Calcium binding and voltage gating in Cx46 hemichannels.

The opening of connexin (Cx) hemichannels in the membrane is tightly regulated by calcium (Ca²⁺) and membrane voltage. Electrophysiological and atomic force microscopy experiments indicate that Ca²⁺ stabilizes the hemichannel closed state. However, structural data show that Ca²⁺ 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 Ca²⁺ sensitivity is increased as the voltage is made more negative. Moreover, the voltage and Ca²⁺ dependence of the channel kinetics indicate that the voltage sensor movement and Ca²⁺ binding are allosterically coupled. An allosteric kinetic model in which the Ca²⁺ decreases the energy necessary to deactivate the voltage sensor reproduces the effects of Ca²⁺ and voltage in Cx46 hemichannels. In agreement with the model and suggesting a conformational change that narrows the pore, Ca²⁺ inhibits the water flux through Cx hemichannels. We conclude that Ca²⁺ and voltage act allosterically to stabilize the closed conformation of Cx46 hemichannels.

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