Innexin

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.

Innexin
Identifiers
SymbolInnexin
PfamPF00876
InterProIPR000990
TCDB1.A.25
OPM superfamily194
OPM protein5h1r

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]

Function

Innexins form gap junctions found in invertebrates. They also form non-junctional membrane channels with properties similar to those of pannexons.[12] 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.[13]

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.[14]

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
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gollark: Smart and yet vaguely evl.
gollark: I wasn't even aware that it was valentine's day until I was made aware that it was valentine's day by people talking about valentine's day.
gollark: I don't see how that works. That's just putting your brain into bizarre edge-case states, it wouldn't give you visibility into the afterlife or lack thereof (unless whatever controls access to that is very badly designed and easily tricked).

See also

References
  1. Hasegawa DK, Turnbull MW (April 2014). "Recent findings in evolution and function of insect innexins". FEBS Letters. 588 (8): 1403–10. doi:10.1016/j.febslet.2014.03.006. PMID 24631533.
  2. Phelan P, Stebbings LA, Baines RA, Bacon JP, Davies JA, Ford C (January 1998). "Drosophila Shaking-B protein forms gap junctions in paired Xenopus oocytes". Nature. 391 (6663): 181–4. Bibcode:1998Natur.391..181P. doi:10.1038/34426. PMID 9428764.
  3. Panchin Y, Kelmanson I, Matz M, Lukyanov K, Usman N, Lukyanov S (June 2000). "A ubiquitous family of putative gap junction molecules". Current Biology. 10 (13): R473-4. doi:10.1016/S0960-9822(00)00576-5. PMID 10898987.
  4. Kelmanson IV, Shagin DA, Usman N, Matz MV, Lukyanov SA, Panchin YV (December 2002). "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. doi:10.1046/j.1460-9568.2002.02423.x. PMID 12492443.
  5. 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
  6. Bao L, Samuels S, Locovei S, Macagno ER, Muller KJ, Dahl G (December 2007). "Innexins form two types of channels". FEBS Letters. 581 (29): 5703–8. doi:10.1016/j.febslet.2007.11.030. PMC 2489203. PMID 18035059.
  7. Oshima A, Matsuzawa T, Murata K, Tani K, Fujiyoshi Y (March 2016). "Hexadecameric structure of an invertebrate gap junction channel". Journal of Molecular Biology. 428 (6): 1227–1236. doi:10.1016/j.jmb.2016.02.011. PMID 26883891.
  8. Michalski K, Syrjanen JL, Henze E, Kumpf J, Furukawa H, Kawate T (February 2020). "The cryo-EM structure of a pannexin 1 reveals unique motifs for ion selection and inhibition". eLife. 9: e54670. doi:10.7554/eLife.54670. PMC 7108861. PMID 32048993.
  9. Qu R, Dong L, Zhang J, Yu X, Wang L, Zhu S (March 2020). "Cryo-EM structure of human heptameric Pannexin 1 channel". Cell Research. 30: 446–448. doi:10.1038/s41422-020-0298-5. PMC 7196123. PMID 32203128.
  10. Turnbull M, Webb B (2002). "Perspectives on polydnavirus origins and evolution". Advances in Virus Research. 58: 203–54. doi:10.1016/S0065-3527(02)58006-4. ISBN 9780120398584. PMID 12205780. Cite journal requires |journal= (help)
  11. Kroemer JA, Webb BA (2004). "Polydnavirus genes and genomes: emerging gene families and new insights into polydnavirus replication". Annual Review of Entomology. 49 (1): 431–56. doi:10.1146/annurev.ento.49.072103.120132. PMID 14651471.
  12. Bao L, Samuels S, Locovei S, Macagno ER, Muller KJ, Dahl G (December 2007). "Innexins form two types of channels". FEBS Letters. 581 (29): 5703–8. doi:10.1016/j.febslet.2007.11.030. PMC 2489203. PMID 18035059.
  13. Chen YB, Xiao W, Li M, Zhang Y, Yang Y, Hu JS, Luo KJ (May 2016). "N-TERMINALLY ELONGATED SpliInx2 AND SpliInx3 REDUCE BACULOVIRUS-TRIGGERED APOPTOSIS VIA HEMICHANNEL CLOSURE". Archives of Insect Biochemistry and Physiology. 92 (1): 24–37. doi:10.1002/arch.21328. PMID 27030553.
  14. Shestopalov VI, Panchin Y (February 2008). "Pannexins and gap junction protein diversity". Cellular and Molecular Life Sciences. 65 (3): 376–94. doi:10.1007/s00018-007-7200-1. PMID 17982731.

Further reading
This article incorporates text from the public domain Pfam and InterPro: IPR000990

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