In animals, endocytosis of a seven-transmembrane GPCR is mediated by arrestins to propagate or arrest cytoplasmic G protein–mediated signaling, depending on the bias of the receptor or ligand, which determines how much one transduction pathway is used compared to another. In Arabidopsis thaliana, GPCRs are not required for G protein–coupled signaling because the heterotrimeric G protein complex spontaneously exchanges nucleotide. Instead, the seven-transmembrane protein AtRGS1 modulates G protein signaling through ligand-dependent endocytosis, which initiates derepression of signaling without the involvement of canonical arrestins. Here, we found that endocytosis of AtRGS1 initiated from two separate pools of plasma membrane: sterol-dependent domains and a clathrin-accessible neighborhood, each with a select set of discriminators, activators, and candidate arrestin-like adaptors. Ligand identity (either the pathogen-associated molecular pattern flg22 or the sugar glucose) determined the origin of AtRGS1 endocytosis. Different trafficking origins and trajectories led to different cellular outcomes. Thus, in this system, compartmentation with its associated signalosome architecture drives biased signaling.

No abstract

The endoplasmic reticulum (ER) stress response is triggered by any condition that disrupts protein folding and promotes the accumulation of unfolded proteins in the lumen of the organelle. In eukaryotic cells, the evolutionarily conserved unfolded protein response is activated to clear unfolded proteins and restore ER homeostasis. The recovery from ER stress is accomplished by decreasing protein translation and loading into the organelle, increasing the ER protein processing capacity and ER-associated protein degradation activity. However, if the ER stress persists and cannot be reversed, the chronically prolonged stress leads to cellular dysfunction that activates cell death signaling as an ultimate attempt to survive. Accumulating evidence implicates ER stress-induced cell death signaling pathways as significant contributors for stress adaptation in plants, making modulators of ER stress pathways potentially attractive targets for stress tolerance engineering. Here, we summarize recent advances in understanding plant-specific molecular mechanisms that elicit cell death signaling from ER stress. We also highlight the conserved features of ER stress-induced cell death signaling in plants shared by eukaryotic cells.

flg22 is a 22 amino peptide released from bacterial flagellin a Microbe Associated Molecular 51 Pattern ( that is recognized by the plant cell as a signal indicating that bacteria are present. On its own, flg22 initiates a rapid increase in cytoplasmic calcium, extracellular reactive oxygen species, and activation of a Mitogen Activated Protein Kinase (cascade all of which are activated within 15 minutes after the cell perceives flg22. Here we show a massive change in protein abundance and phosphorylation state of the Arabidopsis root cell proteome within this 15 minute duration in wildtype and a mutant deficient in G protein coupled signaling Integration of phosphoproteome with protein protein interactome data followed by network topology analyses discovered that many of the flg22 induced phosphoproteome changes fall on proteins that comprise the G protein interactome and on the most highly populated hubs of the immunity network approximately 95% of the phosphorylation changes in the G protein interactome depend on a functional heterotrimeric G protein complex some occur on proteins that interact directly with components of G coupled signal transduction. One of these is ATBα, a substrate recognition sub-unit of the PP2A Ser/Thr phosphatase and an interactor to Arabidopsis thaliana REGULATOR OF G SIGNALING 1 protein (a 7 transmembrane spanning modulator of the nucleotide binding state of the core G protein complex. AtRGS1 is phosphorylated by BAK1, a component of the flg22 receptor, to initiate AtRGS1 endocytosis. A null mutation of ATB α confer s high 67 basal endocytosis of AtRGS1 suggesting sustained phosphorylated status. Loss of ATB α confers 68 trait s associated with loss of AtRGS1. Because the basal level of AtRGS1 is lower in the atbα null mutant in a proteasome dependent manner we propose that phosphorylation dependent endocytosis of AtRGS1 is part of a mechanism to degrade AtRGS1 which then sustains activation of the 71 G protein complex Thus, the role of ATB α is now established as a central component of phosphorylation dependent regulation of system dynamics in innate immunity