Review
GRK2: multiple roles beyond G protein-coupled receptor desensitization

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G protein-coupled receptor kinases (GRKs) regulate numerous G protein-coupled receptors (GPCRs) by phosphorylating the intracellular domain of the active receptor, resulting in receptor desensitization and internalization. GRKs also regulate GPCR trafficking in a phosphorylation-independent manner via direct protein–protein interactions. Emerging evidence suggests that GRK2, the most widely studied member of this family of kinases, modulates multiple cellular responses in various physiological contexts by either phosphorylating non-receptor substrates or interacting directly with signaling molecules. In this review, we discuss traditional and newly discovered roles of GRK2 in receptor internalization and signaling as well as its impact on non-receptor substrates. We also discuss novel exciting roles of GRK2 in the regulation of dopamine receptor signaling and in the activation and trafficking of the atypical GPCR, Smoothened (Smo).

Section snippets

GRKs and β-arrestins attenuate GPCR signal transduction

GPCRs are seven-transmembrane receptors (7TMRs) encoded by approximately 950 genes, representing the largest family of cell-surface receptors [1]. They transmit a wide range of extracellular stimuli into cells, regulating most biological processes. Following agonist stimulation, GPCRs activate heterotrimeric G proteins, which exchange bound GDP for GTP, leading to the dissociation of the G protein into activated Gα and Gβγ subunits. This dissociation promotes downstream signaling through

GRK2 domains

All GRKs share a common structure comprising a highly conserved, centrally located catalytic domain of approximately 270 residues, flanked by an N-terminal domain of approximately 185 amino acids and a C-terminal domain of variable length and structure. Both the N-terminal and the C-terminal domains are involved in the regulation of GRK targeting to the membrane and activity. The N terminus harbors several regulatory motifs, including a regulator of G protein signaling homology (RH) domain. The

GRK2 regulates GPCR internalization and trafficking

Endocytosis of 7TMRs leads to their removal from the plasma membrane, after which they are sorted to either lysosomes for degradation or to recycling endosomes for subsequent reinsertion into the plasma membrane. GRK-dependent recruitment of β-arrestins to the phosphorylated receptor is crucial for clathrin-dependent endocytosis, which is the predominant internalization pathway described for most GPCRs. Interestingly, some receptors are regulated by a single GRK, whereas others appear to be

Phosphorylation-independent roles of GRK2 in GPCR signaling

The direct interactions of GRK2 with various signaling molecules can affect the levels of downstream activity of certain receptors or initiate alternative signal transduction cascades independent of receptor phosphorylation. A prime example of such an interaction is the one between GRK2 and Gβγ. This interaction not only stimulates transient translocation of GRK2 to the membrane 36, 37, but also leads to desensitization of the G protein-coupled inwardly-rectifying potassium channel (GIRK)

Regulation of non-receptor substrates

A growing body of evidence has shown that GRK2 is capable of phosphorylating non-receptor substrates (Table 1). GRK2 is a microtubule-associated kinase that directly phosphorylates tubulin following βAR stimulation 53, 54, 55, 56, suggesting a functional link between GRK2 and the cytoskeleton. Accordingly, GRK2 levels can affect agonist-induced βAR internalization in a mechanism involving microtubule stability [57]. GRK2-mediated phosphorylation of the membrane-cytoskeleton linkers radixin [58]

Emerging role for GRK2 in dopamine receptor signaling

The neurotransmitter dopamine (DA) plays a central role in many physiological processes, such as motor output and reward, via its modulation of dopamine receptors – a family of GPCRs that are highly expressed in the brain. Numerous studies have demonstrated that GRK2 protein levels dynamically regulate the signaling and trafficking of multiple dopamine receptor subtypes 73, 74, 75. For example, the coexpression of GRK2 with the dopamine type 1 receptor (D1R) in HEK293 cells results in enhanced

GRK2 in development: novel roles in the signaling and trafficking of the atypical GPCR Smo

Smo is an atypical GPCR that serves as the main transducer of the conserved Hh signaling pathway, thereby regulating many aspects of embryonic development and growth control 83, 84. Binding of the Hh ligand to its transmembrane receptor Ptch1 relieves Ptch1 inhibition of Smo and allows Smo to signal to a complex of proteins that includes the transcription factors Ci (in Drosophila) and Gli (in vertebrates) [83]. Similar to classic GPCRs, phosphorylation of active Smo by GRK2 and recruitment of

Concluding remarks

GRK2 has long been considered a classic GPCR kinase in that its mechanism of action involves direct phosphorylation of the receptor on agonist stimulation – a process that ultimately leads to the attenuation of at least one arm of GPCR intracellular signaling. Receptor internalization via clathrin-mediated endocytosis was believed to occur by a single mechanism of which the recruitment of β-arrestins to the receptor was a central requirement. Subsequent evidence has revealed a much more complex

Acknowledgements

We are grateful to Dr Richard T. Premont and Dr Melanie Philipp for critically reading this manuscript. The research in our laboratory is supported in part by NIH NS19576 and MH073853. T.D. is a recipient of an NRSA grant from NIDA 1F32-DA030026. T.E. was a recipient of a postdoctoral fellowship from The Machiah Foundation, a supporting foundation of the Jewish Community Federation of San Francisco, the Peninsula, Marin & Sonoma Counties.

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