The Open Question of How GPCRs Interact with GPCR Kinases (GRKs)

Abstract

G protein-coupled receptors (GPCRs), which regulate a vast number of eukaryotic processes, are desensitized by various mechanisms but, most importantly, by the GPCR kinases (GRKs). Ever since GRKs were first identified, investigators have sought to determine which structural features of GRKs are used to select for the agonist-bound states of GPCRs and how this binding event in turn enhances GRK catalytic activity. Despite a wealth of molecular information from high-resolution crystal structures of GRKs, the mechanisms driving activation have remained elusive, in part because the GRK N-terminus and active site tether region, previously proposed to serve as a receptor docking site and to be key to kinase domain closure, are often disordered or adopt inconsistent conformations. However, two recent studies have implicated other regions of GRKs as being involved in direct interactions with active GPCRs. Atomic resolution structures of GPCR–GRK complexes would help refine these models but are, so far, lacking. Here, we assess three distinct models for how GRKs recognize activated GPCRs, discuss limitations in the approaches used to generate them, and then experimentally test a hypothetical GPCR interaction site in GRK2 suggested by the two newest models.

GPCR kinase knockout cells reveal the impact of individual GRKs on arrestin binding and GPCR regulation

“…These results rather demonstrate that the affinities of GRK isoforms to GPCRs differ depending on the individual receptor. Thus, the tissue-specific expression levels 50 of individual GRKs in combination with their affinities to or formed complex configurations 51 with specific receptors determine GPCR regulation.…”

Differential Regulation of GPCRs—Are GRK Expression Levels the Key?

“…This binding mechanism has not been fully understood yet, but possibly features the insertion of a N-terminal α-helix into the cytoplasmic cavity of the GPCR. Although structural evidence is not necessarily conclusive ( Cato et al, 2021 ), this mode of GRK-binding is highly attractive, as G proteins and arrestins probe for active GPCR conformations in a similar fashion ( Figure 1A ). In a cellular context, GRK-binding leads to the phosphorylation of active GPCRs at their intracellular sites.…”

G protein coupled pattern recognition receptors expressed in neutrophils: Recognition, activation/modulation, signaling and receptor regulated functions

“…In addition, and in agreement with the coupling to β‐arrestin, the GRK2 inhibitors had no effect on the response induced by a biased non β‐arrestin recruiting GPR84 agonist. To add complexity to GRK2 biology, it is not clear how the kinase is activated and interacts with activated receptors 122 . This is also true for the precise roles of β‐arrestin as a part of the processes that regulate GPCR‐mediated functions in neutrophils which also remain largely unknown.…”