Fig. 4.

Sodium dependence of BetP for mutations introduced at the Na2 site. (A) Betaine uptake rates in nmol⋅min⁻¹⋅mg⁻¹ cell dry weight (cdw) were measured as a function of the external sodium concentration in E. coli MKH13 cells expressing WT BetP (●) or the mutants T467A (▼), S468A (■), and T467A/S468A (▲). Each point shows the average of at least three independent experiments. Error bars represent SD. (B) The maximum of Trp fluorescence at 340 nm as a function of sodium concentration was measured in proteoliposomes. Each point shows the average of at least three independent experiments. Error bars represent SD. (C) ²²Na⁺ binding by given BetP variants measured with the SPA: 1 μM [²²Na]Cl binding was measured in the presence of increasing concentrations of nonlabeled NaCl. Error bars indicate SD of triplicate determinations.