Anterograde trafficking signals in GABA_A subunits are required for functional expression

Pentameric GABA_A receptors are composed from 19 possible subunits. The GABA_A β subunit is unique because the β₁ and β₃ subunits can assemble and traffic to the cell surface as homomers, whereas most of the other subunits, including β₂, are heteromers. The intracellular domain (ICD) of the GABA_A subunits has been implicated in targeting and clustering GABA_A receptors at the plasma membrane. Here, we sought to test whether and how the ICD is involved in functional expression of the β₃ subunit. Since θ is the most homologous to β but does not form homomers, we created two reciprocal chimeric subunits, swapping the ICD between the β₃ and θ subunits, and expressed them in HEK293 cells. Surface expression was detected with immunofluorescence and functional expression was quantified using whole-cell patch-clamp recording with fast perfusion. Results indicate that, unlike β₃, neither the β₃/θ_IC nor the θ/β_3IC chimera can traffic to the plasma membrane when expressed alone; however, when expressed in combination with either wild-type α₃ or β₃, the β₃/θ_IC chimera was functionally expressed. This suggests that the ICD of α₃ and β₃ each contain essential anterograde trafficking signals that are required to overcome ER retention of assembled GABA_A homo- or heteropentamers.