Fig. 6.

Conformational changes of gating helices. On the intracellular side, the shorter distance induced for the Na⁺/Gal condition of TM5/TM8 indicates that sugar binding induces a gate closure that seals the ligand from the intracellular cavity. This shorter distance agrees well with the MDDS calculation based on the outward-open conformation (cyan dash) (A). TM3/5 on the extracellular side display a very broad distance distribution for the Apo and Na⁺ states. Ligand binding induces a narrowing of the distribution toward shorter distances that was not predicted by our models (B). The intermolecular distance distributions for TM10 on the extracellular side are very wide for the Apo and Na⁺ states, and a clear shift toward shorter distances is observed for the Na⁺/Gal condition (C). The same trend of Na⁺/Gal-induced occlusion is seen for TM5/TM10 (D).