Innexin

Innexins (short for invertebrate nexins, originally designated the OPUS family after the founding Drosophila mutants Ogre, Passover, Unc-7, and Shaking-B) are a family of four-pass transmembrane proteins that form gap-junction channels and non-junctional hemichannels (innexons) across invertebrate phyla. Whereas deuterostome chordates utilize connexins to construct intercellular electrical synapses and metabolic gap junctions, protostome invertebrates—including nematodes, arthropods, annelids, mollusks, and flatworms—as well as basal non-bilaterians such as ctenophores and cnidarians lack connexin genes altogether and rely exclusively on innexins to couple adjacent cells electrically and metabolically.

Despite sharing zero primary amino-acid sequence identity with chordate connexins, innexins converge on an analogous four-transmembrane (4-TM) subunit topology that oligomerizes into wide aqueous pores connecting the cytoplasms of neighboring cells across a 20 to 30 Å extracellular gap. Comparative genomics and cryo-electron microscopy (cryo-EM) have demonstrated that invertebrate innexins are the direct evolutionary orthologs of vertebrate pannexins (and distantly related to volume-regulated anion channels, LRRC8), uniting the Pannexin/Innexin superfamily across Eumetazoa while highlighting one of the clearest examples of molecular convergence in membrane biophysics.

Lineage-Specific Gene Expansions Across Invertebrates

Innexin gene repertoires vary substantially across animal lineages, reflecting diverse anatomical demands for electrical coupling during embryogenesis, epithelial homeostasis, and rapid escape neurotransmission:

• Nematoda (Caenorhabditis elegans): Encodes 25 distinct innexin paralogs (inx-1 through inx-22, eat-5, unc-7, and unc-9). Individual C. elegans neurons and muscle cells co-express multiple innexins to assemble homomeric, heteromeric, and heterotypic gap junctions—most notably UNC-9 and UNC-7, which wire the command interneuron-to-motor-neuron electrical synapses governing forward and backward sinusoidal locomotion, and INX-6, which couples pharyngeal muscles for synchronized pumping.
• Arthropoda (Drosophila melanogaster): Encodes 8 innexin genes (inx1/ogre, inx2, inx3, inx4/zero population growth, inx5, inx6, inx7, and inx8/shaking-B). Alternate promoter usage and splicing of shaking-B (Passover) generate distinct neural and muscular isoforms essential for the Giant Fiber (GF) escape circuit, which couples visual lobula interneurons to the tergotrochanteral jump muscle (TTM) and dorsal longitudinal flight muscle (DLM) with sub-millisecond synaptic delay.
• Ctenophora (Mnemiopsis leidyi and Hormiphora californensis): Encodes 12 to 20 innexin paralogs despite the complete absence of innexin genes in sponges (Porifera) and placozoans. In comb jellies, innexins express broadly across the ciliated comb-plate (ctene) pacemakers, endodermal gastrovascular canals, and mesogleal muscle fibers, functioning alongside syncytial plasma-membrane fusions to coordinate antiplectic metachronal swimming waves.

Four-Transmembrane Bundle and Extracellular Two-Cysteine Loops

Each innexin monomer (typically 360 to 450 amino acids, ~42–50 kDa) threads across the lipid bilayer four times (TM1–TM4), placing both the N-terminus (NT) and C-terminus (CT) inside the cytoplasm while projecting two loops (E1 and E2) into the extracellular space and one loop (CL) into the cytosol:

1. Conserved Extracellular Disulfide Bridges (2 × Cys-Cys): A defining diagnostic signature separating the Innexin/Pannexin superfamily from chordate connexins is the cysteine count in the extracellular loops. While connexins possess three invariant cysteines per extracellular loop (forming three inter-loop disulfide bonds per subunit), innexins and vertebrate pannexins possess exactly two conserved cysteines in E1 and two in E2 (separated by a canonical Cys-X_13–17-Cys spacing in E1 and Cys-X_16–20-Cys in E2). These two intramolecular disulfide bridges rigidify the extracellular docking surfaces required for head-to-head hemichannel pairing across the intercellular cleft.
2. Proline/Tryptophan Motif in TM2: Innexins carry a highly conserved pentapeptide sequence (YYQWV or YYxWV) near the cytoplasmic boundary of TM2 that stabilizes the packing of the four-helix bundle and modulates mechanosensitive and voltage-dependent channel gating.
3. N-Terminal Funnel Helix: The first 20 residues at the cytoplasmic N-terminus fold into an amphipathic α-helix that bends back into the vestibule of the channel mouth, creating the narrowest steric and electrostatic constriction along the permeation pathway.

Octameric Innexons and 16-Subunit Intercellular Channels

For decades, biochemical cross-linking studies debated whether innexin hemichannels assembled as hexamers like connexins. High-resolution single-particle cryo-electron microscopy of Caenorhabditis elegans INX-6 (Oshima et al., 2016; PDB: 5H1Q, resolved at 3.3–3.6 Å) settled the stoichiometry definitively:

• Octameric Hemichannel (Innexon): Eight INX-6 protomers arrange symmetrically around a central eight-fold rotational axis (C₈ symmetry), creating a barrel with an outer diameter of ~115 Å and a height of ~100 Å.
• Hexadecameric Gap-Junction Channel: Two opposing octameric innexons dock head-to-head across the ~30 Å extracellular gap to form a 16-subunit intercellular channel (~205 Å total axial length, D₈ dihedral symmetry) with a molecular mass exceeding 700 kDa.
• Extracellular β-Sheet Docking Crown: Within the extracellular gap, the E1 loop contains a short α-helix and a β-hairpin, while the E2 loop forms a two-stranded antiparallel β-hairpin. Across the intercellular junction, the E2 β-hairpins of opposing octamers interdigitate via an extensive ring of hydrogen bonds and hydrophobic contacts, forming a continuous double-walled β-barrel gasket that seals the central aqueous pore against extracellular ion leakage.

Pore Constrictions, Dome Structures, and Voltage/pH Gating

Along the central permeation axis of the INX-6 hexadecamer, the internal pore widens to ~45 to 54 Å inside the transmembrane cavity before narrowing at two symmetric constriction zones:

1. N-Terminal Cytoplasmic Funnel (~18 Å Diameter): Eight amphipathic N-terminal α-helices project inward from TM1 into the cytoplasmic entrance, narrowing the aperture to ~18 Å. This constriction is markedly wider than the ~10–14 Å N-terminal funnel of hexameric connexin-26 (PDB: 2ZW3), explaining why invertebrate gap junctions readily pass larger second messengers, fluorescent tracers, and morphogenic peptides up to 3–5 kDa.
2. Extracellular E1 Bridge (~14–18 Å Diameter): Loops of E1 line the extracellular vestibule, imposing electrostatic selectivity (cation- vs. anion-preferring channels depending on charged side chains at the E1 crown).
3. Cytoplasmic Dome & C-Terminal Regulatory Core: Unlike connexins (whose C-termini are largely intrinsically disordered), the cytoplasmic loop (CL) and C-terminal domain (CT) of INX-6 fold into six α-helices per monomer that oligomerize into a structured 24-helix cytoplasmic dome encircling the intracellular vestibule. Upon intracellular acidification (low pH_i), millimeter-range cytoplasmic Ca²⁺ elevation, or transjunctional voltage gradients (V_j), concerted rearrangements between the N-terminal funnel and the cytoplasmic dome plug the pore to isolate damaged cells from healthy neighbors.

Why Vertebrate Pannexins Stopped Forming Gap Junctions

At the base of the chordate lineage (~550 million years ago), a newly evolved family of 4-TM proteins—the connexins—arose de novo and rapidly expanded to take over intercellular gap-junction communication across vertebrate tissues. Rather than going extinct, the ancestral eumetazoan innexin lineage persisted in vertebrates as the pannexin family (PANX1, PANX2, and PANX3 in mammals).

Structural comparison of C. elegans INX-6 (octamer, PDB: 5H1Q) with human PANX1 (heptamer, PDB: 6WBF) and LRRC8A/SWELL1 (hexamer, PDB: 6NZW) reveals a shared core fold across all four transmembrane helices and the extracellular two-cysteine disulfide subdomain. However, vertebrate pannexins acquired a conserved asparagine-linked (N-linked) glycosylation site on their extracellular loop (Asn254 in human PANX1, Asn86 in PANX2, Asn71 in PANX3). The bulky branched carbohydrate tree attached to the extracellular crown sterically blocks two pannexons on adjacent cells from docking head-to-head. Consequently, vertebrate pannexins were evolutionarily repurposed into non-junctional plasma-membrane ATP-release channels (activated by mechanical stretch, caspase-3 cleavage during apoptosis, or P2X7 purinergic signaling), while connexins assumed the ancestral gap-junction role still performed by innexins throughout the invertebrate world.

Are invertebrate innexins and human connexins evolutionarily related?

No. Although both innexins and connexins possess four transmembrane helices and assemble into intercellular gap-junction channels, they share no detectable amino-acid sequence homology and represent convergent evolution. The true human evolutionary counterparts (orthologs) of invertebrate innexins are the pannexins (PANX1–PANX3), which lost the ability to dock across cells due to extracellular glycosylation and instead function as single-membrane ATP-release channels.

Why can larger molecules pass through invertebrate gap junctions than through vertebrate gap junctions?

Vertebrate connexin hemichannels assemble from 6 subunits (hexamers) with a narrowest pore diameter of ~10 to 14 Å, restricting diffusion to ions and small metabolites below ~1.0 to 1.2 kDa. In contrast, cryo-EM structures of C. elegans INX-6 (PDB: 5H1Q) demonstrate that invertebrate innexin hemichannels assemble from 8 subunits (octamers), creating a wider central pore (~14 to 18 Å at the N-terminal funnel) that accommodates molecules up to 3 to 5 kDa.

How do ctenophores and arthropods use innexins for rapid neural signaling?

Electrical synapses formed by innexin hexadecamers transmit action potentials directly via ionic current flow without the 0.5-to-2.0-millisecond delay required for chemical vesicle exocytosis and neurotransmitter diffusion. In Drosophila, the shaking-B innexin couples the Giant Fiber escape pathway for sub-millisecond jump-and-flight evasion; in ctenophores, 12 to 20 innexin paralogs couple comb-row pacemaker cells and muscle fibers in lineages that lack acetylcholine, serotonin, and dopamine.

1. Phelan, P., Bacon, J. P., Davies, J. A., Stebbings, L. A., Todman, M. G., Avery, L., ... & Starich, T. A. (1998). "Innexins: a family of invertebrate gap-junction proteins." Trends in Genetics, 14(9), 348–349. https://doi.org/10.1016/S0168-9525(98)01547-901547-9)
2. Oshima, A., Tani, K., & Fujiyoshi, Y. (2016). "Atomic structure of the innexin-6 gap junction channel determined by cryo-EM." Nature Communications, 7, 13681. https://doi.org/10.1038/ncomms13681
3. Burendei, B., Shinozaki, R., Watanabe, M., Terada, T., Tani, K., Fujiyoshi, Y., & Oshima, A. (2020). "Cryo-EM structures of undocked innexin-6 hemichannels in phospholipids." Science Advances, 6(7), eaax3157. https://doi.org/10.1126/sciadv.aax3157
4. Michalski, K., Syrjanen, J. L., Henrickson, E., Infield, D. T., & Kawate, T. (2020). "The Cryo-EM structure of pannexin 1 reveals unique motifs for ion selection and inhibition." eLife, 9, e54670. https://doi.org/10.7554/eLife.54670