Aminosulfonate Modulated pH-induced Conformational Changes in Connexin26 Hemichannels*

Gap junction channels regulate cell-cell communication by passing metabolites, ions, and signaling molecules. Gap junction channel closure in cells by acidification is well documented; however, it is unknown whether acidification affects connexins or modulating proteins or compounds that in turn act on connexins. Protonated aminosulfonates directly inhibit connexin channel activity in an isoform-specific manner as shown in previously published studies. High-resolution atomic force microscopy of force-dissected connexin26 gap junctions revealed that in HEPES buffer, the pore was closed at pH < 6.5 and opened reversibly by increasing the pH to 7.6. This pH effect was not observed in non-aminosulfonate buffers. Increasing the protonated HEPES concentration did not close the pore, indicating that a saturation of the binding sites occurs at 10 mm HEPES. Analysis of the extracellular surface topographs reveals that the pore diameter increases gradually with pH. The outer connexon diameter remains unchanged, and there is a ∼6.5° rotation in connexon lobes. These observations suggest that the underlying mechanism closing the pore is different from an observed Ca2+-induced closure.