Abstract

We examined the role of putative trafficking sequences in two GABA_A receptor subunits: α₄ and δ. These subunits assemble with a β subunit to form a subtype of GABA_A receptor involved in generating the “tonic” outward current. Both α₄ and δ subunits contain dibasic retention motifs in homologous positions. When basic residues are mutated to alanine in the α₄ subunit, surface expression of epitope-tagged δ subunits is increased. When basic residues in homologous regions of the δ subunit are mutated, however, surface expression is reduced. We focused on the mutants that had the maximal effects to increase (in α₄) or reduce (in δ) surface expression. The total expression of δ subunits is significantly decreased by the δ mutation, suggesting an effect on subunit maturation. We also examined surface expression of the β₂ subunit. Expression of the mutated α4 subunit resulted in increased surface expression of β₂ compared with wild-type α₄, indicating enhanced forward trafficking. In contrast, mutated δ resulted in decreased surface expression of β₂ compared with wild-type δ and to α₄ and β₂ in the absence of any δ. This observation suggests that the mutated δ incorporates into multimeric receptors and reduces the overall forward trafficking of receptors. These observations indicate that the roles of trafficking motifs are complex, even when located in homologous positions in related subunits. The physiologic properties of receptors containing mutated subunits were not significantly affected, indicating that the mutations in the α₄ subunit will be useful to enhance surface expression.