Molecular basis of calcium regulation in connexin-32 hemichannels

Abstract

In addition to forming gap-junction channels, a subset of connexins (Cxs) also form functional hemichannels. Most hemichannels are activated by depolarization, and opening depends critically on the external Ca 2+ concentration. Here we describe the mechanisms of action and the structural determinants underlying the Ca 2+ regulation of Cx32 hemichannels. At millimolar calcium concentrations, hemichannel voltage gating to the full open state of ≈90 pS is inhibited, and ion conduction at negative voltages of the partially open hemichannels (≈18 pS) is blocked. Thus, divalent cation blockage should be considered as a physiological mechanism to protect the cell from the potentially adverse effects of leaky hemichannels. A ring of 12 Asp residues within the external vestibule of the pore is responsible for the binding of Ca 2+ that accounts for both pore occlusion and blockage of gating. The residue Asp-169 of one subunit and the Asp-178 of an adjacent subunit must be arranged precisely to allow interactions with Ca 2+ to occur. Interestingly, a naturally occurring mutation (D178Y) that causes an inherited peripheral neuropathy induces a complete Ca 2+ deregulation of Cx32 hemichannel activity, suggesting that this dysfunction may be involved in the pathogenesis of the neuropathy.