Formate-nitrite transporter

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 E. coli FocA ([http://www.tcdb.org/search/result.php?tc=1.A.16.1.1 TC# 1.A.16.1.1]) protein, a 6 TMS topology has been established.{{cite journal | vauthors = Wang Y, Huang Y, Wang J, Cheng C, Huang W, Lu P, Xu YN, Wang P, Yan N, Shi Y | title = Structure of the formate transporter FocA reveals a pentameric aquaporin-like channel | journal = Nature | volume = 462 | issue = 7272 | pages = 467–72 | date = November 2009 | pmid = 19940917 | doi = 10.1038/nature08610 | bibcode = 2009Natur.462..467W | s2cid = 4370839 }} The yeast protein has a similar apparent topology but has a large C-terminal hydrophilic extension of about 400 residues.

FocA of E. coli is a symmetric pentamer, with each subunit consisting of six TMSs.

The phylogenetic tree shows clustering according to function and organismal phylogeny. The putative formate efflux transporters (FocA; TC#s [http://www.tcdb.org/search/result.php?tc=1.A.16.1.1 1.A.16.1.1] and [http://www.tcdb.org/search/result.php?tc=1.A.16.1.3 1.A.16.1.3]) of bacteria associated with pyruvate-formate lyase (pfl) comprise cluster I; the putative formate uptake permeases (FdhC; TC#s [http://www.tcdb.org/search/result.php?tc=1.A.16.2.1 1.A.16.2.1] and [http://www.tcdb.org/search/result.php?tc=1.A.16.2.3 1.A.16.2.3]) of bacteria and archaea associated with formate dehydrogenase comprise cluster II; the nitrite uptake permeases (NirC, TC#s [http://www.tcdb.org/search/result.php?tc=1.A.16.2.5 1.A.16.2.5], [http://www.tcdb.org/search/result.php?tc=1.A.16.3.1 1.A.16.3.1], and [http://www.tcdb.org/search/result.php?tc=1.A.16.2.4 1.A.16.3.4]) of bacteria comprise cluster III, and a yeast protein comprises cluster IV.{{Cite web|url = http://www.tcdb.org/search/result.php?tc=1.A.16#ref9123|title = 1.A.16 The Formate-Nitrite Transporter (FNT) Family|date = |access-date = |website = Transporter Classification Database|publisher = Saier Lab Bioinformatics Group / SDSC|last = Saier|first = MH Jr.}}

The energy coupling mechanisms for proteins of the FNT family have not been extensively characterized. HCO{{su|b=2|p=−}} and NO{{su|b=2|p=−}} uptakes may be coupled to H⁺ symport. HCO{{su|b=2|p=−}} efflux may be driven by the membrane potential by a uniport mechanism or by H⁺ antiport. FocA of E. coli catalyzes bidirectional formate transport and may function by a channel-type mechanism.{{cite journal | vauthors = Falke D, Schulz K, Doberenz C, Beyer L, Lilie H, Thiemer B, Sawers RG | title = Unexpected oligomeric structure of the FocA formate channel of Escherichia coli : a paradigm for the formate-nitrite transporter family of integral membrane proteins | journal = FEMS Microbiology Letters | volume = 303 | issue = 1 | pages = 69–75 | date = February 2010 | pmid = 20041954 | doi = 10.1111/j.1574-6968.2009.01862.x | doi-access = free }}

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 Salmonella typhimurium FocA at pH 4.0 shows that this switch involves a major rearrangement of the amino termini of individual protomers in the pentameric channel.{{cite journal | vauthors = Lü W, Du J, Wacker T, Gerbig-Smentek E, Andrade SL, Einsle O | title = pH-dependent gating in a FocA formate channel | journal = Science | volume = 332 | issue = 6027 | pages = 352–4 | date = April 2011 | pmid = 21493860 | doi = 10.1126/science.1199098 | bibcode = 2011Sci...332..352L | s2cid = 20059830 }} 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.

The probable transport reactions catalyzed by different members of the FNT family are:

(1) RCO{{su|b=2|p=−}} or NO{{su|b=2|p=−}} (out) ⇌ RCO{{su|b=2|p=−}} or NO{{su|b=2|p=−}} (in),

(2) HCO{{su|b=2|p=−}} (in) ⇌ HCO{{su|b=2|p=−}} (out),

(3) HS⁻ (out) ⇌ HS⁻ (in).

A representative list of the currently classified members belonging to the FNT family can be found in the [http://www.tcdb.org/search/result.php?tc=1.A.16.5 Transporter Classification Database]. Some characterized members include:

• FocA and FocB (TC#s [http://www.tcdb.org/search/result.php?tc=1.A.16.1.1 1.A.16.1.1] and [http://www.tcdb.org/search/result.php?tc=1.A.16.1.2 1.A.16.1.2], respectively), from Escherichia coli, transporters involved in the bidirectional transport of formate.
• FdhC, from Methanobacterium maripaludis (TC# [http://www.tcdb.org/search/result.php?tc=1.A.16.2.3 1.A.16.2.3]) and Methanothermobacter thermoformicicum (TC# [http://www.tcdb.org/search/result.php?tc=1.A.16.2.1 1.A.16.2.1]), a probable formate transporter.
• NirC, from E. coli (TC# [http://www.tcdb.org/search/result.php?tc=1.A.16.3.1 1.A.16.3.1]), a probable nitrite transporter.
• Nar1 (TC# [http://www.tcdb.org/search/result.php?tc=1.A.16.2.4 1.A.16.2.4]) of Chlamydomonas reinhardtii (Chlamydomonas smithii), a nitrite uptake porter of 355 amino acyl residues.
• B. subtilis hypothetical protein YwcJ (ipa-48R) (TC# [http://www.tcdb.org/search/result.php?tc=1.A.16.3.2 1.A.16.3.2]).

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{{InterPro content|IPR000292}}

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Category:Protein families

Category:Membrane proteins

Category:Transmembrane proteins

Category:Transmembrane transporters

Category:Transport proteins

Category:Integral membrane proteins