FIGURE 3

Heterodimeric structure of the GABA_B receptor. The two 7TM receptor subunits (GABA_B1 and GABA_B2) are coupled via their intracellular C-termini. The binding domain for GABA is located in the extracellular domain of GABA_B1. The heptahelical domain of GABA_B2 contains an allosteric modulator site as well as the G-protein-coupling site. Neither of these appear to be present in the GABA_B1 subunit. The receptor is coupled indirectly to K⁺ and Ca²⁺ channels, the former of which predominates postsynaptically while the latter is mainly presynaptic in origin. Note that the ligand binding domain is believed to be similar to the leucine/isoleucine/valine-binding protein (LIV-BP), one of the bacterial periplasmic binding proteins (modified from The GABA receptors, in Encyclopedia of Neuroscience, 3rd edn. Amsterdam: Elsevier, with permission).