PROSITE documentation PDOC51013

Pannexin family profile

The pannexin family combines invertebrate gap junction proteins (innexins) and their vertebrate homologs. Gap junctions are composed of membrane proteins, which form a channel permeable for ions and small molecules connecting cytoplasm of adjacent cells. Although gap junctions provide similar functions in all multicellular organisms, until recently it was believed that vertebrates and invertebrates use unrelated proteins for this purpose. While the connexins family of gap junction proteins (see <PDOC00341>) is well-characterized in vertebrates, no homologs heve been found in invertebrates. In turn, gap junction molecules with no sequence homology to connexins have been identified in insects and nematodes. It has been suggested that these proteins are specific invertebrate gap junctions, and they were thus named innexins (invertebrate analog of connexins) [1,2]. As innexins homologs were recently identified in other taxonomic groups including vertebrates, indicating their ubiquitous distribution in the animal kingdom, they were called pannexins (from the Latin pan-all, throughout, and nexus-connection, bond) [3,4,5]. Genomes of vertebrates carry probably a conserved set of 3 pannexin paralogs (PANX1, PANX2 and PANX3). Invertebrates genome 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 [6,7].

Structurally pannexins are simillar 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. The schematic representation of this structure is shown below.

                 NH2-***        ***          *********-COOH
                       **     **   **      **
                       **    **     **    **   Cytoplasmic
                    ---**----**-----**----**----------------
                       **    **     **    **   Membrane
                       **    **     **    **
                    ---**----**-----**----**----------------
                       **    **     **    **   Extracellular
                        **  **       **  **
                          **           **

The profile we developed spans the entire pannexin family.