The C-Terminal Domains of the GABABReceptor Subunits Mediate Intracellular Trafficking But Are Not Required for Receptor Signaling

Abstract

GABA_B receptors are G-protein-coupled receptors that mediate slow synaptic inhibition in the brain and spinal cord. These receptors are heterodimers assembled from GABA_B1 and GABA_B2 subunits, neither of which is capable of producing functional GABA_B receptors on homomeric expression. GABA_B1, although able to bind GABA, is retained within the endoplasmic reticulum (ER) when expressed alone. In contrast, GABA_B2 is able to access the cell surface when expressed alone but does not couple efficiently to the appropriate effector systems or produce any detectable GABA-binding sites. In the present study, we have constructed chimeric and truncated GABA_B1and GABA_B2 subunits to explore further GABA_Breceptor signaling and assembly. Removal of the entire C-terminal intracellular domain of GABA_B1 results in plasma membrane expression without the production of a functional GABA_Breceptor. However, coexpression of this truncated GABA_B1subunit with either GABA_B2 or a truncated GABA_B2 subunit in which the C terminal has also been removed is capable of functional signaling via G-proteins. In contrast, transferring the entire C-terminal tail of GABA_B1 to GABA_B2 leads to the ER retention of the GABA_B2subunit when expressed alone. These results indicate that the C terminal of GABA_B1 mediates the ER retention of this protein and that neither of the C-terminal tails of GABA_B1or GABA_B2 is an absolute requirement for functional coupling of heteromeric receptors. Furthermore although GABA_B1 is capable of producing GABA-binding sites, GABA_B2 is of central importance in the functional coupling of heteromeric GABA_B receptors to G-proteins and the subsequent activation of effector systems.