K⁺ binding induced transient current signal. (A) Transient current induced by the application of extracellular K⁺ (140 mM, gray bar) to the wild-type (black trace) and D454N glutamate transporter (red trace) in a single-turnover K⁺-exchange experiment (only K⁺ inside). (B) Comparison of select mutants’ results. Error bars represent SD. (C) Conceptual illustration of the single-turnover K⁺-exchange experiment shown in A. Proposed electrogenic steps that are responsible for the transient charge movement are indicated in red.