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Formate-nitrite transporter
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<h2 id="structure">Structure</h2> <p>With the exception of the yeast protein (627 amino acyl residues), all characterized members of the family are of 256-285 residues in length and exhibit 6-8 putative transmembrane α-helical spanners (TMSs). In one case, that of the <i>E. coli</i> FocA (<a href="http://www.tcdb.org/search/result.php?tc=1.A.16.1.1">TC# 1.A.16.1.1</a>) protein, a 6 TMS topology has been established.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> The yeast protein has a similar apparent topology but has a large C-terminal hydrophilic extension of about 400 residues. </p><p>FocA of <i>E. coli</i> is a symmetric <a href="/facts/Pentamer/JUdMmxlg">pentamer</a>, with each subunit consisting of six TMSs.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <h2 id="phylogeny">Phylogeny</h2> <p>The phylogenetic tree shows clustering according to function and organismal phylogeny. The putative formate efflux transporters (FocA; TC#s <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.1.1">1.A.16.1.1</a> and <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.1.3">1.A.16.1.3</a>) of bacteria associated with pyruvate-formate lyase (pfl) comprise cluster I; the putative formate uptake permeases (FdhC; TC#s <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.2.1">1.A.16.2.1</a> and <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.2.3">1.A.16.2.3</a>) of bacteria and archaea associated with formate dehydrogenase comprise cluster II; the nitrite uptake permeases (NirC, TC#s <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.2.5">1.A.16.2.5</a>, <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.3.1">1.A.16.3.1</a>, and <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.2.4">1.A.16.3.4</a>) of bacteria comprise cluster III, and a yeast protein comprises cluster IV.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> <h2 id="function">Function</h2> <p>The energy coupling mechanisms for proteins of the FNT family have not been extensively characterized. HCO−2 and NO−2 uptakes may be coupled to H+ symport. HCO−2 efflux may be driven by the membrane potential by a uniport mechanism or by H+ antiport. FocA of <i>E. coli</i> catalyzes bidirectional <a href="/facts/Formate/Py3GhI7L">formate</a> transport and may function by a channel-type mechanism.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p><p>FocA, transports short-chain acids. FocA may be able to switch its mode of operation from a passive export channel at high external pH to a secondary active formate/H+ importer at low pH. The crystal structure of <i><a href="/facts/Salmonella_enterica_subsp._enterica/aj5Ax3uG">Salmonella typhimurium</a></i> FocA at pH 4.0 shows that this switch involves a major rearrangement of the amino termini of individual protomers in the pentameric channel.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> The amino-terminal helices open or block transport in a concerted, cooperative action that indicates how FocA is gated in a pH-dependent way. Electrophysiological studies show that the protein acts as a specific formate channel at pH 7.0 and that it closes upon a shift of pH to 5.1. </p> <h3>Transport Reaction</h3> <p>The probable transport reactions catalyzed by different members of the FNT family are: </p><p>(1) RCO−2 or NO−2 (out) ⇌ RCO−2 or NO−2 (in), </p><p>(2) HCO−2 (in) ⇌ HCO−2 (out), </p><p>(3) HS− (out) ⇌ HS− (in). </p> <h2 id="members">Members</h2> <p>A representative list of the currently classified members belonging to the FNT family can be found in the <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.5">Transporter Classification Database</a>. Some characterized members include: </p> <ul><li>FocA and FocB (TC#s <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.1.1">1.A.16.1.1</a> and <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.1.2">1.A.16.1.2</a>, respectively), from <i>Escherichia coli</i>, <a href="/facts/Membrane_transporter_protein/h1aczVxg">transporters</a> involved in the bidirectional <a href="/facts/Transport/npstdhvc">transport</a> of formate.</li> <li>FdhC, from <i>Methanobacterium maripaludis</i> (TC# <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.2.3">1.A.16.2.3</a>) and <i>Methanothermobacter</i> thermoformicicum (TC# <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.2.1">1.A.16.2.1</a>), a probable formate transporter.</li> <li>NirC, from E. coli (TC# <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.3.1">1.A.16.3.1</a>), a probable nitrite transporter.</li> <li>Nar1 (TC# <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.2.4">1.A.16.2.4</a>) of <i>Chlamydomonas reinhardtii (Chlamydomonas smithii),</i> a nitrite uptake porter of 355 amino acyl residues.</li> <li>B. subtilis hypothetical protein YwcJ (ipa-48R) (TC# <a href="http://www.tcdb.org/search/result.php?tc=1.A.16.3.2">1.A.16.3.2</a>).</li></ul> This article incorporates text from the public domain <a href="/facts/Pfam/BKJa293w">Pfam</a> and <a href="/facts/InterPro/pKwNm8t7">InterPro</a>: <a href="https://www.ebi.ac.uk/interpro/entry/IPR000292">IPR000292</a> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Reizer J, Reizer A, Saier MH (1993-01-01). "The MIP family of integral membrane channel proteins: sequence comparisons, evolutionary relationships, reconstructed pathway of evolution, and proposed functional differentiation of the two repeated halves of the proteins". Critical Reviews in Biochemistry and Molecular Biology. 28 (3): 235–57. doi:10.3109/10409239309086796. PMID 8325040. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Park JH, Saier MH (October 1996). "Phylogenetic characterization of the MIP family of transmembrane channel proteins". The Journal of Membrane Biology. 153 (3): 171–80. doi:10.1007/s002329900120. PMID 8849412. S2CID 1559932. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Suppmann B, Sawers G (March 1994). "Isolation and characterization of hypophosphite--resistant mutants of Escherichia coli: identification of the FocA protein, encoded by the pfl operon, as a putative formate transporter". Molecular Microbiology. 11 (5): 965–82. doi:10.1111/j.1365-2958.1994.tb00375.x. PMID 8022272. S2CID 6425651. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Wang Y, Huang Y, Wang J, Cheng C, Huang W, Lu P, Xu YN, Wang P, Yan N, Shi Y (November 2009). "Structure of the formate transporter FocA reveals a pentameric aquaporin-like channel". Nature. 462 (7272): 467–72. Bibcode:2009Natur.462..467W. doi:10.1038/nature08610. PMID 19940917. S2CID 4370839. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Wang Y, Huang Y, Wang J, Cheng C, Huang W, Lu P, Xu YN, Wang P, Yan N, Shi Y (November 2009). "Structure of the formate transporter FocA reveals a pentameric aquaporin-like channel". Nature. 462 (7272): 467–72. Bibcode:2009Natur.462..467W. doi:10.1038/nature08610. PMID 19940917. S2CID 4370839. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Saier, MH Jr. "1.A.16 The Formate-Nitrite Transporter (FNT) Family". Transporter Classification Database. Saier Lab Bioinformatics Group / SDSC. <a href="http://www.tcdb.org/search/result.php?tc=1.A.16#ref9123" target="_blank">http://www.tcdb.org/search/result.php?tc=1.A.16#ref9123</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Falke D, Schulz K, Doberenz C, Beyer L, Lilie H, Thiemer B, Sawers RG (February 2010). "Unexpected oligomeric structure of the FocA formate channel of Escherichia coli : a paradigm for the formate-nitrite transporter family of integral membrane proteins". FEMS Microbiology Letters. 303 (1): 69–75. doi:10.1111/j.1574-6968.2009.01862.x. PMID 20041954. <a href="https://doi.org/10.1111%2Fj.1574-6968.2009.01862.x" target="_blank">https://doi.org/10.1111%2Fj.1574-6968.2009.01862.x</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Lü W, Du J, Wacker T, Gerbig-Smentek E, Andrade SL, Einsle O (April 2011). "pH-dependent gating in a FocA formate channel". Science. 332 (6027): 352–4. Bibcode:2011Sci...332..352L. doi:10.1126/science.1199098. PMID 21493860. S2CID 20059830. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> </ol>