Figure 4: The Ca²⁺-bound and Ca²⁺-free structures are nearly identical, with the same pore diameter.

The Ca²⁺-bound (cyan) and Ca²⁺-free (orange) structures, as well as the Ca²⁺ ions (yellow) are coloured identically to Figs 2 and 3. (a) Side views of superimposed Ca²⁺-bound and Ca²⁺-free GJC structures. (b) Plots of distance from the centre of the extracellular gap versus pore radius for the Ca²⁺-bound channel (cyan line) and the Ca²⁺-free channel (orange line). The solid horizontal lines at ±28 Å indicate the locations of the Ca²⁺-binding sites within each hemichannel. (c) Overlay of the Ca²⁺-bound (cyan) and Ca²⁺-free (orange) hemichannels, viewed from the extracellular gap and shown at the level of the minimum pore diameter (∼15 Å, as measured between Nζ atoms of K41) within the hemichannel, located between K41 residues of subunits that are directly across from one another. The side chains for the K41 residues are shown as sticks. A K⁺ ion is shown at the centre of the hemichannel with its first (7 Å diameter) and second (12 Å diameter) hydration shells shown in dark and light blue, respectively. (d) A van der Waals surface representation of the Ca²⁺-free Cx26 hemichannel. The K⁺ ion and hydration spheres are shown as in c.