UDP-glucose Activation of a G-protein/Sucrose Synthase Signaling Supercomplex

Arabidopsis thaliana REGULATOR OF G PROTEIN SIGNALING 1 (AtRGS1, hereafter, RGS1) within the heterotrimeric G protein complex plays a role in sensing extracellular signals. High millimolar concentrations of some sugars induce RGS1 endocytosis and therefore G protein activation. However, this is not likely a direct, rather an indirect effect through a sugar metabolite. One metabolite at the plasma membrane where RGS1 resides is UDP-glucose (UDPG) produced by SUCROSE SYNTHASE (SUS) enzymes, previously shown to interact with RGS1. Our direct measurements indicate that the K_d for UDPG binding to RGS1 (2.4 µM) is consistent with UDPG activation of RGS1 with an EC₅₀ of 17 µM to rapidly trigger G protein activation within minutes. RGS1 associates directly with SUS1 and SUS4, exhibiting affinities (K_d = 161 nM and 106 nM, respectively) that are markedly tighter than the corresponding K_m values for UDPG formation (40 µM and 70 µM, respectively). Therefore, SUS may be one source for this RGS1 ligand in a manner analogous to “metabolite funneling” in enzymology. UDPG induces rapid phosphorylation of RGS1 at the C-terminal phosphorylation cluster, a site previously implicated in RGS1 endocytosis and G protein activation. UDPG activation of RGS1 changes the global transcript profiles in various processes, largely involving plant defense. These results provide the first molecular evidence for UDPG as a newly recognized low-molecular weight signal controlling key aspects of plant physiology.