Molecular determinants involved in the allosteric control of agonist affinity in the GABAB receptor by the GABAB2 subunit

The gamma-aminobutyric acid type B (GABAB) receptor is an allosteric complex made of two subunits, GABAB1 (GB1) and GABAB2 (GB2). Both subunits are composed of an extracellular Venus flytrap domain (VFT) and a heptahelical domain (HD). GB1 binds GABA, and GB2 plays a major role in G-protein activation as well as in the high agonist affinity state of GB1. How agonist affinity in GB1 is regulated in the receptor remains unknown. Here, we demonstrate that GB2 VFT is a major molecular determinant involved in this control. We show that isolated versions of GB1 and GB2 VFTs in the absence of the HD and C-terminal tail can form hetero-oligomers as shown by time-resolved fluorescence resonance energy transfer (based on HTRF technology). GB2 VFT and its association with GB1 VFT controlled agonist affinity in GB1 in two ways. First, GB2 VFT exerted a direct action on GB1 VFT, as it slightly increased agonist affinity in isolated GB1 VFT. Second and most importantly, GB2 VFT prevented inhibitory interaction between the two main domains (VFT and HD) of GB1. According to this model, we propose that GB1 HD prevents the possible natural closure of GB1 VFT. In contrast, GB2 VFT facilitates this closure. Finally, such inhibitory contacts between HD and VFT in GB1 could be similar to those important to maintain the inactive state of the receptor.

💡 Research Summary

The GABAB receptor is a class‑C G‑protein‑coupled receptor composed of two distinct subunits, GB1 and GB2, each containing an extracellular Venus flytrap domain (VFT) and a seven‑transmembrane heptahelical domain (HD). GB1 binds the endogenous agonist γ‑aminobutyric acid (GABA), while GB2 is essential for G‑protein activation and for maintaining the high‑affinity state of GB1. Although it is known that GB2 somehow enhances GB1 agonist affinity, the precise molecular mechanism has remained elusive. In this study the authors dissected the receptor into isolated VFT fragments, eliminating the HD and C‑terminal tails, and examined their interactions and functional consequences using time‑resolved fluorescence resonance energy transfer (TR‑FRET) based on HTRF technology, radioligand binding assays, and mutagenesis.

First, the isolated GB1‑VFT and GB2‑VFT were shown to heterodimerize in the absence of any membrane‑spanning regions, as evidenced by a robust FRET signal. This demonstrates that the VFTs alone are sufficient for subunit association, confirming a direct extracellular interface that precedes or accompanies full‑length receptor assembly.

Second, the presence of GB2‑VFT modestly increased the agonist affinity of isolated GB1‑VFT, indicating a direct allosteric influence of GB2 on the ligand‑binding pocket of GB1. More strikingly, when the full‑length GB1 protein was examined, the authors discovered an intramolecular inhibitory interaction between GB1’s own HD and its VFT. This HD‑VFT contact prevents the VFT from adopting the fully closed conformation that is required for high‑affinity agonist binding.

Third, the binding of GB2‑VFT to GB1‑VFT disrupts the inhibitory HD‑VFT interaction. By sterically or conformationally uncoupling the two domains, GB2‑VFT permits the GB1‑VFT to close around the agonist, thereby converting GB1 into its high‑affinity state. The authors propose a model in which GB2‑VFT acts as a “molecular wedge” that releases the GB1 VFT from the restraining influence of its own HD, effectively switching the receptor from an inactive to an active conformation.

The study provides several key insights. (1) The extracellular VFTs are the primary determinants of GB1‑GB2 heterodimerization, independent of the transmembrane regions. (2) GB2 exerts a dual allosteric effect: a modest direct enhancement of GB1 ligand affinity and a dominant indirect effect through disruption of an intramolecular inhibitory clamp within GB1. (3) The HD‑VFT inhibitory interface in GB1 likely represents a conserved structural motif that stabilizes the inactive state of class‑C GPCRs, analogous to the “ionic lock” described for class‑A receptors.

These findings have important implications for drug discovery. Positive allosteric modulators (PAMs) that mimic the GB2‑VFT action—either by stabilizing the GB1‑VFT closed conformation or by preventing the HD‑VFT clamp—could selectively potentiate GABAB signaling without directly competing with GABA at the orthosteric site. Such agents may offer therapeutic advantages in the treatment of anxiety, epilepsy, chronic pain, and other neurological disorders where GABAB modulation is beneficial. Future work will need to map the precise residues involved in the HD‑VFT interface, explore the dynamics of VFT closure in full‑length receptors using cryo‑EM or single‑molecule FRET, and develop small molecules or peptide mimetics that can target this allosteric hub. In summary, the paper elucidates a previously unappreciated structural mechanism by which the GB2 VFT controls agonist affinity in GB1, advancing our understanding of GABAB receptor activation and opening new avenues for pharmacological intervention.