The Formate-Nitrite Transporter (FNT) Family belongs to the Major Intrinsic Protein (MIP) Superfamily. FNT family members have been sequenced from

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

and

Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

, plants and lower eukaryotes. The

prokaryotic A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek language, Greek wikt:πρό#Ancient Greek, πρό (, 'before') a ...

proteins of the FNT family probably function in the transport of the structurally related compounds,

formate Formate (IUPAC name: methanoate) is the conjugate base of formic acid. Formate is an anion () or its derivatives such as ester of formic acid. The salts and esters are generally colorless.Werner Reutemann and Heinz Kieczka "Formic Acid" in ''Ull ...

and

nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...

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. 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 A pentamer is an entity composed of five sub-units. In chemistry, it applies to molecules made of five monomers. In biochemistry, it applies to macromolecules, in particular to pentameric proteins, made of five proteic sub-units. In microbiolog ...

, with each subunit consisting of six TMSs.

Phylogeny

The phylogenetic tree shows clustering according to function and organismal phylogeny. The putative formate efflux transporters (FocA; TC#
1.A.16.1.1
an
1.A.16.1.3
of bacteria associated with pyruvate-formate lyase (pfl) comprise cluster I; the putative formate uptake permeases (FdhC; TC#
1.A.16.2.1
an
1.A.16.2.3
of bacteria and archaea associated with formate dehydrogenase comprise cluster II; the nitrite uptake permeases (NirC, TC#
1.A.16.2.51.A.16.3.1
an
1.A.16.3.4
of bacteria comprise cluster III, and a yeast protein comprises cluster IV.

Function

The energy coupling mechanisms for proteins of the FNT family have not been extensively characterized. HCO and NO uptakes may be coupled to H⁺ symport. HCO efflux may be driven by the membrane potential by a uniport mechanism or by H⁺ antiport. FocA of ''E. coli'' catalyzes bidirectional

transport and may function by a channel-type mechanism. 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 ''Salmonella enterica'' subsp. ''enterica'' is a subspecies of ''Salmonella enterica'', the rod-shaped, flagellated, aerobic, Gram-negative bacterium. Many of the pathogenic serovars of the ''S. enterica'' species are in this subspecies, includi ...

'' FocA at pH 4.0 shows that this switch involves a major rearrangement of the amino termini of individual protomers in the pentameric channel. 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 or NO (out) ⇌ RCO or NO (in), (2) HCO (in) ⇌ HCO (out), (3) HS⁻ (out) ⇌ HS⁻ (in).

Members

A representative list of the currently classified members belonging to the FNT family can be found in th
Transporter Classification Database
Some characterized members include: *FocA and FocB (TC#
1.A.16.1.1
an
1.A.16.1.2
respectively), from ''Escherichia coli'', transporters involved in the bidirectional

transport Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land (rail and road), water, cable, pipeline, an ...

of formate. *FdhC, from ''Methanobacterium maripaludis'' (TC
1.A.16.2.3
and ''Methanothermobacter'' thermoformicicum (TC
1.A.16.2.1
, a probable formate transporter. *NirC, from E. coli (TC
1.A.16.3.1
, a probable nitrite transporter. *Nar1 (TC
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
1.A.16.3.2
.

References

{{Portal bar, Biology, border=no Protein families Membrane proteins Transmembrane proteins Transmembrane transporters Transport proteins Integral membrane proteins