Functional and structural analysis of the GABA_A receptor α₁ subunit during channel gating and alcohol modulation

The substituted cysteine accessibility method has proven useful for investigating structural changes of the γ-aminobutyric acid type A (GABA_A) receptor during channel gating and allosteric modulation. In the present study, the surface accessibility and reaction rate of propyl- and hexyl-methanethiosulfonate to cysteine residues introduced into the third transmembrane segment of the GABA_A receptor α₁ subunit were examined. GABA-induced currents in Xenopus oocytes expressing wild type and cysteine mutant GABA_A receptors were recorded before and after application of methanethiosulfonate (MTS) reagents in the resting, GABA- or alcohol-bound (ethanol or hexanol) states. Our results indicate that a water-filled cavity exists around the Ala²⁹¹ and Tyr²⁹⁴ residues of the third transmembrane segment, in agreement with previous results. Furthermore, our data indicate that a conformational change produced by alcohols (200 mM ethanol or 0.5 mm hexanol) exposure induces the water cavity around the A291C and Y294C residues to extend deeper, causing the A295C and F296C residues to become accessible to the MTS reagents. In addition, exposure of the A291C, Y294C, F296C, and V297C mutants to MTS reagents in the presence of GABA had significant effects on their GABA-induced currents, indicating that the water cavity around A291C and Y294C residues expanded to F296C and V297C by a structural movement caused by GABA binding. Our data show that GABA _A receptor is a dynamic protein during alcohol modulation and channel gating.