Ion concentration dependence of phosphorylation and ATPase activity. (A) Na⁺ dependence of phosphorylation determined as described in Fig. 4 C. Line plots represent the best fit of a Hill function (Eq. 1) with K_0.5 ± SD and number of independent experiments reported in Table 1. (B) K⁺ dependence of Na⁺,K⁺-ATPase activity determined as in Fig. 4 D. Line plots represent the best fit of a double Hill function to the data (see Materials and methods), with K_0.5 ± SD and number of independent experiments corresponding to the rising part reported in Table 1. (C) K⁺ affinity determined by K⁺ inhibition of phosphorylation. Phosphorylation was carried out as in A, with 100 mM NaCl and the indicated concentration of K⁺ added as KCl (with various concentrations of choline chloride to maintain a constant ionic strength), but without oligomycin. Line plots represent the best fit of a negative Hill function (Eq. 2). The extracted K_0.5 values ± SD and the number of independent experiments are reported in Table 1. For all panels, error bars (seen only when larger than the size of the symbols) represent SEM.