The SF structure of NavAb N49K, S178T ^NK , S178A ^NK , and S178G ^NK mutants in high sodium conditions. (A–D) Horizontal (left side) and vertical (right side) views from the extracellular side of the ion pathway of NavAb N49K, S178T^NK, S178A^NK, and S178G^NK mutants with electron density map, respectively. Background (BG) indicates N49K mutant without the Ser178 mutation. The carbon atoms of NavAb N49K, S178T^NK, S178A^NK, and S178G^NK mutants are colored cyan, magenta, green, and yellow, respectively. The 2Fo-Fc electron density of NavAb N49K, S178T^NK, S178A^NK, and S178G^NK mutants is colored blue, dark magenta, grey, and orange, respectively. Black arrows indicate the small vicinal electron densities. Black arrowheads indicate the large vicinal electron density connected to the Met181 side chain. An asterisk indicates the large vicinal electron density isolated from the Met181 side chain. The green particles indicated the calcium ions. The residues of the center subunit are numbered with prime symbols. The residues of the left subunit are numbered with double prime symbols. In vertical view, Fo-Fc electron density map is colored green and contoured at 3σ. In C and G, the dual conformation side chain of Met181 was depicted as a white stick.