Calcium binding and voltage gating in Cx46 hemichannels.

The opening of connexin (Cx) hemichannels in the membrane is tightly regulated by calcium (Ca ²⁺ ) and membrane voltage. Electrophysiological and atomic force microscopy experiments indicate that Ca ²⁺ stabilizes the hemichannel closed state. However, structural data show that Ca ²⁺ binding induces an electrostatic seal preventing ion transport without significant structural rearrangements. In agreement with the closed-state stabilization hypothesis, we found that the apparent Ca ²⁺ sensitivity is increased as the voltage is made more negative. Moreover, the voltage and Ca ²⁺ dependence of the channel kinetics indicate that the voltage sensor movement and Ca ²⁺ binding are allosterically coupled. An allosteric kinetic model in which the Ca ²⁺ decreases the energy necessary to deactivate the voltage sensor reproduces the effects of Ca ²⁺ and voltage in Cx46 hemichannels. In agreement with the model and suggesting a conformational change that narrows the pore, Ca ²⁺ inhibits the water flux through Cx hemichannels. We conclude that Ca ²⁺ and voltage act allosterically to stabilize the closed conformation of Cx46 hemichannels.