G protein coupling and activation of the metabotropic GABAB heterodimer

Abstract

Metabotropic γ-aminobutyric acid receptor (GABABR), a class C G protein-coupled receptor (GPCR) heterodimer, plays a crucial role in the central nervous system. Cryo-electron microscopy studies revealed a drastic conformational change upon activation and a unique G protein (GP) binding mode. However, little is known about the mechanism for GP coupling and activation for class C GPCRs. Here, we use molecular metadynamics computations to predict the mechanism by which the inactive GP induces conformational changes in the GABABR transmembrane domain (TMD) to form an intermediate pre-activated state. We find that the inactive GP first interacts with TM3, which further leads to the TMD rearrangement and deeper insertion of the α5 helix that causes the Gα subunit to open, releasing GDP, and forming the experimentally observed activated structure. This mechanism provides fresh insights into the mechanistic details of class C GPCRs activation expected to be useful for designing selective agonists and antagonists.

Cholesterol in Class C GPCRs: Role, Relevance, and Localization

“…They are made up of three unique structural elements: a seven-transmembrane domain which is responsible for allosteric ligand recognition but is uniquely dimeric in the case of class C GPCRS [ 27 ]; an unusually large extracellular venus flytrap domain (VFT) which has a double-lobed structure with a crevice between them to serve as the orthosteric binding site; and a cysteine-rich domain (CRD) that links the VFT region to the 7TM region ( Figure 1 ) [ 3 ]. However, some class C GPCRs, like GABA receptors, lack the cysteine-rich domain [ 24 , 25 , 28 , 29 , 30 ]. Due to these distinct structural features and mandatory dimerization, the class C GPCRs have been the most complex of the GPCRs in terms of understanding their activation mechanism [ 31 , 32 , 33 , 34 , 35 ].…”

The Molecular Theory of Sweet Taste: Revisit, Update, and Beyond

“…An equilibrium between active folded and extended conformers could derive from the mutual efficacy of TAS1R2 and TAS1R3 in allosteric modulation of receptor activation . Alternatively, there could be a conformational transition between two conformers accompanied by receptor activation process, which coincides with the signal transmission direction from VFD2 to TMD3. − In support of this speculation, an aspartame analogue with an exterior hydrophobic crown-ether adopted virtually all extended conformations in apolar environment, and ligand reorientation to TMD has been found in other class C GPCRs . In both cases, the previously defined relatively broad range of torsion angles requirement for hydrophobic X moiety of aspartame is in accord with its relaxed topological orientation constraints in VFD2 (Figure k) .…”

“…However, although the taste active conformer of aspartame identified by crystallography studies was proved to be the folded type, another research group insisted on that the active conformer is the extended type, based on their NMR studies and the crystal structure of an aspartame analogue . Functional differentiation of distinct domains between TAS1R2 and TAS1R3 and the suggested signal transmission pathway from VFD2 to VFD3, CRD3, and TMD3 strongly argue that TAS1R2/TAS1R3 adopts an allosteric trans-activation mechanism that is unique for heterodimeric class C GPCRs. ,,− Notably, conformations of both peptidic ligand and receptor are very flexible, and bioactive conformers of aspartame could be an ensemble of both folded and extended types . An equilibrium between active folded and extended conformers could derive from the mutual efficacy of TAS1R2 and TAS1R3 in allosteric modulation of receptor activation .…”

“…However, the overall architecture of heterodimeric TAS1R2/TAS1R3 displays a certain degree of asymmetry. This configuration could suitably accommodate various sweeteners and facilitate allosteric communication via a trans-activation across two subunits. − It has been exemplified that TAS1R2s/TAS1R3s from different species share a common interaction mechanism with sweeteners, whereas the species-dependent sweet taste toward specific sweetener is determined by the variations of residues at nonconserved positions . Hence, the analogous intramolecular organization and spatial topology between glucophores and its counterparts in the receptor as well as their complementary interaction imply an inherent correlation of probable congenerous origins of sweet compounds and TAS1R2/TAS1R3.…”

Modulated protein-sterol interactions drive oxysterol-induced impaired CXCR4 signalling

“…Membrane oxysterols modulate lipid packing, influence microdomain formation (13) and can potentially alter the protein-sterol binding landscape. Molecular dynamics (MD) simulations at both allatom and coarse grained levels have been widely used to study several aspects of GPCR activation and dynamics (14)(15)(16). Using a combination of experiments and all-atom MD simulations we explore the altered protein-sterol interaction landscape to explain the influence of oxysterol on CXCL12 mediated CXCR4 signalling…”