Charged residues in the alpha1 and beta2 pre-M1 regions involved in GABAA receptor activation

For Cys-loop ligand-gated ion channels (LGIC), the protein movements that couple neurotransmitter binding to channel gating are not well known. The pre-M1 region, which links the extracellular agonist-binding domain to the channel-containing transmembrane domain, is in an ideal position to transduce binding site movements to gating movements. A cluster of cationic residues in this region is observed in all LGIC subunits, and in particular, an arginine residue is absolutely conserved. We mutated charged pre-M1 residues in the GABAA receptor alpha1 (K219, R220, K221) and beta2 (K213, K215, R216) subunits to cysteine and expressed the mutant subunits with wild-type beta2 or alpha1 in Xenopus oocytes. Cysteine substitution of beta2R216 abolished channel gating by GABA without altering the binding of the GABA agonist [3H]muscimol, indicating that this residue plays a key role in coupling GABA binding to gating. Tethering thiol-reactive methanethiosulfonate (MTS) reagents onto alpha1K219C, beta2K213C, and beta2K215C increased maximal GABA-activated currents, suggesting that structural perturbations of the pre-M1 regions affect channel gating. GABA altered the rates of sulfhydryl modification of alpha1K219C, beta2K213C, and beta2K215C, indicating that the pre-M1 regions move in response to channel activation. A positively charged MTS reagent modified beta2K213C and beta2K215C significantly faster than a negatively charged reagent, and GABA activation eliminated modification of beta2K215C by the negatively charged reagent. Overall, the data indicate that the pre-M1 region is part of the structural machinery coupling GABA binding to gating and that the transduction of binding site movements to channel movements is mediated, in part, by electrostatic interactions.