Figure 4. S2—S4 salt bridges/hydrogen bonds in the NMR and cryo-EM structures of the KCNQ1 VSD.

(A) Intermediate state conformation of human KCNQ1 VSD (1^st structure in the PDB 6MIE ensemble). Of particular note are the ionic interactions of E1-R2 and E2-R4, as well as, the close packing of Q3 and F0. (B) Activated state conformation of the Xenopus KCNQ1 VSD (PDB 5VMS). Note the gating charge residue pairings of E1-R4 and H5-E2. In all panels, the Cα atoms of the S4 polar residues are shown as yellow spheres and the transmembrane helices are labeled in vermillion text. S0 and S1 are not shown to improve clarity of side chain interactions.