An innexin is a member of a family of proteins that create gap junctions in invertebrates. The innexin proteins have four transmembrane spanning units and, like the vertebrate connexin gap junction protein, six innexin subunits together form a channel, an "innexon", in the plasma membrane between the inside and outside of the cell. Two innexons in apposed plasma membranes can form a gap junction. Innexin genes have homologues in vertebrates called pannexins. However, increasing evidence suggests that pannexons do not form gap junctions unless overexpressed in tissue. Thus, pannexins and innexins differ functionality and should be treated as separate families.
These proteins have been named innexins. 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, until the late 1990s it was not known what proteins invertebrates used for this purpose. While the connexin family of gap junction proteins was well-characterised in vertebrates, no homologs were found in non-chordates. Gap junction molecules 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. Once the human genome was sequenced, innexin homologs were identified in humans and then in other vertebrates, indicating their ubiquitous distribution in the animal kingdom. They were called "pannexins" (from the Greek pan - all, throughout, and Latin nexus - connection, bond).
Genomes of vertebrates carry probably a conserved set of three pannexin paralogs (PANX1, PANX2 and PANX3). Invertebrate genomes may contain more than a dozen pannexin (innexin) genes. Vinnexins, viral homologs of pannexins/innexins, were identified in Polydnaviruses that occur in obligate symbiotic associations with parasitoid wasps. It was suggested that virally encoded vinnexin proteins may function to alter gap junction proteins in infected host cells, possibly modifying cell-cell communication during encapsulation responses in parasitized insects. Structurally pannexins are similar to connexins. Both types of protein consist of a cytoplasmic N-terminal domain, followed by four transmembrane segments that delimit two extracellular and one cytoplasmic loops; the C- terminal domain is cytoplasmic.
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