Formate-nitrite transporter

Form_Nir_trans
Identifiers
Symbol Form_Nir_trans
Pfam PF01226
InterPro IPR000292
PROSITE PDOC00769
TCDB 2.A.44
OPM superfamily 7
OPM protein 3tdp

The Formate-Nitrite Transporter (FNT) Family (TC# 1.A.16) is a family of homologous transmembrane proteins belonging to the Major Intrinsic Protein (MIP) Superfamily.[1][2] FNT family members have been sequenced from Gram-negative and Gram-positive bacteria, archaea, yeast, plants and lower eukaryotes. The prokaryotic proteins of the FNT family probably function in the transport of the structurally related compounds, formate and nitrite.[3]

Structure

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 (TC# 1.A.16.1.1) protein, a 6 TMS topology has been established. Wang et al. (2009) reported the crystal structure of the E. coli FocA at 2.25 Å resolution.[4] The yeast protein has a similar apparent topology but has a large C-terminal hydrophilic extension of about 400 residues.

FocA of E. coli has a pentameric quaternary structure. FocA forms a symmetric pentamer, with each protomer consisting of six TMSs. Despite a lack of sequence homology, the overall structure of the FocA protomer closely resembles that of aquaporin, indicating that FocA is a channel rather than a carrier. Structural analysis identified potentially important channel residues, defined the channel path and revealed two constriction sites. Unlike aquaporin, FocA is impermeable to water but allows the passage of formate.[5]

Phylogeny

The phylogenetic tree shows clustering according to function and organismal phylogeny. The putative formate efflux transporters (FocA; TC#s 1.A.16.1.1 and 1.A.16.1.3) of bacteria associated with pyruvate-formate lyase (pfl) comprise cluster I; the putative formate uptake permeases (FdhC; TC#s 1.A.16.2.1 and 1.A.16.2.3) of bacteria and archaea associated with formate dehydrogenase comprise cluster II; the nitrite uptake permeases (NirC, TC#s 1.A.16.2.5, 1.A.16.3.1, and 1.A.16.3.4) of bacteria comprise cluster III, and a yeast protein comprises cluster IV.[6]

Function

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 E. coli catalyzes bidirectional formate transport and may function by a channel-type mechanism.[7]

FocA, a representative member of the formate-nitrite transporter family, transports short-chain acids in bacteria, archaea, fungi, algae and certain eukaryotic parasites. 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.[8] 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.

Transport Reaction

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

(1) RCO−
2 or NO−
2 (out) ⇌ RCO−
2 or NO−
2 (in),

(2) HCO−
2 (in) ⇌ HCO−
2 (out),

(3) HS− (out) ⇌ HS− (in).

Members

A representative list of the currently classified members belonging to the FNT family can be found in the Transporter Classification Database. Some characterized members include:

References

  1. ↑ Reizer, J.; Reizer, A.; Saier, M. H. (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): 235–257. doi:10.3109/10409239309086796. ISSN 1040-9238. PMID 8325040.
  2. ↑ Park, J. H.; Saier, M. H. (1996-10-01). "Phylogenetic characterization of the MIP family of transmembrane channel proteins". The Journal of Membrane Biology. 153 (3): 171–180. doi:10.1007/s002329900120. ISSN 0022-2631. PMID 8849412.
  3. ↑ 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". Mol. Microbiol. 11 (5): 965–82. doi:10.1111/j.1365-2958.1994.tb00375.x. PMID 8022272.
  4. ↑ Wang, Yi; Huang, Yongjian; Wang, Jiawei; Cheng, Chao; Huang, Weijiao; Lu, Peilong; Xu, Ya-Nan; Wang, Pengye; Yan, Nieng (2009-11-26). "Structure of the formate transporter FocA reveals a pentameric aquaporin-like channel". Nature. 462 (7272): 467–472. doi:10.1038/nature08610. ISSN 1476-4687. PMID 19940917.
  5. ↑ Wang, Yi; Huang, Yongjian; Wang, Jiawei; Cheng, Chao; Huang, Weijiao; Lu, Peilong; Xu, Ya-Nan; Wang, Pengye; Yan, Nieng (2009-11-26). "Structure of the formate transporter FocA reveals a pentameric aquaporin-like channel". Nature. 462 (7272): 467–472. doi:10.1038/nature08610. ISSN 0028-0836.
  6. ↑ Saier, MH Jr. "1.A.16 The Formate-Nitrite Transporter (FNT) Family". Transporter Classification Database. Saier Lab Bioinformatics Group / SDSC.
  7. ↑ Falke, Dörte; Schulz, Kristin; Doberenz, Claudia; Beyer, Lydia; Lilie, Hauke; Thiemer, Barbara; Sawers, Robert Gary (2010-02-01). "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. ISSN 1574-6968. PMID 20041954.
  8. ↑ Lü, Wei; Du, Juan; Wacker, Tobias; Gerbig-Smentek, Elke; Andrade, Susana L. A.; Einsle, Oliver (2011-04-15). "pH-dependent gating in a FocA formate channel". Science (New York, N.Y.). 332 (6027): 352–354. doi:10.1126/science.1199098. ISSN 1095-9203. PMID 21493860.

This article incorporates text from the public domain Pfam and InterPro IPR000292

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