Fig. 1

Overall architecture and the sialic acid binding site of SiaT. a Side-view of SiaT in the membrane plane. Transmembrane helices that coordinate with Neu5Ac and Na⁺ ions are depicted in colour, while the remaining helices are coloured in white. Neu5Ac is shown as grey spheres coloured by atom type and Na⁺ ions are shown as blue spheres. b Neu5Ac forms hydrogen bonds with Thr58 (Tm1), Thr63 (Tm1), Ser60 (Tm1) and Gln82 (Tm2) and a salt bridge with Arg135 (Tm3). Neu5Ac also forms water-mediated hydrogen bonds with Gln82 (Tm2), Asn247 (Tm6), Gln250 (Tm6) and Phe78 (Tm2). c Omit maps for Neu5Ac 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) and −3σ (red). d The SiaT–Neu5Ac interaction network represented as a Ligplot⁺ diagram. Hydrogen bonds (dashed lines), hydrophobic contacts (arcs with spokes) and interacting water molecules (yellow) are shown