Figure 4: The Na2-binding site in outward-open states of BetP-G153D.
(a) The CeS state structure (protomer B) in side view with Na2 and adjacent central substrate-binding site in stick representation, coordinating Na+ (violet sphere) and arseno-choline (black, red and purple sticks), respectively. The inset shows the coordination of a Na+ ion (violet sphere) in the Na2 site. The Fo−Fc electron density map is shown in green at a level of 3.0σ. (b–e) The Na2 site in simulations of the substrate-bound CeS (b,c) and substrate-free Ce (d,e) states: (b,d) Example configurations in simulations of the CeS state (b) and the Ce state (d). Proteins and ions are presented as in (a), waters are shown as balls-and-sticks and water densities are shown as green surfaces. (c,e) Distance between Na+ ion and Na2-site oxygen atoms during three molecular dynamics simulations of the substrate-bound CeS state (c), each 200 ns long, and the substrate-free Ce state (e), each 100 ns long. (f) 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 BetP WT or the mutant N309A. Each point shows the average of at least three independent experiments. Error bars represent s.d.
