Biology:Innexin
| Innexin | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | Innexin | ||||||||
| Pfam | PF00876 | ||||||||
| InterPro | IPR000990 | ||||||||
| TCDB | 1.A.25 | ||||||||
| OPM superfamily | 194 | ||||||||
| OPM protein | 5h1r | ||||||||
| |||||||||
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.
Innexins or related proteins are widespread among Eumetazoa, with the exception of echinoderms.[1]
Discovery
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).[2] 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).[3][4] However, increasing evidence suggests that pannexins do not form gap junctions unless overexpressed in tissue and thus, differ functionally from innexins.[5]
Structure
Innexins have four transmembrane segments (TMSs) and, like the vertebrate connexin gap junction protein, innexin subunits together form a channel (an "innexon") in the plasma membrane of the cell.[6] Two innexons in apposed plasma membranes can form a gap junction. Innexons are made from eight subunits, instead of the six subunits of connexons.[7] Structurally, innexins and connexins are very similar, consisting of 4 transmembrane domains, 2 extracellular and 1 intracellular loop, along with intracellular N- and C-terminal tails. Despite this shared topology, the protein families do not share enough sequence similarity to confidently infer common ancestry.
Pannexins are similar to innexins and are usually considered a sub-group, but they do not participate in the formation of gap junctions and the channels have seven subunits.[8][9]
Vinnexins, viral homologues of innexins, were identified in polydnaviruses that occur in obligate symbiotic associations with parasitoid wasps. It was suggested that vinnexins may function to alter gap junction proteins in infected host cells, possibly modifying cell-cell communication during encapsulation responses in parasitized insects.[10][11][12]
Function
Innexins form gap junctions found in invertebrates. They also form non-junctional membrane channels with properties similar to those of pannexons.[13] N-terminal- elongated innexins can act as a plug to manipulate hemichannel closure and provide a mechanism connecting the effect of hemichannel closure directly to apoptotic signal transduction from the intracellular to the extracellular compartment.[14]
The vertebrate homolog pannexin do not form gap junctions. They only form the hemichannel "pannexons". These hemichannels can be present in plasma, ER and Golgi membranes. They transport Ca2+, ATP, inositol triphosphate and other small molecules and can form hemichannels with greater ease than connexin subunits.[15]
Transport reaction
The transport reactions catalyzed by innexin gap junctions is:
- Small molecules (cell 1 cytoplasm) ⇌ small molecules (cell 2 cytoplasm)
Or for hemichannels:
- Small molecules (cell cytoplasm) ⇌ small molecules (out)
Examples
- Caenorhabditis elegans
- unc-7
- unc-9
- inx-3
- Drosophila melanogaster
- Inx2
- Inx3
- Inx4 (zero population growth, zpg)
- Ogre
- shaking-B
- Hirudo medicinalis
- Hm-inx1
- Hm-inx2
- Hm-inx3
- Hm-inx6
See also
References
- ↑ "Recent findings in evolution and function of insect innexins". FEBS Letters 588 (8): 1403–10. April 2014. doi:10.1016/j.febslet.2014.03.006. PMID 24631533.
- ↑ "Drosophila Shaking-B protein forms gap junctions in paired Xenopus oocytes". Nature 391 (6663): 181–4. January 1998. doi:10.1038/34426. PMID 9428764. Bibcode: 1998Natur.391..181P.
- ↑ "A ubiquitous family of putative gap junction molecules". Current Biology 10 (13): R473-4. June 2000. doi:10.1016/S0960-9822(00)00576-5. PMID 10898987.
- ↑ "Altering electrical connections in the nervous system of the pteropod mollusc Clione limacina by neuronal injections of gap junction mRNA". The European Journal of Neuroscience 16 (12): 2475–6. December 2002. doi:10.1046/j.1460-9568.2002.02423.x. PMID 12492443.
- ↑ Dahl G. & Harris A. 2009. Pannexins or Connexins? Chapter 12. In: A. Harris, D. Locke (eds.), Connexins: A Guide doi:10.1007/978-1-59745-489-6_12
- ↑ "Innexins form two types of channels". FEBS Letters 581 (29): 5703–8. December 2007. doi:10.1016/j.febslet.2007.11.030. PMID 18035059.
- ↑ "Hexadecameric structure of an invertebrate gap junction channel". Journal of Molecular Biology 428 (6): 1227–1236. March 2016. doi:10.1016/j.jmb.2016.02.011. PMID 26883891.
- ↑ "The cryo-EM structure of a pannexin 1 reveals unique motifs for ion selection and inhibition". eLife 9: e54670. February 2020. doi:10.7554/eLife.54670. PMID 32048993.
- ↑ "Cryo-EM structure of human heptameric Pannexin 1 channel". Cell Research 30 (5): 446–448. March 2020. doi:10.1038/s41422-020-0298-5. PMID 32203128.
- ↑ Perspectives on polydnavirus origins and evolution. Advances in Virus Research. 58. 2002. pp. 203–54. doi:10.1016/S0065-3527(02)58006-4. ISBN 9780120398584.
- ↑ "Polydnavirus genes and genomes: emerging gene families and new insights into polydnavirus replication". Annual Review of Entomology 49 (1): 431–56. 2004. doi:10.1146/annurev.ento.49.072103.120132. PMID 14651471.
- ↑ Marziano N.K.; Hasegawa D.K.; Phelan P.; Turnbull M.W. (October 2011). "Functional Interactions between Polydnavirus and Host Cellular Innexins". Journal of Virology 85 (19): 10222–9. doi:10.1128/jvi.00691-11. PMID 21813607.
- ↑ "Innexins form two types of channels". FEBS Letters 581 (29): 5703–8. December 2007. doi:10.1016/j.febslet.2007.11.030. PMID 18035059.
- ↑ "N-TERMINALLY ELONGATED SpliInx2 AND SpliInx3 REDUCE BACULOVIRUS-TRIGGERED APOPTOSIS VIA HEMICHANNEL CLOSURE". Archives of Insect Biochemistry and Physiology 92 (1): 24–37. May 2016. doi:10.1002/arch.21328. PMID 27030553.
- ↑ "Pannexins and gap junction protein diversity". Cellular and Molecular Life Sciences 65 (3): 376–94. February 2008. doi:10.1007/s00018-007-7200-1. PMID 17982731.
Further reading
- "Innexins: a family of invertebrate gap-junction proteins". Trends in Genetics 14 (9): 348–9. September 1998. doi:10.1016/S0168-9525(98)01547-9. PMID 9769729.
- "Drosophila Shaking-B protein forms gap junctions in paired Xenopus oocytes". Nature 391 (6663): 181–4. January 1998. doi:10.1038/34426. PMID 9428764. Bibcode: 1998Natur.391..181P.
- "Molecular characterization and embryonic expression of innexins in the leech Hirudo medicinalis". Development Genes and Evolution 216 (4): 185–97. April 2006. doi:10.1007/s00427-005-0048-1. PMID 16440200.
External links
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