Mechanism of Cx26-G45E Deafness Mutant Dysregulation Explored by Molecular Dynamics Simulations

Abstract

To explore the mechanism by which Ca2+ blocks ionic conductance during tissue injury, we recently solved the X-ray crystal structures of the Cx26 gap junction channel with and without bound Ca2+. The two structures were nearly identical, ruling out a large-scale steric mechanism for channel block. Also, the pore diameter was ∼15 A, sufficient for the passage of hydrated ions. The sites for Ca2+ coordination reside at the interface between adjacent subunits, near the entrance to the extracellular gap. Ca2+ binding occurs by local conformational shifts of Ca2+-chelating residues. Molecular dynamics simulations and electrostatic calculations suggest that Ca2+ induces an electrostatic barrier to the passage of cations. We used MD simulations to explore the mechanism of channelopathy in the G45E mutant. The simulations suggest that the additional acidic side-chain at each of the channel-lining Ca2+ binding sites is unable to contribute to Ca2+ coordination. We propose that the additional negative charge contributed by the glutamate carboxylate disrupts normal Ca2+-dependent electrostatic regulation of Cx26 ion selectivity and permeability.