An Asymmetric Contribution to γ-Aminobutyric Type A Receptor Function of a Conserved Lysine within TM2–3 of α1, β2, and γ2 Subunits

Mutations that impair the expression and/or function of γ-aminobutyric acid type A (GABAA) receptors can lead to epilepsy. The familial epilepsy γ2(K289M) mutation affects a basic residue conserved in the TM2–3 linker of most GABAA subunits. We investigated the effect on expression and function of the Lys → Met mutation in mouse α1(K278M), β2(K274M), and γ2(K289M) subunits. Compared with cells expressing wild-type and α1β2γ2(K289M) receptors, cells expressing α1(K278M)β2γ2 and α1β2(K274M)γ2 receptors exhibited reduced agonist-evoked current density and reduced GABA potency, with no change in single channel conductance. The low current density of α1β2(K274M)γ2 receptors coincided with reduced surface expression. By contrast the surface expression of α1(K278M)β2γ2 receptors was similar to wild-type and α1β2γ2(K289M) receptors suggesting that the α1(K278M) impairs function. In keeping with this interpretation GABA-activated channels mediated by α1(K278M)β2γ2 receptors had brief open times. To a lesser extent γ2(K289M) also reduced mean open time, whereas β2(K274M) had no effect. We used propofol as an alternative GABAA receptor agonist to test whether the functional deficits of mutant subunits were specific to GABA activation. Propofol was less potent as an activator of α1(K278M)β2γ2 receptors. By contrast, neither β2(K274M) nor γ2(K289M) affected the potency of propofol. The β2(K274M) construct was unique in that it reduced the efficacy of propofol activation relative to GABA. These data suggest that the α1 subunit Lys-278 residue plays a pivotal role in channel gating that is not dependent on occupancy of the GABA binding site. Moreover, the conserved TM2–3 loop lysine has an asymmetric function in different GABAA subunits.