The GTP-dependent step is not integral to the heterotrimeric extra-large Gα subunit, XLG2, in regulation of plant immune response

N Maruta, Y Trusov and J Botella

University of Queensland St Lucia Brisbane Queensland Australia 4072

Heterotrimeric GTP-binding proteins (G proteins), consisting of Gα, Gβ and Gγ subunits, initiate cell-signaling upon ligand recognition by their receptors. They mediate multiple responses in eukaryotes. Animal Gα subunits function as molecular switches utilizing exchange of guanine nucleotides (GTP/GDP). In plants, Gα proteins evolved into extra-large Gα subunits (XLGs). XLGs retained interaction with the Gβγ dimer and form a heterotrimer at the plasma membrane. We found that Arabidopsis mutants lacking XLG2 and XLG3 were hyper-susceptible to diverse pathogens, indicating that XLG2 and XLG3 are required for plant disease resistance. Study of the quadruple mutants lacking G protein components indicated that XLGs and Gβγ interact functionally during immune signaling, confirming that XLGs are functional Gα subunits. However, from the biochemical studies and structural analysis, it remains questionable whether XLGs utilize GTP/GDP exchange to transduce the signal. We mutated the conserved GTP binding residue of XLG2 (XLG2T475N), rendering a protein incapable of GTP binding and found that immune response was not affected. We conclude that XLG2 functions independently of GTP/GDP exchange during immune response. We and others also established that XLG2 binds to several pathogenesis-related receptor-like kinases (RLKs), and mediates signaling in phosphorylation-dependent manner. We hypothesize that in plants XLGα subunits replaced the activation mechanism from GTP-dependent to phosphorylation-dependent.