The C-terminal domains of the GABAB receptor subunits mediate intracellular trafficking but are not required for receptor signaling

Abstract

GABAB receptors are G-protein-coupled receptors that mediate slow synaptic inhibition in the brain and spinal cord. These receptors are heterodimers assembled from GABAB1 and GABAB2 subunits, neither of which is capable of producing functional GABAB receptors on homomeric expression. GABAB1, although able to bind GABA, is retained within the endoplasmic reticulum (ER) when expressed alone. In contrast, GABAB2 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 GABAB1 and GABAB2 subunits to explore further GABAB receptor signaling and assembly. Removal of the entire C-terminal intracellular domain of GABAB1 results in plasma membrane expression without the production of a functional GABAB receptor. However, coexpression of this truncated GABAB1 subunit with either GABAB2 or a truncated GABAB2 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 GABAB1 to GABAB2 leads to the ER retention of the GABAB2 subunit when expressed alone. These results indicate that the C terminal of GABAB1 mediates the ER retention of this protein and that neither of the C-terminal tails of GABAB1 or GABAB2 is an absolute requirement for functional coupling of heteromeric receptors. Furthermore although GABAB1 is capable of producing GABA-binding sites, GABAB2 is of central importance in the functional coupling of heteromeric GABAB receptors to G-proteins and the subsequent activation of effector systems.