View source for P2X purinoreceptor

{{Redirect-distinguish|P2X|Power-to-X}}
{{Pfam_box
| Symbol = P2X_receptor
| Name = ATP P2X receptor
| image = SchematicP2XRSubunitV2.png
| width =
| caption = Figure 1. Schematic representation showing the membrane topology of a typical P2X receptor subunit. First and second transmembrane domains are labeled TM1 and TM2.
| Pfam= PF00864
| InterPro= IPR001429
| SMART=
| Prosite = PDOC00932
| SCOP =
| TCDB = 1.A.7
| OPM family=181
| OPM protein=3h9v
| PDB= 
}}

The '''P2X receptors''', also '''ATP-gated P2X receptor cation channel family''',<ref name="TCDB" /> is a [[protein family]] that consists of cation-permeable [[ligand-gated ion channel]]s that open in response to the binding of extracellular [[adenosine 5'-triphosphate]] (ATP). They belong to a larger family of receptors known as the ENaC/P2X superfamily.<ref name="TCDB">{{cite web | url = http://www.tcdb.org/search/result.php?tc=1.A.7 | title = ATP-gated P2X Receptor Cation Channel (P2X Receptor) Family | work = Functional and Phylogenetic Classification of Membrane Transport Proteins | publisher = Saier Lab. Group, UCSD and SDSC  }}</ref> ENaC and P2X receptors have similar 3-D structures and are homologous.<ref name="pmid22286036">{{cite journal | vauthors = Chen JS, Reddy V, Chen JH, Shlykov MA, Zheng WH, Cho J, Yen MR, Saier MH | title = Phylogenetic characterization of transport protein superfamilies: superiority of SuperfamilyTree programs over those based on multiple alignments | journal = J. Mol. Microbiol. Biotechnol. | volume = 21 | issue = 3–4 | pages = 83–96 | year = 2011 | pmid = 22286036 | pmc = 3290041 | doi = 10.1159/000334611 }}</ref> P2X receptors are present in a diverse array of organisms including [[humans]], [[mouse]], [[rat]], [[rabbit]], [[chicken]], [[zebrafish]], [[American bullfrog|bullfrog]], [[Trematoda|fluke]], and [[amoeba]].<ref name="ReferenceA"/>

[[File:FullStructureV2.png|thumb|270px|Figure 2. Crystal structure of the zebrafish P2X<sub>4</sub> receptor (deltaP2X4-B) channel as viewed from the side (left), extracellular (top right), and intracellular (bottom right) perspectives({{PDB|3I5D}})]]

==Physiological roles==
P2X receptors are involved in a variety of physiological processes,<ref name="ReferenceA"/><ref name="pmid16885977">{{cite journal | vauthors = Khakh BS, North RA | title = P2X receptors as cell-surface ATP sensors in health and disease | journal = Nature | volume = 442 | issue = 7102 | pages = 527–32 | year = 2006 | pmid = 16885977 | doi = 10.1038/nature04886 | bibcode = 2006Natur.442..527K | s2cid = 4422150 }}</ref> including:
* Modulation of cardiac rhythm and [[contractility]]<ref name="pmid11274344">{{cite journal | vauthors = Vassort G | title = Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium | journal = Physiol. Rev. | volume = 81 | issue = 2 | pages = 767–806 | year = 2001 | pmid = 11274344 | doi =  10.1152/physrev.2001.81.2.767}}</ref>
* Modulation of [[vascular resistance|vascular tone]]<ref name="ReferenceA"/>
* Mediation of [[Pain and nociception|nociception]], especially [[chronic pain]]<ref name="pmid11734618">{{cite journal | vauthors = Chizh BA, Illes P | title = P2X receptors and nociception | journal = Pharmacol. Rev. | volume = 53 | issue = 4 | pages = 553–68 | year = 2001 | pmid = 11734618 | url = http://pharmrev.aspetjournals.org/cgi/content/abstract/53/4/553 }}</ref>
* Contraction of the [[vas deferens]] during [[ejaculation]]<ref name="ReferenceA"/>
* Contraction of the [[urinary bladder]] during [[micturition]]<ref>{{cite journal | vauthors = Fowler CJ, Griffiths D, de Groat WC | title = The neural control of micturition | journal = Nat Rev Neurosci | volume = 9 | issue = 6 | pages = 453–466 | year = 2008 | pmid = 18490916 | pmc = 2897743 | doi = 10.1038/nrn2401 }}</ref>
* [[Platelet]] aggregation<ref>{{cite journal | vauthors = Gachet C | title = Regulation of Platelet Functions by P2 Receptors | journal = Annual Review of Pharmacology and Toxicology | volume = 46 | pages = 277–300 | year = 2006 | pmid = 16402906 | doi = 10.1146/annurev.pharmtox.46.120604.141207 }}</ref>
* [[Macrophage]] activation<ref>{{cite journal | vauthors = Wewers MD, Sarkar A | title = P2X7 receptor and macrophage function | journal = Purinergic Signalling | volume = 5 | issue = 2 | pages = 189–195 | year = 2009 | pmid = 19214778 | pmc = 2686821 | doi = 10.1007/s11302-009-9131-9 }}</ref>
* [[Apoptosis]]<ref>{{cite journal | vauthors = Kawano A, Tsukimoto M, Noguchi T, Hotta N, Harada H, Takenouchi T, Kitani H, Kojima S | title = Involvement of P2X4 receptor in P2X7 receptor-dependent cell death of mouse macrophages | journal = Biochemical and Biophysical Research Communications | volume = 419 | issue = 2 | pages = 374–380 | year = 2012 | pmid = 22349510 | doi = 10.1016/j.bbrc.2012.01.156 }}</ref>
* [[Neuron]]al-[[neuroglia|glial]] integration<ref>{{cite book | vauthors = Burnstock G | title = Glioma Signaling | chapter = Introduction to Purinergic Signalling in the Brain | series = Advances in Experimental Medicine and Biology | volume = 986 | pages = 1–12 | year = 2013 | pmid = 22879061 | doi = 10.1007/978-94-007-4719-7_1 | isbn = 978-94-007-4718-0 }}</ref>

==Tissue distribution==
P2X receptors are expressed in cells from a wide variety of animal [[biological tissue|tissues]]. On presynaptic and postsynaptic [[chemical synapse|nerve terminals]] and [[neuroglia|glial]] cells throughout the [[central nervous system|central]], [[peripheral nervous system|peripheral]] and [[autonomic nervous system|autonomic]] nervous systems, P2X receptors have been shown to modulate [[chemical synapse|synaptic transmission]].<ref name="ReferenceA"/><ref name="pmid10823099">{{cite journal | vauthors = Burnstock G | title = P2X receptors in sensory neurones | journal = Br J Anaesth | volume = 84 | issue = 4 | pages = 476–88 | year = 2000 | pmid = 10823099 | doi = 10.1093/oxfordjournals.bja.a013473 | doi-access = free }}</ref> Furthermore, P2X receptors are able to initiate [[muscle contraction|contraction]] in cells of the [[myocardium|heart muscle]], [[skeletal muscle]], and various [[smooth muscle]] tissues, including that of the [[circulatory system|vasculature]], [[vas deferens]] and [[urinary bladder]]. P2X receptors are also expressed on [[White blood cell|leukocytes]], including lymphocytes and macrophages, and are present on blood [[platelet]]s. There is some degree of subtype specificity as to which P2X receptor subtypes are expressed on specific cell types, with P2X<sub>1</sub> receptors being particularly prominent in smooth muscle cells, and P2X<sub>2</sub> being widespread throughout the autonomic nervous system. However, such trends are very general and there is considerable overlap in subunit distribution, with most cell types expressing more than one subunits. For example, P2X<sub>2</sub> and P2X<sub>3</sub> subunits are commonly found co-expressed in [[sensory neuron]]s, where they often co-assemble into functional P2X<sub>2/3</sub> receptors.

==Basic structure and nomenclature==
To date, seven separate genes coding for P2X subunits have been identified, and named as '''P2X<sub>1</sub>''' through '''P2X<sub>7</sub>''', based on their pharmacological properties.<ref name="ReferenceA"/><ref name="Gever">{{cite journal | vauthors = Gever JR, Cockayne DA, Dillon MP, Burnstock G, Ford AP | title = Pharmacology of P2X channels | journal = Pflügers Arch. | volume = 452 | issue = 5 | pages = 513–37 | year = 2006 | pmid = 16649055 | doi = 10.1007/s00424-006-0070-9 | s2cid = 15837425 }}</ref>

{| class="wikitable" style="text-align:center"
|-
! receptor subtype
! [[HUGO Gene Nomenclature Committee|HGNC]] gene name
! [[Locus (genetics)|chromosomal location]]
|-
| [[P2RX1|P2X<sub>1</sub>]]
| [https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:8533 P2RX1]
| [https://omim.org/search/?index=geneMap&search=17p13.3 17p13.3]
|-
| [[P2RX2|P2X<sub>2</sub>]]
| [https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:15459 P2RX2]
| [https://omim.org/search/?index=geneMap&search=12q24.33 12q24.33]
|-
| [[P2RX3|P2X<sub>3</sub>]]
| [https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:8534 P2RX3]
| [https://omim.org/search/?index=geneMap&search=11q12 11q12]
|-
| [[P2RX4|P2X<sub>4</sub>]]
| [https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:8535 P2RX4]
| [https://omim.org/search/?index=geneMap&search=12q24.32 12q24.32]
|-
| [[P2RX5|P2X<sub>5</sub>]]
| [https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:8536 P2RX5]
| [https://omim.org/search/?index=geneMap&search=17p13.3 17p13.3]
|-
| [[P2RX6|P2X<sub>6</sub>]]
| [https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:8538 P2RX6]
| [https://omim.org/search/?index=geneMap&search=22p11.21 22p11.21]
|-
| [[P2RX7|P2X<sub>7</sub>]]
| [https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:8537 P2RX7]
| [https://omim.org/search/?index=geneMap&search=12q24.31 12q24.31 ]
|}

The proteins of the P2X receptors are quite similar in sequence (>35% identity), but they possess 380-1000 amino acyl residues per subunit with variability in length. The subunits all share a common topology, possessing two [[transmembrane domain]]s (one about 30-50 residues from their N-termini, the other near residues 320-340), a large extracellular loop and intracellular [[C-terminal end|carboxyl]] and [[N-terminal end|amino]] termini (Figure 1)<ref name="ReferenceA">{{cite journal | vauthors = North RA | title = Molecular physiology of P2X receptors | journal = Physiological Reviews | volume = 82 | issue = 4 | pages = 1013–1067 | year = 2002 | pmid = 12270951 | doi = 10.1152/physrev.00015.2002 }}</ref> The extracellular receptor domains between these two segments (of about 270 residues) are well conserved with several conserved glycyl residues and 10 conserved cysteyl residues. The amino termini contain a consensus site for [[protein kinase C]] phosphorylation, indicating that the phosphorylation state of P2X subunits may be involved in receptor functioning.<ref>{{cite journal | vauthors = Boué-Grabot E, Archambault V, Séguéla P | title = A protein kinase C site highly conserved in P2X subunits controls the desensitization kinetics of P2X(2) ATP-gated channels | journal = The Journal of Biological Chemistry | volume = 275 | issue = 14 | pages = 10190–10195 | year = 2000 | pmid = 10744703 | doi = 10.1074/jbc.275.14.10190 | doi-access = free }}</ref> Additionally, there is a great deal of variability (25 to 240 residues) in the C termini, indicating that they might serve subunit specific properties.<ref>{{cite journal | vauthors = Surprenant A, North RA | title = Signaling at Purinergic P2X Receptors | journal = Annual Review of Physiology | volume = 71 | pages = 333–359 | year = 2009 | pmid = 18851707 | doi = 10.1146/annurev.physiol.70.113006.100630 }}</ref>

Generally speaking, most subunits can form functional [[homomeric]] or [[heteromer]]ic receptors.<ref name="ReferenceB">{{cite journal | vauthors = Kaczmarek-Hájek K, Lörinczi E, Hausmann R, Nicke A | title = Molecular and functional properties of P2X receptors—recent progress and persisting challenges | journal = Purinergic Signalling | volume = 8 | issue = 3 | pages = 375–417 | year = 2012 | pmid = 22547202 | pmc = 3360091 | doi = 10.1007/s11302-012-9314-7 }}</ref> Receptor nomenclature dictates that naming is determined by the constituent subunits; e.g. a homomeric P2X receptor made up of only P2X<sub>1</sub> subunits is called a P2X<sub>1</sub> receptor, and a heteromeric receptor containing P2X<sub>2</sub> and P2X<sub>3</sub> subunits is called a P2X<sub>2/3</sub> receptor. The general consensus is that P2X<sub>6</sub> cannot form a functional homomeric receptor and that P2X<SUB>7</SUB> cannot form a functional heteromeric receptor.<ref>{{cite journal | vauthors = Barrera NP, Ormond SJ, Henderson RM, Murrell-Lagnado RD, Edwardson JM | title = Atomic Force Microscopy Imaging Demonstrates that P2X2 Receptors Are Trimers but That P2X6 Receptor Subunits Do Not Oligomerize | journal = Journal of Biological Chemistry | volume = 280 | issue = 11 | pages = 10759–10765 | year = 2005 | pmid = 15657042 | doi = 10.1074/jbc.M412265200 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Torres GE, Egan TM, Voigt MM | title = Hetero-oligomeric assembly of P2X receptor subunits. Specificities exist with regard to possible partners | journal = The Journal of Biological Chemistry | volume = 274 | issue = 10 | pages = 6653–6659 | year = 1999 | pmid = 10037762 | doi = 10.1074/jbc.274.10.6653 | doi-access = free }}</ref>

Topologically, they resemble the [[Epithelial sodium channel|epithelial Na<sup>+</sup> channel proteins]] in possessing (a) N- and C-termini localized intracellularly, (b) two putative transmembrane segments, (c) a large extracellular loop domain, and (d) many conserved extracellular cysteyl residues. P2X receptor channels transport small monovalent cations, although some also transport Ca<sup>2+</sup>.<ref>{{cite patent | country = US | number = 6498022 | status = Lapsed | title = Isolated nucleic acid molecules encoding human carbonate transporter proteins, and uses thereof | fdate = March 13, 2001 | inventor = Yale University School Of Medicine | assign1 = Applera Corporation, Connecticut | Celera Corporation, California class =  | url = https://www.google.com/patents/US6498022}} {{PD-notice}}</ref>

Evidence from early molecular biological and functional studies has strongly indicated that the functional P2X receptor protein is a [[Trimer (biochemistry)|trimer]], with the three peptide [[protein subunit|subunits]] arranged around an ion-permeable channel pore.<ref name="pmid9606184">{{cite journal | vauthors = Nicke A, Bäumert HG, Rettinger J, Eichele A, Lambrecht G, Mutschler E, Schmalzing G | title = P2X1 and P2X3 receptors form stable trimers: a novel structural motif of ligand-gated ion channels | journal = EMBO J. | volume = 17 | issue = 11 | pages = 3016–28 | year = 1998 | pmid = 9606184 | pmc = 1170641 | doi = 10.1093/emboj/17.11.3016 }}</ref> This view was recently confirmed by the use of [[X-ray crystallography]] to resolve the [http://www.rcsb.org/pdb/explore/explore.do?structureId=3I5D three-dimensional structure] of the [[zebrafish]] P2X<sub>4</sub> receptor<ref name="pmid19641588">{{cite journal | vauthors = Kawate T, Michel JC, Birdsong WT, Gouaux E | title = Crystal structure of the ATP-gated P2X4 ion channel in the closed state | journal = Nature | volume = 460 | issue = 7255 | pages = 592–598 | year = 2009 | pmid = 19641588 | pmc = 2720809 | doi = 10.1038/nature08198 | bibcode = 2009Natur.460..592K }}</ref>(Figure 2). These findings indicate that the second transmembrane domain of each subunit lines the ion-conducting pore and is therefore responsible for channel [[Gating (electrophysiology)|gating]].<ref>{{cite journal | vauthors = Migita K, Haines WR, Voigt MM, Egan TM | title = Polar Residues of the Second Transmembrane Domain Influence Cation Permeability of the ATP-gated P2X2 Receptor | journal = Journal of Biological Chemistry | volume = 276 | issue = 33 | pages = 30934–30941 | year = 2001 | pmid = 11402044 | doi = 10.1074/jbc.M103366200 | doi-access = free }}</ref>

The relationship between the structure and function of P2X receptors has been the subject of considerable research using [[site-directed mutagenesis]] and [[fusion protein|chimeric channels]], and key protein domains responsible for regulating ATP binding, ion permeation, pore dilation and desensitization have been identified.<ref name="pmid16708237">{{cite journal | vauthors = Egan TM, Samways DS, Li Z | title = Biophysics of P2X receptors | journal = Pflügers Arch. | volume = 452 | issue = 5 | pages = 501–12 | year = 2006 | pmid = 16708237 | doi = 10.1007/s00424-006-0078-1 | s2cid = 20394414 }}</ref><ref name="pmid16607539">{{cite journal | vauthors = Roberts JA, Vial C, Digby HR, Agboh KC, Wen H, Atterbury-Thomas A, Evans RJ | title = Molecular properties of P2X receptors | journal = Pflügers Arch. | volume = 452 | issue = 5 | pages = 486–500 | year = 2006 | pmid = 16607539 | doi = 10.1007/s00424-006-0073-6 | s2cid = 15079763 }}</ref>

==Activation and channel opening==
Three ATP molecules are thought to be required to activate a P2X receptor, suggesting that ATP needs to bind to each of the three subunits in order to open the channel pore, though recent evidence suggests that ATP binds at the three subunit interfaces.<ref>{{cite journal | vauthors = Evans RJ | title = Orthosteric and allosteric binding sites of P2X receptors | journal = Eur. Biophys. J. | volume = 38 | issue = 3 | pages = 319–27 | year = 2008 | pmid = 18247022 | doi = 10.1007/s00249-008-0275-2 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Ding S, Sachs F | title = Single channel properties of P2X2 purinoceptors | journal = The Journal of General Physiology | volume = 113 | issue = 5 | pages = 695–720 | year = 1999 | pmid = 10228183 | pmc = 2222910 | doi = 10.1085/jgp.113.5.695 }}</ref> Once ATP binds to the extracellular loop of the P2X receptor, it evokes a [[conformational change]] in the structure of the ion channel that results in the opening of the ion-permeable pore. The most commonly accepted theory of channel opening involves the rotation and separation of the second transmembrane domain (TM) helices, allowing cations such as [[Sodium|Na<sup>+</sup>]] and [[calcium|Ca<sup>2+</sup>]] to access the ion-conducting pore through three lateral [[fenestra|fenestrations]] above the TM domains.<ref>{{cite journal | vauthors = Cao L, Broomhead HE, Young MT, North RA | title = Polar Residues in the Second Transmembrane Domain of the Rat P2X2 Receptor That Affect Spontaneous Gating, Unitary Conductance, and Rectification | journal = Journal of Neuroscience | volume = 29 | issue = 45 | pages = 14257–14264 | year = 2009 | pmid = 19906973 | pmc = 2804292 | doi = 10.1523/JNEUROSCI.4403-09.2009 }}</ref><ref>{{cite journal | vauthors = Kawate T, Robertson JL, Li M, Silberberg SD, Swartz KJ | title = Ion access pathway to the transmembrane pore in P2X receptor channels | journal = The Journal of General Physiology | volume = 137 | issue = 6 | pages = 579–590 | year = 2011 | pmid = 21624948 | pmc = 3105519 | doi = 10.1085/jgp.201010593 }}</ref> The entry of cations leads to the [[depolarization]] of the cell membrane and the activation of various Ca<sup>2+</sup>-sensitive intracellular processes.<ref>{{cite journal | vauthors = Shigetomi E, Kato F | title = Action Potential-Independent Release of Glutamate by Ca2+ Entry through Presynaptic P2X Receptors Elicits Postsynaptic Firing in the Brainstem Autonomic Network | journal = Journal of Neuroscience | volume = 24 | issue = 12 | pages = 3125–3135 | year = 2004 | pmid = 15044552 | pmc =  6729830| doi = 10.1523/JNEUROSCI.0090-04.2004 }}</ref><ref>{{cite journal | vauthors = Koshimizu TA, Van Goor F, Tomić M, Wong AO, Tanoue A, Tsujimoto G, Stojilkovic SS | title = Characterization of calcium signaling by purinergic receptor-channels expressed in excitable cells | journal = Molecular Pharmacology | volume = 58 | issue = 5 | pages = 936–945 | year = 2000 | doi = 10.1124/mol.58.5.936 | pmid = 11040040 }}</ref> The channel opening time is dependent upon the subunit makeup of the receptor.  For example, P2X<sub>1</sub> and P2X<sub>3</sub> receptors [[Desensitization (medicine)|desensitize]] rapidly (a few hundred milliseconds) in the continued presence of ATP, whereas the P2X<sub>2</sub> receptor channel remains open for as long as ATP is bound to it.

===Transport reaction===
The generalized transport reaction is: 
:Monovalent cations or Ca<sup>2+</sup> (out) ⇌ monovalent cations or Ca<sup>2+</sup> (in)

==Pharmacology==
The pharmacology of a given P2X receptor is largely determined by its subunit makeup.<ref name="Gever"/> Different subunits exhibit different sensitivities to purinergic agonists such as ATP, α,β-meATP and BzATP; and antagonists such as [[PPADS|pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid]] (PPADS) and [[suramin]].<ref name="ReferenceA" /> Of continuing interest is the fact that some P2X  receptors (P2X<sub>2</sub>, P2X<sub>4</sub>, human P2X<sub>5</sub>, and P2X<sub>7</sub>) exhibit multiple open states in response to ATP, characterized by a time-dependent increase in the permeabilities of large organic ions such as [[meglumine|''N''-methyl-<small>D</small>-glucamine]] (NMDG<sup>+</sup>) and nucleotide binding dyes such as [[propidium iodide]] (YO-PRO-1). Whether this change in permeability is due to a widening of the P2X receptor channel pore itself or the opening of a separate ion-permeable pore is the subject of continued investigation.<ref name="ReferenceA"/>

==Synthesis and trafficking==
P2X receptors are synthesized in the rough [[endoplasmic reticulum]]. After complex glycosylation in the [[Golgi apparatus]], they are transported to the plasma membrane, whereby docking is achieved through specific members of the [[SNARE (protein)|SNARE protein]] family.<ref name="ReferenceB"/>  A YXXXK [[sequence motif|motif]] in the C terminus is common to all P2X subunits and seems to be important for [[Protein targeting|trafficking]] and stabilization of P2X receptors in the membrane.<ref>{{cite journal | vauthors = Chaumont S, Jiang LH, Penna A, North RA, Rassendren F | title = Identification of a Trafficking Motif Involved in the Stabilization and Polarization of P2X Receptors | journal = Journal of Biological Chemistry | volume = 279 | issue = 28 | pages = 29628–29638 | year = 2004 | pmid = 15126501 | doi = 10.1074/jbc.M403940200 | url = http://www.hal.inserm.fr/inserm-00320759/document | doi-access = free }}</ref>  Removal of P2X receptors occurs via [[clathrin]]-mediated [[endocytosis]] of receptors to [[endosomes]] where they are sorted into [[Vesicle (biology and chemistry)|vesicles]] for degradation or recycling.<ref>{{cite journal | vauthors = Royle SJ, Bobanović LK, Murrell-Lagnado RD | title = Identification of a Non-canonical Tyrosine-based Endocytic Motif in an Ionotropic Receptor | journal = Journal of Biological Chemistry | volume = 277 | issue = 38 | pages = 35378–35385 | year = 2002 | pmid = 12105201 | doi = 10.1074/jbc.M204844200 | doi-access = free }}</ref>

==Allosteric modulation==

The sensitivity of P2X receptors to ATP is strongly modulated by changes in extracellular pH and by the presence of heavy metals (e.g. zinc and cadmium). For example, the ATP sensitivity of P2X<sub>1</sub>, P2X<sub>3</sub> and P2X<sub>4</sub> receptors is attenuated when the extracellular pH<7, whereas the ATP sensitivity of P2X<sub>2</sub> is significantly increased. On the other hand, zinc potentiates ATP-gated currents through P2X<sub>2</sub>, P2X<sub>3</sub> and P2X<sub>4</sub>, and inhibits currents through P2X<sub>1</sub>. The [[allosteric regulation|allosteric modulation]] of P2X receptors by pH and metals appears to be conferred by the presence of histidine side chains in the extracellular domain.<ref name="ReferenceA"/> In contrast to the other members of the P2X receptor family, P2X<sub>4</sub> receptors are also very sensitive to modulation by the macrocyclic lactone, [[ivermectin]].<ref>{{cite journal | vauthors = Khakh BS, Proctor WR, Dunwiddie TV, Labarca C, Lester HA | title = Allosteric control of gating and kinetics at P2X(4) receptor channels | journal = J. Neurosci. | volume = 19 | issue = 17 | pages = 7289–99 | year = 1999 | pmid = 10460235 | doi =  10.1523/JNEUROSCI.19-17-07289.1999| pmc = 6782529 }}</ref> Ivermectin potentiates ATP-gated currents through P2X<sub>4</sub> receptors by increasing the open probability of the channel in the presence of ATP, which it appears to do by interacting with the transmembrane domains from within the lipid bilayer.<ref>{{cite journal | vauthors = Priel A, Silberberg SD | title = Mechanism of ivermectin facilitation of human P2X<sub>4</sub> receptor channels | journal = J. Gen. Physiol. | volume = 123 | issue = 3 | pages = 281–93 | year = 2004 | pmid = 14769846 | pmc = 2217454 | doi = 10.1085/jgp.200308986 }}</ref>

==Subfamilies==
*[[P2RX1]] {{InterPro|IPR003044}}
*[[P2RX2]] {{InterPro|IPR003045}}
*[[P2RX3]] {{InterPro|IPR003046}}
*[[P2RX4]] {{InterPro|IPR003047}}
*[[P2RX5]] {{InterPro|IPR003048}}
*[[P2RX6]] {{InterPro|IPR003049}}
*[[P2RX7]] {{InterPro|IPR003050}}

==Human proteins containing this domain==
[[P2RX1]];     [[P2RX2]];     [[P2RX3]];     [[P2RX4]];     [[P2RX5]];     [[P2RX7]];     [[P2RXL1]];    [[TAX1BP3]]

==See also==

* [[Ligand-gated ion channel]]s

==References==
{{Reflist|2}}

==External links==
{{Commons category}}
*[https://web.archive.org/web/20090521020834/http://www.biotrend.com/download/NetBTReview3-9-2008.pdf Ivar von Kügelgen: Pharmacology of mammalian P2X- and P2Y-receptors, BIOTREND Reviews No. 03, September 2008,© 2008 BIOTREND Chemicals AG]
*[http://www.ebi.ac.uk/compneur-srv/LGICdb/LGICdb.php Ligand-gated ion channel Database (European Bioinformatics Institute)]
*[https://sites.google.com/site/thep2xproject/home "The P2X Project"]

{{Ligand-gated ion channels}}
{{CCBYSASource|sourcepath= http://www.tcdb.org/search/result.php?tc=1.A.7#ref34544484|sourcearticle= 1.A.7 ATP-gated P2X Receptor Cation Channel (P2X Receptor) Family |revision=699838558}}

{{DEFAULTSORT:P2x Purinoreceptor}}
[[Category:Ion channels]]
[[Category:Ionotropic receptors]]
[[Category:Cell signaling]]
[[Category:Molecular neuroscience]]
[[Category:Protein families]]
[[Category:Membrane proteins]]
[[Category:Transmembrane proteins]]
[[Category:Transmembrane transporters]]
[[Category:Transport proteins]]
[[Category:Integral membrane proteins]]