Figure 5.

His 124 and His 165 are important for NCX pH regulation. Examples of outward currents recorded from the indicated mutants in the presence of different cytoplasmic pH values. Cytoplasmic Na⁺ and H⁺ concentrations are indicated under the H124A trace. The normalized steady-state currents versus [H⁺] for mutants H124A (♢), H165A (○), H124A-K229Q-E516L (△), and H165A-K229Q-E516L (□) are shown on the right. For comparison, the proton sensitivity of WT NCX is shown before (■) and after chymotrypsin treatment (gray dashed line). The dependence of H124A-K229Q-E516L and H165A-K229Q-E516L currents to changes in proton concentration is instead compared with the pH sensitivity of K229Q-E516L (●). Current values were measured at steady state, which in exchangers H165A, H124A-K229Q-E516L and H165A-K229Q-E516L are essentially the same as peak currents because of the absence of Na⁺-dependent inactivation. Replacement of His 124 and His 165 decreased the exchanger pH sensitivity even in the absence of Na⁺ and Ca²⁺ regulation. Error bars represent SEM.