TCDB is operated by the Saier Lab Bioinformatics Group

1.A.7 ATP-gated P2X Receptor Cation Channel (P2X Receptor) Family

Members of the P2X Receptor family respond to ATP, a functional neurotransmitter released by exocytosis from many types of neurons. They have been placed into seven groups (P2X1 - P2X7) based on their pharmacological properties. These channels, which function at neuron-neuron and neuron-smooth muscle junctions, may play roles in the control of blood pressure and pain sensation. They may also function in lymphocyte and platelet physiology. They are found only in animals. Their ATP binding sites are extracellular and involve residues near ile-67. ATP binding causes the channel to go from the closed to the open state (Kracun et al., 2010). The intracellular amino terminus plays a dominant role in desensitization of P2X receptor ion channels (Allsopp and Evans, 2011). Activation and regulation of purinergic P2X receptor channels has been reviewed by Caddou et al. (2011). The gating mechanism has been proposed (Du et al., 2012). The ion access pathway to the transmembrane pore in P2X receptor channels has been estimated (Kawate et al., 2011). P2X receptor channels show threefold symmetry in ionic charge selectivity and unitary conductance (Browne et al., 2011). The binding of ATP to trimeric P2X receptors (P2XR) causes an enlargement of the receptor extracellular vestibule, leading to opening of the cation-selective transmembrane pore, and specific roles of vestibule amino acid residues in receptor activation have been evaluated (Rokic et al. 2013).

The seven different P2X receptors differ in their sensitivities to ATP and various ATP analogues as well as in their inactivation kinetics. ATP binding initially causes opening of the non-selective cation channel, allowing Ca2+ entry. Prolonged exposure of slowly inactivating forms to ATP leads to dilation of the pore, making it permeable to larger molecules (up to 1000 Da). Then it functions as a cytolytic pore that is permeable to organic cations such as ethidium and N-methyl-D-glucamine. Formation of this cytolytic pore is regulated by the C-terminal hydrophilic domain in at least one of these receptors (P2X7; Smart et al., 2003). The ion-conducting pathway is formed by three transmembrane domain 2 (TM2) alpha-helices, each being provided by the three subunits of the trimer. P2X receptors are trimeric ATP-activated ion channels permeable to Na+, K+ and Ca+2. The seven P2X receptor subtypes are implicated in physiological processes that include modulation of synaptic transmission, contraction of smooth muscle, secretion of chemical transmitters and regulation of immune responses.

The zebrafish chalice-shaped, trimeric P2X(4) receptor (TC#1.A.7.1.4) is knit together by subunit-subunit contacts implicated in ion channel gating and receptor assembly. Extracellular domains, rich in beta-strands, have large acidic patches that may attract cations, through fenestrations, to vestibules near the ion channel. In the transmembrane pore, the 'gate' is defined by an approximately 8 A slab of protein. There are three non-canonical, intersubunit ATP-binding sites. ATP binding may promote subunit rearrangement and ion channel opening (Kawate et al., 2009).

The P2X1 receptor is the dominant P2X type in smooth muscle neurons. P2X2/P2X3 heterooligomers mediate sensory signals in many other sensory neurons. P2X4 and P2X6 receptors are highly expressed in the central nervous system and probably form heterooligomers. P2X7 is expressed in cells of the immune system and of hematopoetic origin. These channels have a homo- or heterotrimeric architecture (Aschrafi et al., 2004). The carboxy termini influence the regulation of P2X1 receptors (Wen and Evans 2011).

The proteins of the P2X Receptor family are quite similar in sequence (>35% identity), but they possess 380-1000 amino acyl residues per subunit with variability in length localized primarily to the C-terminal domains. They possess two transmembrane spanners, one about 30-50 residues from their N-termini, the other near residues 320-340. The extracellular receptor domains between these two spanners (of about 270 residues) are well conserved with several conserved glycyl residues and 10 conserved cysteyl residues. The hydrophilic C-termini vary in length from 25 to 240 residues. Topologically, they resemble the epithelial Na+ channel (ENaC) proteins (TC #1.A.6) in possessing (a) N- and C-termini localized intracellularly, (b) two putative transmembrane spanners, (c) a large extracellular loop domain, and (d) many conserved extracellular cysteyl residues. P2X Receptor family members are, however, not demonstrably homologous with them. P2X Receptor channels are probably hetero- or homomultimers of several subunits and transport small monovalent cations (Me+). Some also transport Ca2+, and after prolonged exposure to ATP, various metabolites as noted above.

The three-dimensional structure of a P2X receptor is known (Burnstock and Kennedy, 2011). When ATP binds the pore opens within milliseconds, allowing the cations to flow. P2X receptors are expressed in both central and peripheral neurons where they are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed in most types of nonneuronal cells and mediate a wide range of actions, such as contraction of smooth muscle, secretion and immunomodulation. Changes in the expression of P2X receptors have been characterized in many pathological conditions of the cardiovascular, gastrointestinal, respiratory, and urinogenital systems and in the brain and special senses. The therapeutic potential of P2X receptor agonists and antagonists is currently being investigated in a range of disorders, including chronic neuropathic and inflammatory pain, depression, cystic fibrosis, dry eye, irritable bowel syndrome, interstitial cystitis, dysfunctional urinary bladder, and cancer.

Hattori and Gouaux (2012) reported the crystal structure of the zebrafish P2X(4) receptor in complex with ATP and a new structure of the apo receptor. The agonist-bound structure reveals an ATP-binding motif and an open ion channel. ATP binding induces cleft closure of the nucleotide-binding pocket, flexing of the lower body β-sheet and a radial expansion of the extracellular vestibule. The structural widening of the extracellular vestibule is directly coupled to the opening of the ion channel pore by way of an iris-like expansion of the transmembrane helices.

Phosphoinositides modulate the functions of most P2X receptor channels in neurons and glia. A dual polybasic motif has been shown to determine phosphoinositide binding and regulation in members of the P2X channel family (Bernier et al., 2012). 

Cys loop, glutamate, and P2X receptors are ligand-gated ion channels (LGICs) with 5, 4, and 3 protomers, respectively. Agonists and competitive antagonists apparently induce opposite motions of the binding pocket (Du et al., 2012b). Agonists, usually small, induce closure of the binding pocket, leading to opening of the channel pore, whereas antagonists, usually large, induce opening of the binding pocket, thereby stabilizing the closed pore.

The strong expression of ATP-gated P2X3 receptors by a subpopulation of sensory neurons indicates the important role of these membrane proteins in nociceptive signaling in health and disease, especially when the latter is accompanied by chronic pain syndromes. These receptors exist mainly as trimeric homomers, and, in part, as heteromers (assembly of two P2X3 subunits with one P2X2). Recent investigations have suggested distinct molecular determinants responsible for agonist binding and channel opening for transmembrane flux of sodium, calcium and potassium ions. Trimeric P2X3 receptors are rapidly activated by ATP and can be strongly desensitized in the continuous presence of the agonist. Endogenous substances, widely thought to be involved in triggering pain, especially in pathological conditions, can potently modulate the expression and function of P2X3 receptors, with differential changes in response amplitude, desensitization and recovery. Strong facilitation of P2X3 receptor function is induced by enodogenous substances like the neuropeptide calcitonin gene-related peptide and the neurotrophins nerve growth factor and brain-derived neurotrophic factor. These substances possess distinct mechanisms of action on P2X3 receptors, generally attributable to discrete phosphorylation of N- or C-terminal P2X3 domains (Fabbretti and Nistri, 2012).

Two structural classes of pore-opening mechanisms have been established: bending of pore-lining helices in the case of tetrameric cation channels, and tilting of such helices in mechanosensitive channels. Expansion of the gate region in the external pore in P2X receptors is accompanied by a narrowing of the inner pore, indicating that pore-forming helices straighten on ATP binding to open the channel (Li et al., 2010). This pore-opening mechanism has implications for the role of subunit interfaces in the gating mechanism and points to a role of the internal pore in ion permeation.  Amino terminal residues are involved in lipid, cholesterol and lipid raft regulation of P2X1 (Allsopp et al. 2010).

The generalized transport reaction is:

Me+ (out) Me+ (in).

This family belongs to the: ENaC/P2X Superfamily.

References associated with 1.A.7 family:

Alexander, S.P.H. and J.A. Peters. (1997). Receptor and ion channel nomenclature supplement. Trends Pharmacol. Sci. 18: 65-68.
Allsopp, R.C. and R.J. Evans. (2011). The Intracellular Amino Terminus Plays a Dominant Role in Desensitization of ATP-gated P2X Receptor Ion Channels. J. Biol. Chem. 286: 44691-44701. 22027824
Allsopp, R.C., S. El Ajouz, R. Schmid, and R.J. Evans. (2011). Cysteine scanning mutagenesis (residues Glu52-Gly96) of the human P2X1 receptor for ATP: mapping agonist binding and channel gating. J. Biol. Chem. 286: 29207-29217. 21690089
Allsopp, R.C., U. Lalo, and R.J. Evans. (2010). Lipid raft association and cholesterol sensitivity of P2X1-4 receptors for ATP: chimeras and point mutants identify intracellular amino-terminal residues involved in lipid regulation of P2X1 receptors. J. Biol. Chem. 285: 32770-32777. 20699225
Aschrafi, A., S. Sadtler, C. Niculescu, J. Rettinger, and G. Schmalzing. (2004). Trimeric architecture of homomeric P2X2 and heteromeric P2X1+2 receptor subtypes. J. Mol. Biol. 342: 333-343. 15313628
Bernier, L.P., D. Blais, E. Boué-Grabot, and P. Séguéla. (2012). A Dual Polybasic Motif Determines Phosphoinositide Binding and Regulation in the P2X Channel Family. PLoS One 7: e40595. 22792379
Browne, L.E., L. Cao, H.E. Broomhead, L. Bragg, W.J. Wilkinson, and R.A. North. (2011). P2X receptor channels show threefold symmetry in ionic charge selectivity and unitary conductance. Nat Neurosci 14: 17-18. 21170052
Browne, L.E., V. Compan, L. Bragg, and R.A. North. (2013). P2X7 receptor channels allow direct permeation of nanometer-sized dyes. J. Neurosci. 33: 3557-3566. 23426683
Burnstock, G. and C. Kennedy. (2011). P2X receptors in health and disease. Adv Pharmacol 61: 333-372. 21586364
Coddou, C., Z. Yan, T. Obsil, J.P. Huidobro-Toro, and S.S. Stojilkovic. (2011). Activation and regulation of purinergic P2X receptor channels. Pharmacol Rev 63: 641-683. 21737531
de Souza CA., Teixeira PC., Faria RX., Krylova O., Pohl P. and Alves LA. (2012). A consensus segment in the M2 domain of the hP2X(7) receptor shows ion channel activity in planar lipid bilayers and in biological membranes. Biochim Biophys Acta. 1818(1):64-71. 21958668
Du, J., H. Dong, and H.X. Zhou. (2012). Gating mechanism of a P2X4 receptor developed from normal mode analysis and molecular dynamics simulations. Proc. Natl. Acad. Sci. USA 109: 4140-4145. 22378652
Du, J., H. Dong, and H.X. Zhou. (2012). Size matters in activation/inhibition of ligand-gated ion channels. Trends Pharmacol Sci. [Epub: Ahead of Print] 22789930
Fabbretti, E. and A. Nistri. (2012). Regulation of P2X3 Receptor Structure and Function. CNS Neurol Disord Drug Targets. [Epub: Ahead of Print] 22963434
Fountain, S.J., K. Parkinson, M.T. Young, L. Cao, C.R. Thompson, and R.A. North. (2007). An intracellular P2X receptor required for osmoregulation in Dictyostelium discoideum. Nature. 448: 200-203. 17625565
Fountain, S.J., L. Cao, M.T. Young, and R.A. North. (2008). Permeation Properties of a P2X Receptor in the Green Algae Ostreococcus tauri. J. Biol. Chem. 283: 15122-15126. 18381285
Hattori, M. and E. Gouaux. (2012). Molecular mechanism of ATP binding and ion channel activation in P2X receptors. Nature. [Epub: Ahead of Print] 22535247
He, Y.Q., J. Chen, X.J. Lu, and Y.H. Shi. (2013). Characterization of P2X7R and its function in the macrophages of ayu, Plecoglossus altivelis. PLoS One 8: e57505. 23437395
Jiang, L.H., F. Rassendren, A. Surprenant, and R.A. North. (2000). Identification of amino acid residues contributing to the ATP-binding site of a purinergic P2X receptor. J. Biol. Chem. 275: 34190-34196. 10940304
Jindrichova, M., K. Khafizov, A. Skorinkin, D. Fayuk, G. Bart, H. Zemkova, and R. Giniatullin. (2011). Highly conserved tyrosine 37 stabilizes desensitized states and restricts calcium permeability of ATP-gated P2X3 receptor. J Neurochem 119: 676-685. 21883226
Kawate, T., J.C. Michel, W.T. Birdsong, and E. Gouaux. (2009). Crystal structure of the ATP-gated P2X(4) ion channel in the closed state. Nature 460: 592-598. 19641588
Kawate, T., J.L. Robertson, M. Li, S.D. Silberberg, and K.J. Swartz. (2011). Ion access pathway to the transmembrane pore in P2X receptor channels. J Gen Physiol 137: 579-590. 21624948
Kracun S., Chaptal V., Abramson J. and Khakh BS. (2010). Gated access to the pore of a P2X receptor: structural implications for closed-open transitions. J Biol Chem. 285(13):10110-21. 20093367
Li, M., T. Kawate, S.D. Silberberg, and K.J. Swartz. (2010). Pore-opening mechanism in trimeric P2X receptor channels. Nat Commun 1: 44. 20975702
Li, Q., X. Luo, and S. Muallem. (2005). Regulation of the P2X7 receptor permeability to large molecules by extracellular Cl- and Na+. J. Biol. Chem. 280: 26922-26927. 15923180
Liang, X., H. Xu, C. Li, S. Yin, T. Xu, J. Liu, and Z. Li. (2013). Functional Identification of Close Proximity Amino Acid Side Chains within the Transmembrane-Spanning Helixes of the P2X2 Receptor. PLoS One 8: e70629. 23936459
Lu, H., D. Burns, P. Garnier, G. Wei, K. Zhu, and W. Ying. (2007). P2X7 receptors mediate NADH transport across the plasma membranes of astrocytes. Biochem. Biophys. Res. Commun. 362: 946-950. 17803959
Ludlow, M.J., L. Durai, and S.J. Ennion. (2009). Functional characterization of intracellular Dictyostelium discoideum P2X receptors. J. Biol. Chem. 284: 35227-35239. 19833731
Marques-da-Silva, C., M.M. Chaves, J.C. Rodrigues, S. Corte-Real, R. Coutinho-Silva, and P.M. Persechini. (2011). Differential modulation of ATP-induced P2X7-associated permeabilities to cations and anions of macrophages by infection with Leishmania amazonensis. PLoS One 6: e25356. 21966508
McCleskey E.W. and M.S. Gold. (1999). Ion channels of nociception. Annu. Rev. Physiol. 61: 835-856. 10099712
North, R.A. (2002). Molecular physiology of P2X receptors. Physiol. Rev. 82: 1013-1067. 12270951
North, R.A. (1996). Families of ion channels with two hydrophobic segments. Curr. Opin. Cell Biol. 8: 474-483. 8791456
Roger, S., P. Pelegrin, and A. Surprenant. (2008). Facilitation of P2X7 receptor currents and membrane blebbing via constitutive and dynamic calmodulin binding. J. Neurosci. 28: 6393-6401. 18562610
Rokic, M.B., S.S. Stojilkovic, V. Vavra, P. Kuzyk, V. Tvrdonova, and H. Zemkova. (2013). Multiple Roles of the Extracellular Vestibule Amino Acid Residues in the Function of the Rat P2X4 Receptor. PLoS One 8: e59411. 23555667
Rokic, M.B., V. Tvrdoňová, V. Vávra, M. Jindřichová, T. Obšil, S.S. Stojilkovic, and H. Zemková. (2010). Roles of conserved ectodomain cysteines of the rat P2X4 purinoreceptor in agonist binding and channel gating. Physiol Res 59: 927-935. 20406028
Samways DS., Khakh BS. and Egan TM. (2012). Allosteric Modulation of Ca2+ flux in Ligand-gated Cation Channel (P2X4) by Actions on Lateral Portals. J Biol Chem. 287(10):7594-602. 22219189
Samways, D.S., B.S. Khakh, S. Dutertre, and T.M. Egan. (2011). Preferential use of unobstructed lateral portals as the access route to the pore of human ATP-gated ion channels (P2X receptors). Proc. Natl. Acad. Sci. USA 108: 13800-13805. 21808018
Smart, M.L., B. Gu, R.G. Panchal, J. Wiley, B. Cromer, D.A. Williams, and S. Petrou. (2003). P2X7 receptor cell surface expression and cytolytic pore formation are regulated by a distal C-terminal region. J. Biol. Chem. 278: 8853-8860. 12496266
Soto, F., M. Garcia-Guzman, and W. Stühmer. (1997). Cloned ligand-gated channels activated by extracellular ATP (P2X receptors). J. Membr. Biol. 160: 91-100. 9354701
Stojilkovic, S.S., Z. Yan, T. Obsil, and H. Zemkova. (2010). Structural insights into the function of P2X4: an ATP-gated cation channel of neuroendocrine cells. Cell Mol Neurobiol 30: 1251-1258. 21107680
Wen, H. and R.J. Evans. (2011). Contribution of the intracellular C terminal domain to regulation of human P2X1 receptors for ATP by phorbol ester and Gq coupled mGlu(1α) receptors. Eur J Pharmacol 654: 155-159. 21172341