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 β(1) and β(3) subunits can assemble and traffic to the cell surface as homomers, whereas most of the other subunits, including β(2), 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 β(3) 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 β(3) 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 β(3), neither the β(3)/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 α(3) or β(3), the β(3)/θ(IC) chimera was functionally expressed. This suggests that the ICD of α(3) and β(3) each contain essential anterograde trafficking signals that are required to overcome ER retention of assembled GABA(A) homo- or heteropentamers.