Innexins
| Innexin | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | Innexin | ||||||||
| Pfam | PF00876 | ||||||||
| InterPro | IPR000990 | ||||||||
| TCDB | 1.A.25 | ||||||||
| OPM superfamily | 215 | ||||||||
| OPM protein | 5h1r | ||||||||
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| Available protein structures: | |
|---|---|
| Pfam | structures |
| PDB | RCSB PDB; PDBe; PDBj |
| PDBsum | structure summary |
Innexins are transmembrane proteins that form gap junctions in invertebrates. Gap junctions are composed of membrane proteins that form a channel permeable to ions and small molecules connecting the cytoplasm of adjacent cells. Although gap junctions provide similar functions in all multicellular organisms, it was not known what proteins invertebrates used for this purpose until the late 1990s. While the connexin family of gap junction proteins was well-characterized in vertebrates, no homologues were found in non-chordates.
Gap junction proteins with no sequence homology to connexins were initially identified in fruit flies. It was suggested that these proteins are specific invertebrate gap junctions, and they were thus named "innexins" (invertebrate analog of connexins). They were later identified in diverse invertebrates. Invertebrate genomes may contain more than a dozen innexin genes. Once the human genome was sequenced, innexin homologues were identified in humans and then in other vertebrates, indicating their ubiquitous distribution in the animal kingdom. These homologues were called "pannexins" (from the Greek pan - all, throughout, and Latin nexus - connection, bond). However, increasing evidence suggests that pannexons do not form gap junctions unless overexpressed in tissue and thus, differ functionally from innexins.
Innexins have four transmembrane segments (TMSs) and, like the vertebrate connexin gap junction protein, six innexin subunits together form a channel (an "innexon") in the plasma membrane of the cell. Two innexons in apposed plasma membranes can form a gap junction. Structurally, pannexins are similar to connexins. Both types of protein consist of a cytoplasmic N-terminal domain, followed by four (TMSs) that delimit one cytoplasmic and two extracellular loops; the C- terminal domain is cytoplasmic. In addition, pannexin1 and pannexin2 channels show quaternary similarities to connexons, but different oligomerization numbers.
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