Hexadecameric structure of an invertebrate gap junction channel

Abstract

Innexins are invertebrate-specific gap junction proteins with four transmembrane helices. These proteins oligomerize to constitute intercellular channels that allow for the passage of small signaling molecules associated with neural and muscular electrical activity. In contrast to the large number of structural and functional studies of connexin gap junction channels, few structural studies of recombinant innexin channels are reported. Here we show the three-dimensional structure of two-dimensionally crystallized Caenorhabditis elegans innexin-6 (INX-6) gap junction channels. The N-terminal deleted INX-6 proteins are crystallized in lipid bilayers. The three-dimensional reconstruction determined by cryo-electron crystallography reveals that a single INX-6 gap junction channel comprises 16 subunits, a hexadecamer, in contrast to chordate connexin channels, which comprise 12 subunits. The channel pore diameters at the cytoplasmic entrance and extracellular gap region are larger than those of connexin26. Two bulb densities are observed in each hemichannel, one in the pore and the other at the cytoplasmic side of the hemichannel in the channel pore pathway. These findings imply a structural diversity of gap junction channels among multicellular organisms.

Gap junction gene and protein families: Connexins, innexins, and pannexins

“…The connexin topologies were originally predicted from hydropathy plots of the cloned sequences and supported by mapping of regions using site-directed antibodies [24,56]. Determination of the crystal structures of some connexins and innexins supported the topological models [32,33,34,52]. Studies confirm that connexins, pannexins and innexins form channels with similar quaternary structures [3,12,19,32,33,52].…”

“…However, while connexin hemi-gap junction channels contain 6 subunits regardless of the connexin isoform, innexin hemi-gap junction channels and pannexin channels can have 6 or 8 subunits depending on the isoform (e.g. C. elegans inx-6 forms octamers, rat Panx1 forms hexamers and rat Panx2 most likely forms octamers) [3,33,34]. …”

Dissection of neuronal gap junction circuits that regulate social behavior in Caenorhabditis elegans

“…The structural components of electrical synapses are intercellular gap junctions consisting of two aligned hemichannels, each composed of six connexin subunits in vertebrates or eight innexin subunits in invertebrates (9)(10)(11). The apposition and opening of these hemichannels form a small protein-lined pore that allows bidirectional ionic flow and electrical coupling between two cells (12).…”

Exciting and not so exciting roles of pannexins

“…Differently from connexins but similarly to innexins, pannexins have 4 conserved cysteines, two in each extracellular loop. In terms of single membrane oligomers (pannexons), Panx1 has been shown to form hexameric structures like connexins, while Panx2 oligomerize as octamers [7], as does the innexin-6 (INX6) of C. elegans [8, 9]. For Panx3, however, the stoichiometry has not yet been determined.…”