BeF_X-bound Na⁺/K⁺ pumps mimic conformational transitions of normal phosphorylated pumps. (A and B) C113Y Na⁺/K⁺ pump–mediated pre–steady-state Na⁺ charge movements (e.g., Nakao and Gadsby, 1986; Meier et al., 2010; Vedovato and Gadsby, 2010) elicited by steps from −20 mV to potentials between −180 and +60 mV, in 125 mM Na⁺_o, 0 K⁺_o before (A) and after (B) oocyte injection with 1 mM BeF_X (as in Fig. 5 A). (C) Ouabain-sensitive charge, ΔQ, on termination of each step is plotted against step voltage, and ΔQ-V is fitted with the Boltzmann relation to determine total charge moved, Q_tot, effective valence, z_q, and midpoint voltage; none was altered by BeF_X injection. The ratio of Q_tot before versus after BeF_X was 1.2 for A and B, and averaged 1.1 ± 0.1 (n = 3). The same results were obtained with C113Y-ΔYY mutants (Q_tot ratio = 1.15 ± 0.02; n = 4). In contrast, the ratio of K⁺_o-activated outward pump current after versus before BeF_X injection was ≤0.2 for C113Y pumps (e.g., outward current was reduced by 88% for the oocyte of A and B, and by ∼90% for C113Y-ΔYY pumps). (D) Mean (n = 3) relaxation rates of the transient currents elicited by the ON voltage steps, plotted against voltage, were not altered by BeF_X. The unaltered transient Na⁺ charge movements demonstrate that BeF_X-bound and inhibited Na⁺/K⁺ pumps remain capable of a subset of normal conformational changes in 125-mM Na⁺_o solutions.