GadC ‘logic-gate mechanism’. (a) Possible protonation states of Glu and GABA and their occurrence at different pHs. The indicated pH is where the species above the value is the most abundant (> 95%). (b) Proposed mechanism for selection of Glu⁰ and GABA⁺: following acidic pH activation, the C-plug of GadC (green triangle with a dashed border) is displaced from the substrate transport channel of GadC. The protein (shaded from blue to dark red) will then recognise the substrates via a charge-based mechanism that will allow exclusively the entry of Glu⁰ (blue border as in A) and the exit of GABA⁺ (red border as in A).