Analgesic α-Conotoxin Binding Site on the Human GABAB Receptor
The analgesic α-conotoxins Vc1.1, RgIA, and PeIA attenuate nociceptive transmission via activation of G protein-coupled GABAB receptors (GABABRs) to modulate N-type calcium channels in primary afferent neurons and recombinantly coexpressed human GABABR and Cav2.2 channels in human embryonic kidney 293T cells. Here, we investigate the effects of analgesic α-conotoxins following the mutation of amino acid residues in the Venus flytrap (VFT) domains of the GABABR subunits predicted through computational peptide docking and molecular dynamics simulations. Our docking calculations predicted that all three of the α-conotoxins form close contacts with VFT residues in both B1 and B2 subunits, comprising a novel GABABR ligand-binding site. The effects of baclofen and α-conotoxins on the peak Ba2+ current (IBa) amplitude were investigated on wild-type and 15 GABABR mutants individually coexpressed with human Cav2.2 channels. Mutations at the interface of the VFT domains of both GABABR subunits attenuated baclofen-sensitive IBa inhibition by the analgesic α-conotoxins. In contrast, mutations located outside the putative peptide-binding site (D380A and R98A) did not. The key GABABR residues involved in interactions with the α-conotoxins are K168 and R207 on the B2 subunit and S130, S153, R162, E200, F227, and E253 on the B1 subunit. The double mutant, S130A + S153A, abolished inhibition by both baclofen and the α-conotoxins. Depolarization-activated IBa mediated by both wild-type and all GABABR mutants were inhibited by the selective GABABR antagonist CGP 55845. This study identifies specific residues of GABABR involved in the binding of the analgesic α-conotoxins to the VFT domains of the GABABR. SIGNIFICANCE STATEMENT: This study defines the binding site of the analgesic α-conotoxins Vc1.1, RgIA, and PeIA on the human GABAB receptor to activate Gi/o proteins and inhibit Cav2.2 channels. Computational docking and molecular dynamics simulations of GABABR identified amino acids of the Venus flytrap (VFT) domains with which the α-conotoxins interact. GABABR alanine mutants attenuated baclofen-sensitive Cav2.2 inhibition by the α-conotoxins. We identify an allosteric binding site at the interface of the VFT domains of the GABABR subunits for the analgesic α-conotoxins.
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