Abstract

In this study, we investigated the regulation of different G protein-coupled receptor (GPCR)-stimulated signaling pathways by GPCR kinase 2 (GRK2). We used thyrotropin receptor, which is coupled to different G proteins, to investigate the regulation of Gαs- and Gαq-mediated signaling (assessed by cAMP and inositol phosphate production, respectively). In transfected cells, both pathways were desensitized by GRK2. However a kinase-dead GRK2 mutant (GRK2-K220R) only decreased inositol phosphate production, indicating that GRK2 could regulate Gαq signaling through a phosphorylation-independent mechanism. Similar results were obtained with serotonin receptor 5-hydroxytryptamine_2C, which is coupled to Gαq. This effect was mimicked by the N-terminal domain of GRK2 (GRK2-Nter), but not by the C-terminal domain. In cells transfected with Gαq, direct activation of Gαq signaling (by AlF₄ ⁻) was desensitized by GRK2-Nter, indicating an effect at the Gα-level. For comparison, in parallel samples we studied a protein regulator of G protein signaling RGS4 and we found a similar regulatory profile. We therefore hypothesized that the GRK2-Nter could directly interact with the Gαq subunit to regulate its signaling, as demonstrated for several RGS proteins. This hypothesis is further supported by the presence, within the GRK2-Nter, of an RGS homology domain. In direct binding experiments, we found that GRK2-Nter interacts with Gαq (only when activated) but not with Gαs and Gαo. We conclude that GRK2, besides desensitizing the GPCR by phosphorylation, is able to selectively bind to Gαq and to regulate its signaling.