Proc Natl Acad Sci U S A 2016 Dec 06;11349:E7986-E7995. doi: 10.1073/pnas.1609378113.

Mechanism of gating by calcium in connexin hemichannels.

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Aberrant opening of nonjunctional connexin hemichannels at the plasma membrane is associated with many diseases, including ischemia and muscular dystrophy. Proper control of hemichannel opening is essential to maintain cell viability and is achieved by physiological levels of extracellular Ca 2+ , which drastically reduce hemichannel activity. Here we examined the role of conserved charged residues that form electrostatic networks near the extracellular entrance of the connexin pore, a region thought to be involved in gating rearrangements of hemichannels. Molecular dynamics simulations indicate discrete sites for Ca 2+ interaction and consequent disruption of salt bridges in the open hemichannels. Experimentally, we found that disruption of these salt bridges by mutations facilitates hemichannel closing. Two negatively charged residues in these networks are putative Ca 2+ binding sites, forming a Ca 2+ -gating ring near the extracellular entrance of the pore. Accessibility studies showed that this Ca 2+ -bound gating ring does not prevent access of ions or small molecules to positions deeper into the pore, indicating that the physical gate is below the Ca 2+ -gating ring. We conclude that intra- and intersubunit electrostatic networks at the extracellular entrance of the hemichannel pore play critical roles in hemichannel gating reactions and are tightly controlled by extracellular Ca 2+ Our findings provide a general mechanism for Ca 2+ gating among different connexin hemichannel isoforms.

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