Sandbox Reserved 1703: Difference between revisions

Cryo-EM studies of mGlu2 have yielded adequate structures that have acted as maps to aid in producing a better structural understanding of the inactive and active states of mGlu2<ref name="Lin" />. The overall structure of the mGlu2 is composed of 3 main parts: a ligand binding Venus FlyTrap Domain(VFT), followed by a Cysteine Rich Domain linker to the Transmembrane Domain that contains 7 alpha helices (7TM) on both the <scene name='90/905587/Alphaandbetachain/2'>alpha and beta chains</scene> that aid in the binding of the G-Protein. Class C CPCRs such as mGlu2, are activated by their ability to form dimers. MGlu2 is a homodimer which is imperative to the receptor’s ability to relay signals induced by glutamate from the extracellular domain(ECD) to its transmembrane domain(TMD). The homodimer of mGlu2 contains an alpha chain and a beta chain. Occupation of both ECDs with the agonist, glutamate, is necessary for a fully active mGlu2<ref name="Du">Du, Juan, et al. “Structures of Human mglu2 and mglu7 Homo- and Heterodimers.” Nature News, Nature Publishing Group, 16 June 2021, https://www.nature.com/articles/s41586-021-03641-w.></ref>. However, only one chain in the dimer is responsible for activation of the G-protein, this suggests an asymmetrical signal transduction mechanism for mGlu2<ref name="Lin" />.  

A few hallmarks of the inactive structure of mGlu2 are the Venus FlyTrap Domain in the open conformation, well separated Cysteine-Rich Domains, and distinct orientation of the 7 Transmembrane Domains (7TM). Perhaps the most critical component of the inactive form is the asymmetric TM3-TM4 interface formed by both of the 7 alpha helices in the alpha and beta chains in the transmembrane domain. The transmembrane domain is mediated mainly by helix IV on the alpha chain and helix lll on the beta chain of the dimer through hydrophobic interactions. These hydrophobic interactions between both transmembrane helices stabilize inactive conformation of mGlu2<ref name="Lin" />.

A positive allosteric modulator (PAM) or a negative allosteric modulator (NAM) can bind to mGlu2. PAM binds to the receptor, induces conformational changes, which help promote greater affinity for G protein binding. PAM binds in a binding pocket that is created by alpha helices III, V, VI, VII in the transmembrane domain. Upon binding of PAM, it interacts with helix VI, including residues W773, F776, L777, and F780. Due to spatial hindrance, helix VI is shifted downward, causing conformational changes. NAM, however, reduces the affinity for G protein binding. NAM binds to the same binding pocket as PAM and also interacts with residue W773. Due to the structure of NAM, it occupies the binding site a little deeper than PAM. This causes NAM to push on the side chain of W773 towards helix VII<ref name="Lin" />. PAM and NAM induce different conformational changes, which result in different outcomes.

  Reorientation positions helix lll on either the alpha or beta chain because both have the ability to bind to the G-protein but only one chain is required for full receptor activation. The intracellular region of helix lll mainly contributes to the interactions with the alpha subunit of the G-protein. Intracellular Loop 2 plays a key role in G-protein coupling as well by building polar interaction networks through its ionic interactions with the alpha subunit of the G-protein. Lastly, mGlu2 residue E666 forms a salt bridge with an alpha N residue (R32) on the alpha subunit which further destabilizes the inactive conformation<ref name="Lin" />.