Fig 1.
Schematic representation of the GABAB1 (grey) and GABAB2 (blue) subunits of the GABAB receptor.
X-ray crystal structure of extracellular VFT domain in complex with the antagonist CGP46381 (PDB id 4MS1) (ribbon representation), 7TM domain (cylinder representation) with the PAM ADX71943 representing the allosteric binding pocket, and intracellular C-terminal domain with the coil-coil interaction, are shown.
Fig 2.
Overview of the ligand-guided homology modelling approach.
IFD, induced-fit docking; BEDROC, Boltzmann-enhanced discrimination of receiver operating characteristic; SIFt, structural interaction fingerprints.
Fig 3.
Structures of PAM cluster 1–5 representatives.
(A) GS39783 (Cluster 1), (B) 27 (BHF177) (Cluster 2), (C) COR627 (Cluster 3), (D) ADX71943 (Cluster 4), and (E) rac-BHFF (Cluster 5). Cluster 1 Emax range: 20%–78% (11 PAMs), cluster 2 EC50 range: 870–5000 nM (10 PAMs), cluster 3 range of increase (%) compared to 10μM [GABA]: 5.9%–19% (13 PAMs), cluster 4 EC50 range: <100–1000 nM (32 PAMs), and cluster 5 EC50 range: 4.6–4600 nM (6 PAMs). Please see S1 Table for full activity data.
Table 1.
Putative GABAB2 allosteric binding site.
SIFt(8), SIFt(7) profiles and corresponding amino acids in GABAB2 subunit and mGlu1 and mGlu5 receptors. GPCRdb(C), GPCR database numbering. Amino acids in bold have been shown to change binding/potency of allosteric ligands > 5-fold when mutated in other class C human receptors (data extracted from GPCRdb [5,6] accessed Nov. 25, 2015).
Fig 4.
Putative GABAB2 allosteric binding pocket.
(A) The 7TM domain of model c1_m2_4009 (mGlu5-based model) with putative allosteric binding site shown as orange mesh. (B) Location of the allosteric binding site identified through docking of PAMs shown in model c1_m2_4009 (mGlu5-based model). Hotspot residues shown in bold/italics (SIFt(8) hotspots) and bold/italics/underlined (SIFt(7) hotspots) (Table 1). The figure has been generated using ICM software version 3.8–0 [49].
Fig 5.
(A) Cluster 1 PAMs in model c1_m2_4OO9 (mGlu5-based model), (B) cluster 2 PAMs in model c2_m1_4OO9 (mGlu5-based model), (C) cluster 4 PAMs in model c4_m2_4OR2 (mGlu1-based model), and (D) cluster 5 PAMs in model c5_m1_4OO9 (mGlu5-based model). Intermolecular hydrogen bonds shown as dotted lines. Images generated using ICM software version 3.8–0 [49].