G protein-coupled receptors (GPCRs) are vital components of cell signal transduction, forming the largest protein family targeted by drugs. They orchestrate intricate downstream effects by activating G proteins and arrestins upon agonist stimulation, resulting in diverse physiological outcomes. A notable function of arrestins is modulating GPCR activities by terminating G protein signaling and promoting internalization.
Class B GPCRs play pivotal roles in severe diseases such as diabetes, obesity, and osteoporosis, making them attractive drug targets. Researchers are keen on developing biased drugs that selectively activate G protein-dependent pathways or arrestin recruitment to enhance efficacy and reduce side effects.
However, our understanding of arrestin-bound structures within the class B GPCR group remains limited, as existing knowledge is confined to class A GPCRs. This knowledge gap impedes our comprehension of arrestin-mediated modulation and hampers drug discovery efforts.
In a recent breakthrough, Wu Beili and Zhao Qiang’s research team at the Shanghai Institute of Materia Medica, Chinese Academy of Sciences, published a study in Nature. They employed cryo-electron microscopy (cryo-EM) to unveil the interaction between the class B glucagon receptor (GCGR) and b-arrestin 1 (barr1) in both glucagon-bound and ligand-free states. This marks the first time such structures have been elucidated, offering intricate details about the interaction between a class B GPCR and arrestin. Surprisingly, these structures reveal distinctive features previously unseen.
The GCGR-barr1 structures exhibit an unconventional “tail” conformation of the complex, where the receptor predominantly engages barr1 via helix VIII in its C-terminal tail. This diverges from previously studied GPCR-arrestin structures, where arrestin adopts a “core” conformation by binding to the receptor’s transmembrane core and C terminus. The tail-binding mode is defined further by the close proximity between barr1’s C-edge loops and the transmembrane helical bundle in GCGR. Additionally, a phosphoinositide derivative reinforces tail engagement by bridging barr1 and the receptor’s helix VIII.
This study suggests that the different tail and core conformations of arrestin govern distinct aspects of receptor signaling and cellular trafficking, resulting in varied cellular responses. The GCGR-barr1 structures provide unprecedented insights into the mechanism of arrestin-mediated GPCR regulation through a tail conformation.
Another intriguing distinction observed in the GCGR-barr1 structures is that the receptor maintains an inactive conformation, even in the presence of the natural agonist glucagon. The agonist either loosely attaches or is absent, binding to a shallower site compared to active GCGR structures. This peculiar feature is attributed to the tail-binding mode of arrestin, which doesn’t necessitate contact with the receptor’s core and doesn’t rely on the receptor being in an active state.
These findings open up new avenues for developing biased ligands that can selectively recognize different GCGR conformations, presenting opportunities for pathway-specific drug development. To delve deeper into the significance of tail engagement by arrestin, the scientists conducted extensive functional studies using mutagenesis and bioluminescence resonance energy transfer (BRET). By studying arrestin recruitment and endocytosis of wild-type GCGR and its mutants at the receptor-barr1 binding interface, they confirmed the critical role of the tail conformation in governing receptor trafficking.