ammonia transporter
{{Pfam_box
| Symbol = AmtB
| Name = Ammonia transporter
| image = Rhcg 3hd6.png
| width =
| caption = Human Rhesus C Glycoprotein. PDB {{PDBe|3hd6}}{{Cite journal
| last1 = Gruswitz | first1 = F.
| last2 = Chaudhary | first2 = S.
| last3 = Ho | first3 = J. D.
| last4 = Schlessinger | first4 = A.
| last5 = Pezeshki | first5 = B.
| last6 = Ho | first6 = C. -M.
| last7 = Sali | first7 = A.
| last8 = Westhoff | first8 = C. M.
| last9 = Stroud | first9 = R. M.
| doi = 10.1073/pnas.1003587107
| title = Function of human Rh based on structure of RhCG at 2.1 A
| journal = Proceedings of the National Academy of Sciences
| volume = 107
| issue = 21
| pages = 9638–9643
| year = 2010
| pmid = 20457942
| pmc =2906887
| doi-access = free
}}
| Pfam= PF00909
| InterPro= IPR001905
| SMART=
| Prosite =
| SCOP =
| TCDB = 1.A.11
| OPM family= 13
| OPM protein= 2ns1
| PDB=
}}
Ammonia transporters ([http://www.tcdb.org/search/result.php?tc=1.a.11 TC# 1.A.11]) are structurally related membrane transport proteins called Amt proteins (ammonia transporters) in bacteria and plants, methylammonium/ammonium permeases (MEPs) in yeast, or Rhesus (Rh) proteins in chordates. In humans, the RhAG, RhBG, and RhCG Rhesus proteins constitute solute carrier family 42{{cite journal | vauthors = Nakhoul NL, Hamm LL | title = Non-erythroid Rh glycoproteins: a putative new family of mammalian ammonium transporters | journal = Pflügers Archiv | volume = 447 | issue = 5 | pages = 807–12 | date = Feb 2004 | pmid = 12920597 | doi = 10.1007/s00424-003-1142-8 | s2cid = 24601165 }} whilst RhD and RhCE form the Rh blood group system. The three-dimensional structure of the ammonia transport protein AmtB from Escherichia coli has been determined by x-ray crystallography{{PDBe|1xqe}}; {{cite journal | vauthors = Khademi S, O'Connell J, Remis J, Robles-Colmenares Y, Miercke LJ, Stroud RM | title = Mechanism of ammonia transport by Amt/MEP/Rh: structure of AmtB at 1.35 A | journal = Science | volume = 305 | issue = 5690 | pages = 1587–94 | date = Sep 2004 | pmid = 15361618 | doi = 10.1126/science.1101952 | citeseerx = 10.1.1.133.6480 | s2cid = 11436509 }}{{PDBe|2u7c}}; {{cite journal | vauthors = Zheng L, Kostrewa D, Bernèche S, Winkler FK, Li XD | title = The mechanism of ammonia transport based on the crystal structure of AmtB of Escherichia coli | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 49 | pages = 17090–5 | date = Dec 2004 | pmid = 15563598 | pmc = 535379 | doi = 10.1073/pnas.0406475101 | bibcode = 2004PNAS..10117090Z | doi-access = free }} revealing a hydrophobic ammonia channel.{{cite journal | vauthors = Khademi S, Stroud RM | title = The Amt/MEP/Rh family: structure of AmtB and the mechanism of ammonia gas conduction | journal = Physiology | volume = 21 | issue = 6 | pages = 419–29 | date = Dec 2006 | pmid = 17119155 | doi = 10.1152/physiol.00051.2005 }} The human RhCG ammonia transporter was found to have a similar ammonia-conducting channel structure. It was proposed{{Citation needed|date=November 2013}} that the erythrocyte Rh complex is a heterotrimer of RhAG, RhD, and RhCE subunits in which RhD and RhCE might play roles in anchoring the ammonia-conducting RhAG subunit to the cytoskeleton. Based on reconstitution experiments, purified RhCG subunits alone can function to transport ammonia.{{cite journal | vauthors = Mouro-Chanteloup I, Cochet S, Chami M, Genetet S, Zidi-Yahiaoui N, Engel A, Colin Y, Bertrand O, Ripoche P | title = Functional reconstitution into liposomes of purified human RhCG ammonia channel | journal = PLOS ONE | volume = 5 | issue = 1 | pages = e8921 | year = 2010 | pmid = 20126667 | pmc = 2812482 | doi = 10.1371/journal.pone.0008921 | bibcode = 2010PLoSO...5.8921M | editor1-last = Fatouros | editor1-first = Dimitris | doi-access = free }} RhCG is required for normal acid excretion by the mouse kidney{{cite journal | vauthors = Wagner CA, Devuyst O, Belge H, Bourgeois S, Houillier P | title = The rhesus protein RhCG: a new perspective in ammonium transport and distal urinary acidification | journal = Kidney International | volume = 79 | issue = 2 | pages = 154–61 | date = Jan 2011 | pmid = 20927037 | doi = 10.1038/ki.2010.386 | url = https://www.zora.uzh.ch/id/eprint/38490/4/The_Rhesus_proteins_RgCG_a_new_perspective_-_accepted_versionV.pdf }} and epididymis.{{cite journal | vauthors = Biver S, Belge H, Bourgeois S, Van Vooren P, Nowik M, Scohy S, Houillier P, Szpirer J, Szpirer C, Wagner CA, Devuyst O, Marini AM | title = A role for Rhesus factor Rhcg in renal ammonium excretion and male fertility | journal = Nature | volume = 456 | issue = 7220 | pages = 339–43 | date = Nov 2008 | pmid = 19020613 | doi = 10.1038/nature07518 | bibcode = 2008Natur.456..339B | s2cid = 205215412 }}
Structure
The structure of the ammonia channel from E. coli, was, at the time of its publication, the highest resolution structure of any integral membrane protein. It shows a trimer of subunits, each made up of 11 transmembrane segments (TMSs) and containing a pseudo two-fold symmetry.{{cite journal | vauthors = Conroy MJ, Jamieson SJ, Blakey D, Kaufmann T, Engel A, Fotiadis D, Merrick M, Bullough PA | title = Electron and atomic force microscopy of the trimeric ammonium transporter AmtB | journal = EMBO Reports | volume = 5 | issue = 12 | pages = 1153–8 | date = Dec 2004 | pmid = 15568015 | pmc = 1299191 | doi = 10.1038/sj.embor.7400296 }} Each monomer contains a hydrophobic ammonia conducting channel.
While prokaryotic ammonia channel proteins have an N-terminal region which acts as a signal sequence and is cleaved in the mature protein,{{cite journal | vauthors = Thornton J, Blakey D, Scanlon E, Merrick M | title = The ammonia channel protein AmtB from Escherichia coli is a polytopic membrane protein with a cleavable signal peptide | journal = FEMS Microbiology Letters | volume = 258 | issue = 1 | pages = 114–20 | date = May 2006 | pmid = 16630265 | doi = 10.1111/j.1574-6968.2006.00202.x | doi-access = free }} the Rhesus glycoproteins retain this as a 12th transmembrane helix in the mature protein.
Substrate specificity
Most functionally characterized members of the family are ammonium uptake transporters.{{Cite journal|last1=Soupene|first1=Eric|last2=King|first2=Natalie|last3=Feild|first3=Eithne|last4=Liu|first4=Phillip|last5=Niyogi|first5=Krishna K.|last6=Huang|first6=Cheng-Han|last7=Kustu|first7=Sydney|date=2002-05-28|title=Rhesus expression in a green alga is regulated by CO(2)|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=99|issue=11|pages=7769–7773|doi=10.1073/pnas.112225599|issn=0027-8424|pmc=124347|pmid=12032358|bibcode=2002PNAS...99.7769S|doi-access=free}} Some, but not other Amt proteins also transport methylammonium.{{Cite journal|last1=Musa-Aziz|first1=Raif|last2=Chen|first2=Li-Ming|last3=Pelletier|first3=Marc F.|last4=Boron|first4=Walter F.|date=2009-03-31|title=Relative CO2/NH3 selectivities of AQP1, AQP4, AQP5, AmtB, and RhAG|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=106|issue=13|pages=5406–5411|doi=10.1073/pnas.0813231106|issn=1091-6490|pmc=2664022|pmid=19273840|bibcode=2009PNAS..106.5406M|doi-access=free}}{{Cite journal|last1=Andrade|first1=Susana L. A.|last2=Einsle|first2=Oliver|date=2007-12-01|title=The Amt/Mep/Rh family of ammonium transport proteins|journal=Molecular Membrane Biology|volume=24|issue=5–6|pages=357–365|doi=10.1080/09687680701388423|issn=0968-7688|pmid=17710640|s2cid=41937253|doi-access=free}} Detailed phylogenetic analyses of plant homologues have been published.{{Cite journal|last1=von Wittgenstein|first1=Neil J. J. B.|last2=Le|first2=Cuong H.|last3=Hawkins|first3=Barbara J.|last4=Ehlting|first4=Jürgen|date=2014-01-01|title=Evolutionary classification of ammonium, nitrate, and peptide transporters in land plants|journal=BMC Evolutionary Biology|volume=14|pages=11|doi=10.1186/1471-2148-14-11|issn=1471-2148|pmc=3922906|pmid=24438197 |doi-access=free }} In E. coli, NH4+, rather than NH3, may be the substrate of AmtB, but controversy still exists.{{Cite journal|last1=Fong|first1=Rebecca N.|last2=Kim|first2=Kwang-Seo|last3=Yoshihara|first3=Corinne|last4=Inwood|first4=William B.|last5=Kustu|first5=Sydney|date=2007-11-20|title=The W148L substitution in the Escherichia coli ammonium channel AmtB increases flux and indicates that the substrate is an ion|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=104|issue=47|pages=18706–18711|doi=10.1073/pnas.0709267104|issn=1091-6490|pmc=2141841|pmid=17998534|bibcode=2007PNAS..10418706F|doi-access=free}}{{Cite journal|last1=Ishikita|first1=Hiroshi|last2=Knapp|first2=Ernst-Walter|date=2007-02-07|title=Protonation states of ammonia/ammonium in the hydrophobic pore of ammonia transporter protein AmtB|journal=Journal of the American Chemical Society|volume=129|issue=5|pages=1210–1215|doi=10.1021/ja066208n|issn=0002-7863|pmid=17263403}}{{Cite journal|last1=Javelle|first1=Arnaud|last2=Lupo|first2=Domenico|last3=Zheng|first3=Lei|last4=Li|first4=Xiao-Dan|last5=Winkler|first5=Fritz K.|last6=Merrick|first6=Mike|date=2006-12-22|title=An unusual twin-his arrangement in the pore of ammonia channels is essential for substrate conductance|journal=The Journal of Biological Chemistry|volume=281|issue=51|pages=39492–39498|doi=10.1074/jbc.M608325200|issn=0021-9258|pmid=17040913|doi-access=free}} If NH4+ is transported, K+ possibly serves as a counter ion in an antiport process with K+, and that one histidine removes a proton off of NH4+ to yield NH3.
=Transport reaction=
The generalized transport reaction catalyzed by members of the Amt family are suggested to be:
:NH4+ (out) ⇌ NH4+ (in)
Mechanism
The X-ray structures have revealed that the pore of the Amt and Rh proteins is characterized by a hydrophobic portion about 12 Å long, in which electronic density was observed in the crystallographic study of AmtB from Escherichia coli. This electronic density was initially only observed when crystals were grown in the presence of ammonium, and was thus attributed to ammonia molecules. The Amt/Rh protein mechanism might involve the single-file diffusion of NH3 molecules. However, the pore could also be filled with water molecules. The possible presence of water molecules in the pore lumen calls for a reassessment of the notion that Amt/Rh proteins work as plain NH3 channels. Indeed, functional experiments on plant ammonium transporters and Rh proteins suggest a variety of permeation mechanisms including the passive diffusion of NH3, the antiport of NH4+/H+, the transport of NH4+, or the cotransport of NH3/H+. Lamoureux et al. discuss these mechanisms in light of functional and simulation studies on the AmtB transporter.{{Cite journal|last1=Lamoureux|first1=G.|last2=Javelle|first2=A.|last3=Baday|first3=S.|last4=Wang|first4=S.|last5=Bernèche|first5=S.|date=2010-09-01|title=Transport mechanisms in the ammonium transporter family|journal=Transfusion Clinique et Biologique|volume=17|issue=3|pages=168–175|doi=10.1016/j.tracli.2010.06.004|issn=1953-8022|pmid=20674437}}
Regulation
In E. coli the AmtB gene is expressed only under limiting nitrogen levels to yield the AmtB protein. It is co-expressed with the GlnK gene which encodes a PII protein. This protein is also trimeric and remains in the cytoplasm.{{cite journal | vauthors = Durand A, Merrick M | title = In vitro analysis of the Escherichia coli AmtB-GlnK complex reveals a stoichiometric interaction and sensitivity to ATP and 2-oxoglutarate | journal = The Journal of Biological Chemistry | volume = 281 | issue = 40 | pages = 29558–67 | date = Oct 2006 | pmid = 16864585 | doi = 10.1074/jbc.M602477200 | doi-access = free }} It is covalently modified by a U/U deuridylylated group at Y51. The hydrolyzed product, adenosine 5'-diphosphate, orients the surface of GlnK for AmtB blockade. When nitrogen levels outside the cell rise, the ammonia channel must be deactivated to prevent excessive ammonia entering the cell (where ammonia would be combined with glutamate to make glutamine, utilizing ATP and thereby depleting the cell's ATP reserves). This deactivation is achieved by deuridylylation of the GlnK protein which then binds to the cytoplasmic face of AmtB and inserts a loop into the ammonia conducting pore. At the tip of this loop is an arginine residue which sterically blocks the channel.{{PDBe|2nuu}}; {{cite journal | vauthors = Conroy MJ, Durand A, Lupo D, Li XD, Bullough PA, Winkler FK, Merrick M | title = The crystal structure of the Escherichia coli AmtB-GlnK complex reveals how GlnK regulates the ammonia channel | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 4 | pages = 1213–8 | date = Jan 2007 | pmid = 17220269 | pmc = 1783118 | doi = 10.1073/pnas.0610348104 | bibcode = 2007PNAS..104.1213C | doi-access = free }}
References
{{Reflist|35em}}
{{CCBYSASource|sourcepath= http://www.tcdb.org/search/result.php?tc=1.a.11|sourcearticle= 1.A.11 The Ammonium Channel Transporter (Amt) Family |revision=699838558}}
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