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The epilepsy associated GABA_A receptor γ₂R43Q mutation increases sensitivity to Zn²⁺ inhibition

GABA_A receptors mediate rapid inhibitory signalling in the central nervous system, and mutations in various subunits of these pentameric receptors are associated with epilepsy. Receptors composed of α and β subunits are highly sensitive to inhibition by extracellular Zn²⁺ ions; however incorporation of a γ subunit disrupts two of three known Zn²⁺ binding sites and greatly reduces Zn²⁺ sensitivity. Here we demonstrate that the epilepsy associated γ₂(R43Q) mutation greatly increases the susceptibility of heterologously expressed receptors to Zn²⁺ inhibition while preserving functional characteristics underpinned by presence of the γ₂ subunit. α₁β₂γ₂ receptors are believed to contain a single N-terminal Zn²⁺ binding site. Mutation of residues contributed to this site by the α subunit ameliorated the effect of γ₂(R43Q) on Zn²⁺ sensitivity, indicating that γ₂(R43Q) allosterically affects Zn²⁺ binding or affects signal transduction, rather than directly interacting with Zn²⁺. This assertion was bolstered by the increased Zn²⁺ sensitivity of mutations predicted, by molecular modelling, to interact with γ₂(R43Q). We also examined other epilepsy-associated γ₂ mutations, γ₂(K289M) and γ₂(R139G), and found that they did not substantially increase sensitivity to Zn²⁺ inhibition. Increased Zn²⁺ sensitivity may be physiologically important in hippocampal neurones, where synaptic Zn²⁺ reaches high enough levels to modulate GABAergic signalling, and may represent a novel mechanism underlying the increased occurrence of febrile seizures reported in patients harbouring the γ₂(R43Q) mutation.