Targeting G protein-coupled receptor kinases (GRKs) to G protein-coupled receptors

Abstract

G protein-coupled receptors (GPCRs) interact with three protein families following agonist binding: heterotrimeric G proteins, G protein-coupled receptor kinases (GRKs) and arrestins. GRK-mediated phosphorylation of GPCRs promotes arrestin binding to uncouple the receptor from G protein, a process called desensitization, and for many GPCRs, arrestin binding also promotes receptor endocytosis and intracellular signaling. Thus, GRKs play a central role in modulating GPCR signaling and localization. Here we review recent advances in this field which include additional insight into how GRKs target GPCRs and bias signaling, and the development of specific inhibitors to dissect GRK function in model systems.

Keywords: arrestin; cell signaling; desensitization; endocytosis; phosphorylation; protein kinase.

Figure 1

Overview of GRK targeting to GPCRs and nodes of GRK regulation. (1) GRKs are recruited to agonist-activated GPCRs at the plasma membrane. (2) Upon binding GPCRs, GRKs undergo a conformational change that promotes kinase activation. (3) The active GRK phosphorylates the GPCR C-terminus. (4) GPCR phosphorylation by GRKs promotes β-arrestin (βarr) recruitment to the receptor, mediating receptor desensitization, endocytosis and arrestin-dependent signaling pathways. GRKs and GRK regulation can play a key role in determining receptor signaling. Recruitment factors such as G protein subunits or acidic lipids can mediate GRK recruitment to the receptor. Regulatory factors can directly bind to or post-translationally modify GRKs to regulate kinase recruitment and activity. Crosstalk receptors or the activated GPCR can signal these recruitment or regulatory factors to modulate GRK function. GRK inhibitors block kinase activity, inhibiting GPCR phosphorylation and phosphorylation-dependent pathways.

Figure 2

GRK domain architecture. GRKs contain a conserved catalytic kinase domain, regulatory N-terminus and regulator of G protein signaling homology (RH) domain. Each GRK subfamily is characterized by a distinct, divergent C-terminal motif that mediates membrane binding. The GRK1 subfamily is prenylated and the GRK2 subfamily contains a PH domain that binds acidic lipids and Gβɣ. At the C-terminus, the GRK4 subfamily is palmitoylated (except for GRK5) and contains a lipid-binding motif. In addition, the GRK4 subfamily has a PIP₂ binding site near the N-terminus. Thick cylinders represent defined structural domains. Domains were defined using previously determined structures for GRK5 (PDB ID: 4TNB), GRK6 (PDB ID: 3NYO), GRK2 (PDB ID: 6U7C) and GRK1 (PDB ID: 3C4Z) and GRK1–7 multiple sequence alignment.

Figure 3

Structural model of the GRK/GPCR complex and GPCR-mediated GRK activation. Models were adapted from docked β₂AR/GRK5 structures based on cross-linking generated by Komolov et al. [13]. (a) Inactive GRK conformation upon recruitment to the active GPCR. The GRK ionic lock establishes contact between the RH and kinase domain to maintain the closed, inactive state. The N-terminal lipid-binding domain (royal blue) [13] and the RH terminal domain (lilac) [14], regions thought to mediate GPCR targeting, are contiguous with the receptor. (b) Domain movements for kinase activation. Arrows indicate GRK domain changes when transitioning from an inactive conformation (as shown) to the active state. Model from (a) is rotated 55° counterclockwise to better visualize domain movements upon activation. The RH domain opens outward and backward from the kinase domain and the kinase N-lobe undergoes closure towards the kinase C-lobe. The entire kinase rotates clockwise relative to the GPCR. (c) GRK active conformation for GPCR phosphorylation.