Gap Junction Channel Structure

Gap junction channels connect the cytoplasms of adjacent cells by the end-to-end docking of single-membrane hemichannels, each formed by a sixfold symmetric ring of connexin monomers. The connexins constitute a multigene family of polytopic membrane proteins that have four transmembrane hydrophobic domains, M1 to M4, two extracellular loops, E1 and E2, with the amino terminus and carboxyl terminus located cytoplasmically. There is a single cytoplasmic loop between M2 and M3. The different connexin isoforms can interact structurally in various ways: homomeric hemichannels are composed of a single connexin isoform; heteromeric hemichannels are composed by at least two different isoforms; homotypic junctional channels are formed by twelve identical connexin subunits; heterotypic channels are formed by two hemichannels that are each homomeric for different isoforms. The expression of multiple connexin isoforms in the same cell type, the multiplicity of isoforms, as well as their different structural combinations, likely provide exquisite functional tuning of this unique family of membrane channels. In spite of the diverse subunit compositions, the fundamental structure of the hemichannel is probably similar in unpaired hemichannels and junctional channels, and for channels formed by different connexin isoforms. The hexameric hemichannel with a central pore is clearly a conserved motif of gap junction channels that can be viewed as modular in design. Electron cryo-crystallography shows that the transmembrane region of each hemichannel is formed by a bundle of 24 α-helices that are staggered with respect to those in the apposed hemichannel. This stagger may be required for interdigitation of the structures formed by the extracellular loops across the gap, which fold at least in part as antiparallel β-strands. Sequence homology in the extracellular domains suggests common mechanisms for hemichannel interactions. The second extracellular loop, E2, guides selectivity in docking between hemichannels formed by different isoforms. However, there is considerably more sequence variability of the portion (i.e., second half) of E2, suggesting that this region dictates specificity of hemichannel docking. The cytoplasmic loop, and especially the carboxyl-terminal domain, are the most divergent regions between different connexins, and confer unique functional or regulatory properties for channels formed by different connexins. The pore itself can accommodate molecules up to ˜1 kDa, which allows the cell–cell exchange of ions, metabolites, and signaling molecules to coordinate the metabolic and electrical activities of tissues. The extracellular surface of the pore is bounded by a continuous wall of protein that forms a tight seal to prevent the loss of permeants to the extracellular space. Mutagenesis, biochemical, dye transfer, and electrophysiological data, combined with computational studies, have suggested possible assignments for the four transmembrane α-helices within each subunit. The map derived by electron cryo-crystallography shows that the transmembrane region of the pore within each hemichannel is bounded by twelve α-helices, two contributed by each connexin subunit. Most current models assign either M1 or M3 as pore-lining α-helices and M4 is agreed to be on the perimeter of the channel. Mapping of human mutations onto a suggested Cα model predicts that mutations that disrupt helix–helix packing impair channel function. In spite of this substantial progress in understanding the structural biology of gap junction channels, an experimentally determined structure at atomic resolution will be essential to confirm and clarify this working model.