Fig. 4

The sodium-binding sites. a Amino-acid residues (sticks coloured by atom type) coordinating the two Na⁺ ions (blue spheres). Inset depicts the omit map for the Na⁺ ions generated by removing respective ligands from the X-ray structure followed by refinement. The 2F_o − F_c electron density map is contoured at lσ (blue), the F_o − F_c map is contoured at 3σ (green). b Ligplot⁺ analysis of the SiaT and Na⁺ ion interactions. Na⁺ ion coordination is indicated by dashed lines between the atoms involved. c The kinetics of Neu5Ac transport by SiaT sodium-binding site variants. The transport of [³H]-Neu5Ac with or without NaCl was measured in proteoliposomes reconstituted with wild-type SiaT and mutated variants (Ser342Ala, Ser343Ala, Ser345Ala, Ser346Ala and Asp182Ala) with an imposed K⁺ diffusion membrane potential. Ser342Ala and Ser343Ala correspond to the Na2 site, whereas Ser345Ala, Ser346Ala and Asp182Ala correspond to the Na3 site. All proteoliposome measurements are presented as means ± SD from five independent experiments