Selective regulation of Gq signaling by G protein-coupled receptor kinase 2: Direct interaction of kinase N terminus with activated G alpha q

In this study, we investigated the regulation of different G protein-couple d receptor (GPCR)-stimulated signaling pathways by GPCR kinase 2 (GRK2). We used thyrotropin receptor, which is coupled to different G proteins, to in vestigate the regulation of G alpha s- and G alpha q-mediated signaling (as sessed by cAMP and inositol phosphate production, respectively). In transfe cted cells, both pathways were desensitized by GRK2. However a kinase-dead GRK2 mutant (GRK2-K220R) only decreased inositol phosphate production, indi cating that GRK2 could regulate G alpha q signaling through a phosphorylati on-independent mechanism. Similar results were obtained with serotonin rece ptor 5-hydroxytryptamine(2C), which is coupled to G alpha q. This effect wa s mimicked by the N-terminal domain of GRK2 (GRK2-Nter), but not by the C-t erminal domain. In cells transfected with G alpha q, direct activation of G alpha q signaling (by AlF4-) was desensitized by GRK2-Nter, indicating an effect at the G alpha-level. For comparison, in parallel samples we studied a protein regulator of G protein signaling RGS4 and we found a similar reg ulatory profile. We therefore hypothesized that the GRK2-Nter could directl y interact with the G alpha q subunit to regulate its signaling, as demonst rated for several RGS proteins. This hypothesis is further supported by the presence, within the GRK2-Nter, of an RGS homology domain. In direct bindi ng experiments, we found that GRK2-Nter interacts with G alpha q (only when activated) but not with G alpha s and G alpha o. We conclude that GRK2, be sides desensitizing the GPCR by phosphorylation, is able to selectively bin d to G alpha q and to regulate its signaling.