1.A.25 The Gap Junction-forming Innexin (Innexin) Family

Innexins comprise a large family of proteins that form intercellular gap junctional channels in invertebrates, but only a few have been functionally characterized. These junctions allow electrical coupling as well as the free flow of small molecules between cells. The C. elegans INX-3, but not a paralogue, EAT-5, induced electrical coupling between Xenopus oocyte pairs (Landesman et al., 1999). Voltage and pH gating of INX-3 channels is functionally similar to that of vertebrate connexin channels (TC# 1.A.24). Many paralogues of the innexin family are found in both C. elegans and D. melanogaster as well as other invertebrates, and these proteins are subject to differential developmental control in various body tissues. Innexins exhibit a 4 TMS topology. Homologues, called pannexins, have been identified in vertebrates (Hua et al., 2003; Yen and Saier, 2007). The LRRC8 family (TC# 1.A.25.3) is a member of the Pfam pannexin-like superfamily. The structures of LRRC8 proteins have been determined, and they resemble connexins (Deneka et al. 2018).

Gap junctions are widespread in immature neuronal circuits. A transient network formed by the innexin gap-junction protein NSY-5 coordinates left-right asymmetry in the developing nervous system of C. elegans. NSY-5 forms hemichannels and intercellular gap-junction channels, consistent with a combination of cell-intrinsic and network functions (Chuang et al., 2007). In addition to making gap junctions, innexins also form non-junctional membrane channels with properties similar to those of pannexons (Bao et al., 2007).  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 signaling transduction from the intracellular to the extracellular compartment (Chen et al. 2016).

Pannexins in vertebrates have been studied in some detail (Shestopalov and Panchin, 2008; Boyce et al. 2013). They can form nonjunctional transmembrane 'hemichannels' for transport of molecules of less than 1000 Da, or intercellular gap junctions. They transport Ca2+, ATP, inositol triphosphate, and other small molecules. They can be present in plasma, ER and golgi membranes. Pannexin1 can form homooligomeric channels and heterooligomeric channels with Pannexin2. They form hemichannels with greater ease than connexin subunits (Shestopalov and Panchin, 2008). Scemes (2011) summarized the published data on hemichannel formation by junctional proteins. Silverman et al. 2008 have showed that probenecid inhibited currents mediated by pannexin 1 channels in the same concentration range as observed for inhibition of transport processes. Probenecid did not affect channels formed by connexins. Thus, probenecid allows for discrimination between channels formed by connexins and pannexins. 

The volume-reglated Anion Channel, VRAC, consists of the leucine-rich repeat-containing protein 8A with N-terminal pannexin-like domain, LRRC8A, together with other LRRC8 subunits (B, C, D and E). The first two TMSs of the 4 TMS LRRC8 proteins appear as DUF3733 in CDD (Abascal and Zardoya, 2012). The C-terminal soluble domain shows sequence similarity to the heme-binding protein Shv (9.A.63.1.1) and pollen-specific leucine-rich repeat extension-like proteins (3.A.20.1.1).  The volume-regulated anion channel, VRAC, has LRRC8A as a VRAC component. It forms heteromers with other LRRC8 membrane proteins (Voss et al. 2014). Genomic disruption of LRRC8A ablated VRAC currents. Cells with disruption of all five LRRC8 genes required LRRC8A cotransfection with other LRRC8 isoforms to reconstitute VRAC currents. The isoform combination determined the VRAC inactivation kinetics. Taurine flux and regulatory volume decrease also depended on LRRC8 proteins. Thus, VRAC defines a class of anion channels, suggests that VRAC is identical to the volume-sensitive organic osmolyte/anion channel VSOAC, and explains the heterogeneity of native VRAC currents (Voss et al. 2014).

Connexins participate in the generation of intercellular calcium waves, in which calcium-mediated signaling responses spread to contiguous cells through gap junction to transmit calcium signaling throughout the airway epithelium. Pannexins in the nasal mucosa contribute not only to ciliary beat modulation via ATP release, but also regulation of mucus blanket components via H2O efflux. The synchronized roles of pannexin and connexin may allow effective mucociliary clearance in nasal mucosa (Ohbuchi and Suzuki 2018).

Using cryo-electron microscopy and X-ray crystallography, Deneka et al. 2018 determined the structure of a homomeric channel of the obligatory subunit LRRC8A (TC# 1.A.25.3.1). This protein conducts ions and has properties in common with endogenous heteromeric channels. Its modular structure consists of a transmembrane pore domain followed by a cytoplasmic leucine-rich repeat domain. The transmembrane domain, which is structurally related to connexins, is wide towards the cytoplasm but constricted on the outside by a structural unit that acts as a selectivity filter. An excess of basic residues in the filter and throughout the pore attracts anions by electrostatic interaction (Deneka et al. 2018).

The transport reaction 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)



This family belongs to the Leucine-rich Repeat-containing Domain.

 

References:

Abascal F. and Zardoya R. (2012). LRRC8 proteins share a common ancestor with pannexins, and may form hexameric channels involved in cell-cell communication. Bioessays. 34(7):551-60.

Ambrosi, C., O. Gassmann, J.N. Pranskevich, D. Boassa, A. Smock, J. Wang, G. Dahl, C. Steinem, and G.E. Sosinsky. (2010). Pannexin1 and Pannexin2 channels show quaternary similarities to connexons and different oligomerization numbers from each other. J. Biol. Chem. 285: 24420-24431.

Bao, L., S. Samuels, S. Locovei, E.R. Macagno, K.J. Muller, and G. Dahl. (2007). Innexins form two types of channels. FEBS Lett. 581: 5703-5708.

Baranova, A., D. Ivanov, N. Petrash, A. Pestova, M. Skoblov, I. Kelmanson, D. Shagin, S. Nazarenko, E. Geraymovych, O. Litvin, A. Tiunova, T.L. Born, N. Usman, D. Staroverov, S. Lukyanov, and Y. Panchin. (2004). The mammalian pannexin family is homologous to the invertebrate innexin gap junction proteins. Genomics 83: 706-716.

Bargiotas, P., A. Krenz, S.G. Hormuzdi, D.A. Ridder, A. Herb, W. Barakat, S. Penuela, J. von Engelhardt, H. Monyer, and M. Schwaninger. (2011). Pannexins in ischemia-induced neurodegeneration. Proc. Natl. Acad. Sci. USA 108: 20772-20777.

Blenski, M. and R.H. Kehlenbach. (2019). Targeting of LRRC59 to the Endoplasmic Reticulum and the Inner Nuclear Membrane. Int J Mol Sci 20:.

Boucher, J., C. Simonneau, G. Denet, J. Clarhaut, A.C. Balandre, M. Mesnil, L. Cronier, and A. Monvoisin. (2018). Pannexin-1 in Human Lymphatic Endothelial Cells Regulates Lymphangiogenesis. Int J Mol Sci 19:.

Boyce AK., Prager RT., Wicki-Stordeur LE. and Swayne LA. (201). Pore positioning: current concepts in Pannexin channel trafficking. Channels (Austin). 8(2):110-7.

Bunse, S., M. Schmidt, S. Hoffmann, K. Engelhardt, G. Zoidl, and R. Dermietzel. (2011). Single cysteines in the extracellular and transmembrane regions modulate pannexin 1 channel function. J. Membr. Biol. 244: 21-33.

Chen, Y.B., W. Xiao, M. Li, Y. Zhang, Y. Yang, J.S. Hu, and K.J. Luo. (2016). N-TERMINALLY ELONGATED SpliInx2 AND SpliInx3 REDUCE BACULOVIRUS-TRIGGERED APOPTOSIS VIA HEMICHANNEL CLOSURE. Arch Insect Biochem Physiol 92: 24-37.

Chuang, C.F., M.K. VanHoven, R.D. Fetter, V.K. Verselis and C.I. Bargmann (2007). An Innexin-Dependent Cell Network Establishes Left-Right Neuronal Asymmetry in C. elegans. Cell 129: 787-799

Curtin, K.D., Z. Zhang and R.J. Wyman (1999). Drosophila has several genes for gap junction proteins. Gene 232: 191-201.

Deneka, D., M. Sawicka, A.K.M. Lam, C. Paulino, and R. Dutzler. (2018). Structure of a volume-regulated anion channel of the LRRC8 family. Nature. [Epub: Ahead of Print]

Firme, C.P., 3rd, R.G. Natan, N. Yazdani, E.R. Macagno, and M.W. Baker. (2012). Ectopic expression of select innexins in individual central neurons couples them to pre-existing neuronal or glial networks that express the same innexin. J. Neurosci. 32: 14265-14270.

Ganfornina, M.D., D. Sanchez, M. Herrera and M.J. Bastiani (1999). Developmental expression and molecular characterization of two gap junction channel proteins during embryogenesis in the grasshopper Schistocerca americana. Dev. Genet. 24: 137-150.

Gunasekar, S.K., L. Xie, and R. Sah. (2019). SWELL signalling in adipocytes: can fat ''feel'' fat? Adipocyte 8: 223-228.

Hua, V.B., A.B. Chang, J.H. Tchieu, P.A. Nielsen, and M.H. Saier, Jr. (2003). Sequence and phylogenetic analysis of 4 TMS junctional proteins: Connexins, innexins, claudins and occludins. J. Mem. Biol. 194: 59-76.

Huang, Y.A. and S.D. Roper. (2010). Intracellular Ca2+ and TRPM5-mediated membrane depolarization produce ATP secretion from taste receptor cells. J. Physiol. 588: 2343-2350.

Kandarian, B., J. Sethi, A. Wu, M. Baker, N. Yazdani, E. Kym, A. Sanchez, L. Edsall, T. Gaasterland, and E. Macagno. (2012). The medicinal leech genome encodes 21 innexin genes: different combinations are expressed by identified central neurons. Dev Genes Evol 222: 29-44.

Karatas, H., S.E. Erdener, Y. Gursoy-Ozdemir, S. Lule, E. Eren-Koçak, Z.D. Sen, and T. Dalkara. (2013). Spreading depression triggers headache by activating neuronal Panx1 channels. Science 339: 1092-1095.

Kasuya, G., T. Nakane, T. Yokoyama, Y. Jia, M. Inoue, K. Watanabe, R. Nakamura, T. Nishizawa, T. Kusakizako, A. Tsutsumi, H. Yanagisawa, N. Dohmae, M. Hattori, H. Ichijo, Z. Yan, M. Kikkawa, M. Shirouzu, R. Ishitani, and O. Nureki. (2018). Cryo-EM structures of the human volume-regulated anion channel LRRC8. Nat Struct Mol Biol 25: 797-804.

Kienitz, M.C., K. Bender, R. Dermietzel, L. Pott, and G. Zoidl. (2011). Pannexin 1 constitutes the large conductance cation channel of cardiac myocytes. J. Biol. Chem. 286: 290-298.

Landesman, Y., T.W. White, T.A. Starich, J.E. Shaw, D.A. Goodenough and D.L. Paul (1999). Innexin-3 forms connexin-like intercellular channels. J. Cell Sci. 112: 2391-2396.

Lee, S.C., V. Arya, X. Yang, D.A. Volpe, and L. Zhang. (2017). Evaluation of transporters in drug development: Current status and contemporary issues. Adv Drug Deliv Rev 116: 100-118.

Llobet, E., J.M. Tomás, and J.A. Bengoechea. (2008). Capsule polysaccharide is a bacterial decoy for antimicrobial peptides. Microbiology 154: 3877-3886.

Locovei, S., E. Scemes, F. Qiu, D.C. Spray, and G. Dahl. (2007). Pannexin1 is part of the pore forming unit of the P2X(7) receptor death complex. FEBS Lett. 581: 483-488.

Maes, M., M.R. McGill, T.C. da Silva, C. Abels, M. Lebofsky, J.L. Weemhoff, T. Tiburcio, I. Veloso Alves Pereira, J. Willebrords, S. Crespo Yanguas, A. Farhood, A. Beschin, J.A. Van Ginderachter, S. Penuela, H. Jaeschke, B. Cogliati, and M. Vinken. (2017). Inhibition of pannexin1 channels alleviates acetaminophen-induced hepatotoxicity. Arch Toxicol 91: 2245-2261.

Ohbuchi, T. and H. Suzuki. (2018). Synchronized roles of pannexin and connexin in nasal mucosal epithelia. Eur Arch Otorhinolaryngol. [Epub: Ahead of Print]

Ohbuchi, T., F. Takenaga, N. Hohchi, T. Wakasugi, Y. Ueta, and H. Suzuki. (2014). Possible contribution of pannexin-1 to ATP release in human upper airway epithelia. Physiol Rep 2: e00227.

Oshima, A., K. Tani, and Y. Fujiyoshi. (2016). Atomic structure of the innexin-6 gap junction channel determined by cryo-EM. Nat Commun 7: 13681.

Oshima, A., T. Matsuzawa, K. Murata, K. Tani, and Y. Fujiyoshi. (2016). Hexadecameric structure of an invertebrate gap junction channel. J. Mol. Biol. [Epub: Ahead of Print]

Oviedo, N.J., and M. Levin. (2007). Gap junctions provide new links in left-right patterning. Cell. 129: 787-799.

Penuela, S., R. Bhalla, X.Q. Gong, K.N. Cowan, S.J. Celetti, B.J. Cowan, D. Bai, Q. Shao, and D.W. Laird. (2007). Pannexin 1 and pannexin 3 are glycoproteins that exhibit many distinct characteristics from the connexin family of gap junction proteins. J. Cell Sci. 120: 3772-3783.

Scemes, E. (2012). Nature of plasmalemmal functional "hemichannels". Biochim. Biophys. Acta. 1818: 1880-1883.

Shestopalov, V.I. and Y. Panchin. (2008). Pannexins and gap junction protein diversity. Cell Mol Life Sci 65: 376-394.

Silverman, W., S. Locovei, and G. Dahl. (2008). Probenecid, a gout remedy, inhibits pannexin 1 channels. Am. J. Physiol. Cell Physiol. 295: C761-767.

Spagnol G., Sorgen PL. and Spray DC. (201). Structural order in Pannexin 1 cytoplasmic domains. Channels (Austin). 8(2):157-66.

Suadicani, S.O., R. Iglesias, J. Wang, G. Dahl, D.C. Spray, and E. Scemes. (2012). ATP signaling is deficient in cultured Pannexin1-null mouse astrocytes. Glia 60: 1106-1116.

Ullrich, F., S.M. Reincke, F.K. Voss, T. Stauber, and T.J. Jentsch. (2016). Inactivation and Anion Selectivity of Volume-Regulated VRAC Channels Depend on Carboxy-Terminal Residues of the First Extracellular Loop. J. Biol. Chem. [Epub: Ahead of Print]

Voss, F.K., F. Ullrich, J. Münch, K. Lazarow, D. Lutter, N. Mah, M.A. Andrade-Navarro, J.P. von Kries, T. Stauber, and T.J. Jentsch. (2014). Identification of LRRC8 heteromers as an essential component of the volume-regulated anion channel VRAC. Science 344: 634-638.

White, T.W. and D.L. Paul (1999). Genetic diseases and gene knockouts reveal diverse connexin functions. Annu. Rev. Physiol. 61: 283-310.

Willebrords, J., M. Maes, I.V.A. Pereira, T.C. da Silva, V.M. Govoni, V.V. Lopes, S. Crespo Yanguas, V.I. Shestopalov, M.S. Nogueira, I.A. de Castro, A. Farhood, I. Mannaerts, L. van Grunsven, J. Akakpo, M. Lebofsky, H. Jaeschke, B. Cogliati, and M. Vinken. (2018). Protective effect of genetic deletion of pannexin1 in experimental mouse models of acute and chronic liver disease. Biochim. Biophys. Acta. 1864: 819-830.

Yamada, T. and K. Strange. (2018). Intracellular and extracellular loops of LRRC8 are essential for volume-regulated anion channel function. J Gen Physiol. [Epub: Ahead of Print]

Yen, M.R. and M.H. Saier, Jr. (2007). Gap junctional proteins of animals: the innexin/pannexin superfamily. Prog. Biophys. Mol. Biol. (945-14).

Zhen, Y., V. Sørensen, C.S. Skjerpen, E.M. Haugsten, Y. Jin, S. Wälchli, S. Olsnes, and A. Wiedlocha. (2012). Nuclear import of exogenous FGF1 requires the ER-protein LRRC59 and the importins Kpnα1 and Kpnβ1. Traffic 13: 650-664.

Zhou, P., M.M. Polovitskaya, and T.J. Jentsch. (2018). LRRC8 amino-termini influence pore properties and gating of volume-regulated VRAC anion channels. J. Biol. Chem. [Epub: Ahead of Print]

Examples:

TC#NameOrganismal TypeExample
1.A.25.1.1Invertebrate innexin, (gap junction protein), INX3 Invertebrates INX3 of C. elegans
 
1.A.25.1.10

Leech innexin, Inx2 (Kandarian et al. 2012; Firme et al. 2012)

Animals

Inx2 of Hirudo verbana

 
1.A.25.1.11

Duplicated innexin of 801 aas and 8 TMSs.

Innexin of Ascaris suum

 
1.A.25.1.12

Duplicated innexin protein of 813 aas and 8 TMSs.

Duplicated innexin of Trichinella spiralis (Trichina worm)

 
1.A.25.1.13

Innexin2, Inx2 of 359 aas and 4 TMSs. N-terminally elongated domains in innexins may act to plug or manipulate hemichannel closure and provide a mechanism connecting the effect of hemichannel closure directly to apoptotic signaling transduction (Chen et al. 2016).

Inx2 of Spodoptera litura (Asian cotton leafworm)

 
1.A.25.1.2Invertebrate innexin, UNC-7 InvertebratesUNC-7 of C. elegans
 
1.A.25.1.3Invertebrate innexin, Ogre InvertebratesOgre of Drosophila melanogaster
 
1.A.25.1.4Invertebrate innexin, passover protein (shaking B locus) InvertebratesPassover protein of Drosophila melanogaster
 
1.A.25.1.5Invertebrate innexin, NSY-5 (INX-19) (Chuang et al., 2007) (establishes left-right neuronal asymmetry) (Oviedo and Levin, 2007)InvertebratesNSY-5 (INX-19) of Caenorhabditis elegans (NP_490983)
 
1.A.25.1.6

Innexin-14 (Protein Opu-14)

Worm

Inx-14 of Caenorhabditis elegans

 
1.A.25.1.7

Innexin-6 protein, Inx-6 or Opu-6, of 389 aas and 4 TMSs. A single INX-6 gap junction channel consists of 16 subunits, a hexadecamer, in contrast to chordate connexin channels, which consist of 12 subunits. The channel pore diameters at the cytoplasmic entrance and extracellular gap region are larger than those of connexin26 (Oshima et al. 2016). Nevertheless, the arrangements of the transmembrane helices and extracellular loops of the INX-6 monomer are highly similar to those of connexin-26 (Cx26). The INX-6 gap junction channel comprises hexadecameric subunits but reveals an N-terminal pore funnel consistent with Cx26. The helix-rich cytoplasmic loop and C-terminus are intercalated through an octameric hemichannel, forming a dome-like entrance that interacts with N-terminal loops in the pore (Oshima et al. 2016).

Worm

Inx-6 of Caenorhabditis elegans

 
1.A.25.1.8

Innexin Inx4 (Innexin-4) (Protein zero population growth)

Animals

Zpg of Drosophila melanogaster

 
1.A.25.1.9

Leech innexin, Inx6 (Kandarian et al. 2012; Firme et al. 2012)

Animals

Inx6 of Hirudo verbana

 
Examples:

TC#NameOrganismal TypeExample
1.A.25.2.1

Pannexin-1 (reported to form functional, single membrane, cell surface channels (Penuela et al., 2007)). Pannexin1 is part of the pore forming unit of the P2X(7) receptor death complex (Locovei et al., 2007). It can catalyze ATP release from cells (Huang and Roper, 2010) and promote ATP signalling in mice (Suadicani et al. 2012). It also promotes acetaminophen liver toxicity by allowing it to enter the cell (Maes et al. 2016).  Pannexin1 and pannexin2 channels show quaternary similarities to connexons but different oligomerization numbers (Ambrosi et al., 2010). Pannexin 1 constitutes the large conductance cation channel of cardiac myocytes (Kienitz et al., 2011). Pannexin 1 (Px1, Panx1) and pannexin 2 (Px2, Panx2) underlie channel function in neurons and contribute to ischemic brain damage (Bargiotas et al., 2011). Single cysteines in the extracellular and transmembrane regions modulate pannexin 1 channel function (Bunse et al., 2011).  Spreading depression triggers migraine headaches by activating neuronal pannexin1 (panx1) channels  (Karatas et al. 2013).  The channel in the mouse orthologue opens upon apoptosis (Spagnol et al. 2014). Transports ATP out of the cell since L-carbenoxolone (a Panx1 channel blocker) inhibits ATP release from the nasal mucosa, but flufenamic acid (a connexin channel blocker) and gadolinium (a stretch-activated channel blocker) do not (Ohbuchi et al. 2014). CALHM1 (TC#1.N.1.1.1) and PANX1 both play roles in ATP release and downstream ciliary beat frequency modulation following a mechanical stimulus in airway epithelial cells (Workman et al. 2017). Pannexin1 may play a role in the pathogenesis of liver disease (Willebrords et al. 2018). Inhibition of pannexin1 channel opening may provide a novel approach for the treatment of drug (acetaminophen-induced)-induced hepatotoxicity (Maes et al. 2017). Pannexin-1 is necessary for capillary tube formation on Matrigel and for VEGF-C-induced invasion. It is highly expressed in HDLECs and is required for in vitro lymphangiogenesis (Boucher et al. 2018).

Vertebrates

Pannexin-1 of Homo sapiens (gi39995064)

 
1.A.25.2.2

Pannexin1 and pannexin2 channels show quaternary similarities to connexons but different oligomerization numbers (Ambrosi et al., 2010). Pannexin 1 (Px1, Panx1) and pannexin 2 (Px2, Panx2) underlie channel function in neurons and contribute to ischemic brain damage (Bargiotas et al., 2011).

Vertebrates

Pannexin-2 of Homo sapiens (Q96RD6)

 
1.A.25.2.3

Pannexin-3 is reported to form functional, single membrane, cell surface channels (Penuela et al., 2007)). It functions as an ER Ca2+ channel, hemichannel, and gap junction to promote osteoblast differentiation (Ishikawa et al., 2011).

Vertebrates

Pannexin-3 of Homo sapiens (gi16418453)

 
Examples:

TC#NameOrganismal TypeExample
1.A.25.3.1

The volume-regulated Anion Channel, VRAC, or volume-sensitive outward rectifying anion channel, VSOR. It is also called the SWELL1 protein. It consists of the leucine-rich repeat-containing protein 8A, with an N-terminal pannexin-like domain, LRRC8A, together with other LRRC8 subunits (B, C, D and E). The first two TMSs of the 4 TMS LRRC8 proteins appear as DUF3733 in CDD (Abascal and Zardoya, 2012). The C-terminal soluble domain shows sequence similarity to the heme-binding protein, Shv, and pollen-specific leucine-rich repeat extension-like proteins (3.A.20.1.1).  The volume-regulated anion channel, VRAC, has LRRC8A as a VRAC component. It forms heteromers with other LRRC8 membrane proteins (Voss et al. 2014). Genomic disruption of LRRC8A ablated VRAC currents. Cells with disruption of all five LRRC8 genes required LRRC8A cotransfection with other LRRC8 isoforms to reconstitute VRAC currents. The isoform combination determined the VRAC inactivation kinetics. Taurine flux and regulatory volume decrease also depended on LRRC8 proteins. Thus, VRAC defines a class of anion channels, suggesting that VRAC is identical to the volume-sensitive organic osmolyte/anion channel VSOAC, and explains the heterogeneity of native VRAC currents (Voss et al. 2014).  Point mutations in two amino-acyl residues (Lys98 and Asp100 in LRRC8A and equivalent residues in LRRC8C and -E) upon charge reversal, alter the kinetics and voltage-dependence of inactivation (Ullrich et al. 2016). Using cryo-electron microscopy and X-ray crystallography, Deneka et al. 2018 and Kasuya et al. 2018  determined the structures of a homomeric channel of the obligatory subunit LRRC8A. This protein conducts ions and has properties in common with endogenous heteromeric channels. Its modular structure consists of a transmembrane pore domain followed by a cytoplasmic leucine-rich repeat domain. The transmembrane domain, which is structurally related to connexins, is wide towards the cytoplasm but constricted on the outside by a structural unit that acts as a selectivity filter. An excess of basic residues in the filter and throughout the pore attracts anions by electrostatic interaction (Deneka et al. 2018). The structure shows a hexameric assembly, and the transmembrane region features a topology similar to gap junction channels. The LRR region, with 15 leucine-rich repeats, forms a long, twisted arc. The channel pore is located along the central axis and constricted on the extracellular side, where highly conserved polar and charged residues at the tip of the extracellular helix contribute to the permeability to anions and other osmolytes. Two structural populations were identified, corresponding to compact and relaxed conformations. Comparing the two conformations suggests that the LRR region is flexible and mobile with rigid-body motions, which might be implicated in structural transitions on pore opening (Kasuya et al. 2018). VRAC is inhibited by Tamoxifen and Mefloquine (Lee et al. 2017). The intracellular loop connecting TMSs 2 and 3 of LRRC8A and the first extracellular loop connecting transmembrane domains 1 and 2 of LRRC8C, LRRC8D, or LRRC8E are essential for VRAC activity (Yamada and Strange 2018). The N termini of the LRRC8 subunits may line the cytoplasmic portion of the VRAC pore, possibly by folding back into the ion permeation pathway (Zhou et al. 2018).  On the adipocyte plasma membrane, the SWELL1-/LRRC8 channel complex activates in response to increases in adipocyte volume in the context of obesity. SWELL1 is required for insulin-PI3K-AKT2 signalling to regulate adipocyte growth and systemic glycaemia (Gunasekar et al. 2019).

Animals

The VRAC channel consisting of LRRC8A together with one or two of the subunits, LRRC8B, LRRC8C, LRRC8D and/or LRRC8E of Homo sapiens (Q8IWT6)

 
1.A.25.3.2

The LRRC8B homologue of 480 aas

Animals

LRRC8B of Ciona intestinalis (Transparent sea squirt) (Ascidia intestinalis)

 
1.A.25.3.3

Uncharacterized protein of 467 aas

Animals

UP of Branchiostoma floridae (Florida lancelet) (Amphioxus)

 
1.A.25.3.4

Uncharacterized ADP-binding protein of 1311 aas and 2 TMSs.  May be involved in defense responses.

Plants

UP of Oryza sativa

 
1.A.25.3.5

Volume-regulated anion channel subunit LRRC8B-likeof 666 aas and 4 TMSs.

LRRC8B of Mizuhopecten yessoensis

 
1.A.25.3.6

Uncharacterized protein of 610 aas and 4 TMSs.  It is of the Pannexin-like Superfamily.

UP of Thelohanellus kitauei

 
1.A.25.3.7

Leucine-rich repeat-containing protein 59, LRRC59, of 307 aas and 1 C-terminal TMS. It is a tail-anchored protein  that localizes to the ER and the nuclear envelope and is required for nuclear import of FGF1. It might regulate nuclear import by facilitating interaction with the nuclear import machinery and by transporting cytosolic FGF1 to, and possibly through, the nuclear pore (TC# 1.I.1) (Zhen et al. 2012). LRRC59 is post-translationally inserted into ER-derived membranes, possibly by diffusion (Blenski and Kehlenbach 2019).


LRRC59 of Homo sapiens