of G42R and activating G-protein mutants.