Surface expression of GABA_B receptors requires heterodimerization of GB1 and GB2 subunits, but little is known about mechanisms that ensure efficient heterodimer assembly. We found that expression of the GB1 subunit on the cell surface is prevented through a C-terminal retention motif RXR(R); this sequence is reminiscent of the ER retention/retrieval motif RKR identified in subunits of the ATP-sensitive K⁺ channel. Interaction of GB1 and GB2 through their C-terminal coiled-coil α helices masks the retention signal in GB1, allowing the plasma membrane expression of the assembled complexes. Because individual GABA_B receptor subunits and improperly assembled receptor complexes are not functional even if expressed on the cell surface, we conclude that a trafficking checkpoint ensures efficient assembly of functional GABA_B receptors.