G protein-coupled receptors (GPCRs) constitute the largest family of membrane proteins in the human body. By interacting with various downstream proteins, they transmit extracellular signals into cells, regulating a wide range of physiological and pathological processes. Beyond the classical G protein-dependent signaling pathway, GPCRs can also undergo phosphorylation by GPCR kinases (GRKs), recruit β-arrestins (βarrs), trigger receptor desensitization and internalization, and modulate receptor function and activity.
Metabotropic glutamate receptors (mGlus) belong to the family C GPCRs. These receptors detect the excitatory neurotransmitter L-glutamate (L-Glu) and play a crucial role in synaptic signal transmission within the central nervous system. They are key therapeutic targets for disorders such as depression, schizophrenia, Alzheimer's disease, and Parkinson's disease. Unlike other GPCR families, family C GPCRs function as homodimers or heterodimers and exhibit distinct structural characteristics when activated and coupled to downstream proteins. However, the molecular mechanism by which these receptors interact with βarrs remains unclear, limiting research into their regulatory mechanisms and the development of selective drugs.
Recently, Prof. Xue Yang and Prof. Yuequan Shen from Nankai University published a groundbreaking study in _Nature Chemical Biology_ titled **"Molecular Basis of β-Arrestin Coupling to the Metabotropic Glutamate Receptor mGlu3."** Their research used cryo-electron microscopy to resolve the structures of the family C GPCR mGlu3 bound to βarr1 in two stoichiometric ratios: 2:1 (mGlu3-βarr1-one) and 2:2 (mGlu3-βarr1-two). This marks the first structural elucidation of a family C GPCR in complex with β-arrestin 1, shedding light on how receptor inactivation is mediated through βarr1 binding and offering valuable insights for designing selective drugs targeting mGlu receptors.
Structural analysis revealed that L-Glu-bound mGlu3 adopts a resting state, with both Venus flytrap (VFT) domains in a closed conformation. The overall structure represents an intermediate "Rcc" state, where **"R"** denotes the resting state, and **"c"** indicates that the VFT domain is closed. mGlu3 interacts with βarr1 in two configurations: asymmetrically (mGlu3-βarr1-one) or symmetrically (mGlu3-βarr1-two) (Figure 1). Previous studies have shown that family C GPCRs, such as mGlu2, mGlu4, and the calcium-sensing receptor, couple to G proteins exclusively in an asymmetric manner. However, this study demonstrates that the mGlu3 homodimer can engage βarr1 and undergo receptor inactivation via both asymmetric and symmetric mechanisms, unveiling a unique mode of βarr signaling in family C GPCRs.
Figure 1 The complex structure of mGlu3-βarr1
Further structural analysis revealed that mGlu3 binds βarr1 through a pocket formed by three intracellular loops (ICL1-3), the C-terminal region, and a phosphorylated tail at the C-terminus, while the transmembrane helix bundle does not participate in binding. Using NanoBRET cell signaling assays on receptor mutants, researchers identified key phosphorylation sites and a characteristic "P-X-P-P" phosphorylation motif in the electron density map. Additionally, βarr1 was found to act as a negative allosteric modulator (NAM), stabilizing the TM3/TM4-TM3/TM4 dimeric interface in the receptor’s resting state and preventing its transition to an active conformation. Notably, in the 2:1 binding ratio, the unbound mGlu3 monomer also fails to form the TM6-TM6 interface required for receptor activation and G protein coupling, leading to a complete shutdown of G protein signaling. This suggests that βarr1 stabilizes the resting state and inhibits receptor activation by preventing G protein interaction.
This study provides key structural insights into the unique mechanism by which mGlu3 couples with βarr1 and mediates receptor inactivation (Figure 2). These findings enhance our understanding of βarr-mediated inactivation in family C GPCRs and establish a foundation for designing selective drugs targeting these receptors.
Figure 2 Model of βarr1coupling to mGlu3