Function of GB1 and GB2 subunits in G protein coupling of GABAB receptors

Many G protein-coupled receptors (GPCRs) have recently been shown to dimerize, and it was suggested that dimerization may be a prerequisite for G protein coupling. gamma -aminobutyric acid type B (GABAB) receptors (GPCRs for GABA, a major inhibitory neurotransmitter in the brain) are obligate heterodimers of homologous GB1 and GB2 subunits, neither of which is functional on its own. This feature of GABA(B) receptors allowed us to examine which of the eight intracellular segments of the heterodimeric receptor were important for G protein activation. Replacing any of the three intracellular loops of GB2 with their GB1 counterparts resulted in nonfunctional receptors. The deletion of the complete GB2 C terminus significantly attenuated the receptor function; however, the proximal 36 residues were sufficient for reconstitution of wild type-like receptor activity. In contrast, the GB1 C terminus could be deleted and GB1 intracellular loops replaced with their GB2 or mGluR1 equivalents without affecting the receptor function. In addition, a large portion of the GB1 i2 loop could be replaced with a random coil peptide without any functional consequences. Thus, GB2 intracellular segments are solely responsible for specific coupling of GABA(B) receptors to their physiologic effectors, G(i) and G protein-activated K+ channels. These findings strongly support a model in which a single GPCR monomer is sufficient for all of the specific G protein contacts.