Alignment of amino acid segments involved in formation of the loop gate. The relevant segments from the three connexins studied in the discussed papers are shown. Cx32*Cx43E1 is the form studied in Tang et al. in this issue. Cx50 is the form studied by Verselis et al. Cx26 is the form that was used in the crystal structure of Maeda et al. By convention, residues 21–42 compose TM1 and 43–70 compose E1, so the segments above correspond to the extracellular portion of TM1 and a contiguous portion of E1. Accessibility referred to below is based on thiol reactivity and/or formation of metal binding sites when the indicated residues are replaced by cysteines. Red, residues accessible/reactive from both sides of the pore when loop gate is open; green, residues not accessible/reactive when the loop gate is open but forms metal binding site and/or can be cross-linked by dibromobimane when loop gate is closed; purple, residues that show no accessibility/reactivity under any tested conditions; pink, residues accessible/reactive when loop gate is open and when closed; blue highlight, residues identified as pore lining in the Maeda et al. crystal structure; box, 3₁₀ helix identified in the Maeda et al. crystal structure.