The N-terminal domain of γ-aminobutyric acidB receptors is sufficient to specify agonist and antagonist binding

The recently identified gamma-aminobutyric acid type B receptors (GABA(B)Rs) share low sequence similarity with the metabotropic glutamate (mGlu) receptors. Like the mGlu receptors, the N-terminal extracellular domain (NTED) of GABA(B)Rs is proposed to be related to bacterial periplasmic binding proteins (PBPs). However, in contrast to the mGlu receptors, the GABA(B)Rs lack a cysteine-rich region that links the PBP-like domain to the first transmembrane domain. This cysteine-rich region is necessary for the PBP-like domain of mGlu receptors to bind glutamate. To delimit the ligand-binding domain of GABA(B)Rs, we constructed a series of chimeric GABA(B)R1/mGluR1 and truncated GABA(B)R1 receptor mutants. We provide evidence that despite the lack of a cysteine-rich region, the NTED of GABA(B)Rs contains all of the structural information that is necessary and sufficient for ligand binding. Moreover, a soluble protein corresponding to the NTED of GABA(B)Rs reproduces the binding pharmacology of wild-type receptors. This demonstrates that the ligand-binding domain of the GABA(B)Rs can correctly fold when dissociated from the transmembrane domains.