Dr. Yeager’s presentation will focus on recent progress related to the structure and function of gap junction channels. Hexameric connexin (Cx) hemichannels from adjacent cells dock end-to-end to form gap junction channels that mediate the passage of ions, second messengers, and metabolites, thereby providing intercellular signaling crucial in normal and pathological physiology. To explore the mechanism by which Ca2+ blocks ionic conductance during tissue injury, we solved X-ray crystal structures of a human gap junction channel with and without bound Ca2+. Three-dimensional crystals of recombinant Cx26 were grown using a new class of detergents designated facial amphiphiles (FAs), which have a cholate backbone with polar groups extending from one face and a short alkyl chain extending from the opposite face. FA-solubilized Cx26 crystallized into the H32 space group with two monomers in the asymmetric unit, and the crystals diffracted isotropically to 3.2 Å resolution. Crystals in the absence of Ca2+ were grown under similar conditions. A cryoEM map of Cx43 at 5.7-Å resolution [Fleishman et al., Mol. Cell 15: 879-888 (2004)] was used as a search model for molecular replacement. The Ca2+-bound and free structures were nearly identical, ruling out a large-scale steric mechanism for channel block. In both cases, the pore diameter was ~15 Å, 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, accompanied 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. A reduction of this electrostatic barrier provides an explanation for some Cx26 channelopathies that cause deafness.