Abstract
Arabidopsis heterotrimeric G proteins regulate diverse processes by coupling to single-transmembrane receptors. One such receptor is the FLS2 receptor kinase, which perceives bacterial flagellin epitope flg22 to activate immunity through a class of cytoplasmic kinases called BIK1/PBLs. Unlike animal and fungal heterotrimeric G proteins that are activated by a ligand-induced guanine nucleotide exchange activity of seven-transmembrane G protein-coupled receptors (GPCRs), plant heterotrimeric G proteins are self-activating. How plant receptors regulate heterotrimeric G proteins in response to external ligands remains unknown. Here we show that RGS1, a GTPase accelerating protein, maintains Arabidopsis G proteins in an inactive state in complex with FLS2. Activation of FLS2 by flg22 induces a BIK1/PBL-mediated phosphorylation of RGS1 at Ser428 and Ser431 and that promotes RGS1 dissociation from the FLS2-G protein complex. This relieves G proteins from the RGS1-mediated repression and enables positive regulation of immune signaling. We additionally show that RGS1 is similarly regulated by multiple immune receptors. Our results uncover ligand-induced de-repression as a mechanism for G protein signaling in plants that is distinct from previously reported mechanism underlying the activation of heterotrimeric G proteins in other systems.
Highlights
Heterotrimeric G proteins are universal signaling modules in eukaryotic organisms, including animals, plants and fungi
The XLG2ΔG1 and XLG2ΔG4 mutants displayed stronger interactions with FLS2 (Supplementary information, Figure S1d). These results indicate that the guanine nucleotide-binding motifs are required for the FLS2-mediated regulation of XLG2–AGB1 interaction, supporting that the guanine nucleotide-binding state of XLG2 is important for the stability of XLG2-Gβγ heterotrimer in the FLS2 receptor complex upon flg[22] treatment
In this study, we show that the flg22-induced dissociation of Gα from Gβγ correlates with guanine nucleotide-binding states and is required for immune signaling
Summary
Heterotrimeric G proteins are universal signaling modules in eukaryotic organisms, including animals, plants and fungi. They regulate transmembrane signaling by coupling to cell surfacelocalized receptors. The animal and fungal heterotrimeric G proteins are directly regulated by seven-transmembrane G protein-coupled receptors (GPCRs). The GTP-bound Gα dissociates from the Gβγ dimer, and each entity goes on to regulate different downstream target referred to as “effectors”. Hydrolysis of GTP by the intrinsic GTPase activity of Gα allows cycling of Gα back to the GDP-bound resting state.[1,2] The GTP hydrolysis is enhanced by the regulator of G protein signaling protein (RGS), a GTPase accelerating protein (GAP).[2,3]
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