Nonenzymatic Rapid Control of GIRK Channel Function by a G Protein-Coupled Receptor Kinase

Abstract

G protein-coupled receptors (GPCRs) respond to agonists to activate downstream enzymatic pathways or to gate ion channel function. Turning off GPCR signaling is known to involve phosphorylation of the GPCR by GPCR kinases (GRKs) to initiate their internalization. The process, however, is relatively slow and cannot account for the faster desensitization responses required to regulate channel gating. Here, we show that GRKs enable rapid desensitization of the G protein-coupled potassium channel (GIRK/Kir3.x) through a mechanism independent of their kinase activity. On GPCR activation, GRKs translocate to the membrane and quench channel activation by competitively binding and titrating G protein βγ subunits away from the channel. Of interest, the ability of GRKs to effect this rapid desensitization depends on the receptor type. The findings thus reveal a stimulus-specific, phosphorylation-independent mechanism for rapidly downregulating GPCR activity at the effector level.

Role of Phosphorylation Sites in Desensitization ofµ-Opioid Receptor

“…Second, ME produced a robust effect but morphine, which does not effectively recruit GRK2 (Doll et al, 2012), produced little very rapid desensitization. Finally, consistent with the mechanism described by Raveh et al (2010), this component was not attenuated in any of the mutants, including 11S/T-A. Morphine recruits GRK5 in HEK293 cells to mediate phosphorylation of S375 (Doll et al, 2012), but this is unlikely to account for either the slow morphine-induced phosphorylation of MOPr or modest very rapid desensitization because expression of mRNA for GRK5 is undetectable in AtT20 cells (Atwood et al, 2011).…”

“…The very rapid component of desensitization was not related to S/T phosphorylation because it persisted in all mutants exposed to ME (see as follows). It resembles the rapid nonenzymatic direct regulation of GIRK by GRK2 reported by Raveh et al (2010). Figure 3C shows a representative example of the time course of desensitization of MOPr coupling to GIRK.…”

“…GRK2 has been shown to nonenzymatically disrupt the PIP2-Gbg interaction needed for GIRK channel opening (Raveh et al, 2010). Supporting evidence includes the initial rapid desensitization includes its rapid time course (,6 seconds), similar to findings of Raveh et al (2010), which is consistent with rapid recruitment of GRK inferred from full phosphorylation of S375 by GRK2 in ,20 seconds. Second, ME produced a robust effect but morphine, which does not effectively recruit GRK2 (Doll et al, 2012), produced little very rapid desensitization.…”

Ion channel regulation by β-secretase BACE1 – enzymatic and non-enzymatic effects beyond Alzheimer's disease

“…For example, the G proteincoupled receptor (GPCR) kinase 2 (GRK2) rapidly desensitizes GPCR-coupled potassium currents independent of its phosphorylation capacity. 69 A whole collection of structural effects has been put together for transglutaminase 2. 70 Further examples are nonenzymatic functions of matrix metalloproteinase 12 and the g-secretase component presenilin 1.…”

Influence of Gαqon the Dynamics of M3-Acetylcholine Receptor–G-Protein–Coupled Receptor Kinase 2 Interaction

“…The GRK surface that binds Ga q (C-site) differs from the binding site RGS proteins use (A-site) (Zhong and Neubig, 2001;Sterne-Marr et al, 2003), which is a possible explanation for the weak or complete lack of GAP (GTPase-activating protein) function of GRK2 (Carman et al, 1999). The sequestration of activated G-protein subunits by GRK2 has been shown to desensitize the signaling of G-protein-coupled receptors independent of phosphorylation by preventing their interaction with downstream effectors (Raveh et al, 2010;Fernandez et al, 2011). However, whether binding of activated Ga q affects the function of GRK2 is still unknown and needs further investigation.…”