Coupling of GABA_B receptor GABA_B2 subunit to G proteins: Evidence from Xenopus oocyte and baby hamster kidney cell expression system

Coupling of functional GABA_B receptors (GABA_BR) to G proteins was investigated with an expression system of baby hamster kidney (BHK) cells and Xenopus oocytes. Fluorescence resonance energy transfer (FRET) analysis of BHK cells coexpressing GABA_B1a receptor (GB _1aR) fused to Cerulean, a brighter variant of cyan fluorescent protein, and GABA_B2 receptor (GB₂R) fused to Venus, a brighter variant of yellow fluorescent protein, revealed that GB _1aR-Cerulean and GB₂R-Venus form a heterodimer. The GABA_BR agonists baclofen and 3-aminopropylphosphonic acid (3-APPA) elicited inward-rectifying K⁺ currents in a concentration-dependent manner in oocytes expressing GB_1aR and GB₂R, or GB _1aR-Cerulean and GB₂R-Venus, together with G protein-activated inward-rectifying K⁺ channels (GIRKs), but not in oocytes expressing GB_1aR alone or GB₂R alone together with GIRKs. Oocytes coexpressing GB_1aR + Gα_i2- fused GB₂R (GB₂R-Gα_i2) caused faster K ⁺ currents in response to baclofen. Furthermore, oocytes coexpressing GB_1aR + GB₂R fused to Gα_qi5 (a chimeric Gα_q protein that activates PLC pathways) caused PLC-mediated Ca²⁺-activated Cl^- currents in response to baclofen. In contrast, these responses to baclofen were not observed in oocytes coexpressing GB_1aR-Gα_i2 or GB_1aR- Gα_qi5 together with GB₂R. BHK cells and Xenopus oocytes coexpressing GB_1aR-Cerulean + a triplet tandem of GB ₂R-Venus-Gα_qi5 caused FRET and Ca ²⁺-activated Cl^- currents, respectively, with a similar potency in BHK cells coexpressing GB_1aR-Cerulean + GB ₂R-Venus and in oocytes coexpressing GB_1aR + GB ₂R-Gα_qi5. Our results indicate that functional GABA_BR forms a heterodimer composed of GB₁R and GB ₂R and that the signal transducing G proteins are directly coupled to GB₂R but not to GB₁R.