2.A.44 The Formate-Nitrite Transporter (FNT) Family

FNT family members have been sequenced from Gram-negative and Gram-positive bacteria, archaea and yeast. The prokaryotic proteins of the FNT family probably function in the transport of the structurally related compounds, formate and nitrite.

With the exception of the yeast protein (627 amino acyl residues), all 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 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.

The phylogenetic tree shows clustering according to function and organismal phylogeny. The putative formate efflux transporters (FocA) of bacteria associated with pyruvate-formate lyase (pfl) comprise cluster I; the putative formate uptake permeases (FdhC) of bacteria and archaea associated with formate dehydrogenase comprise cluster II; the nitrite uptake permeases (NirC) of bacteria comprise cluster III, and a yeast protein comprises cluster IV.

The energy coupling mechanisms for proteins of the FNT family have not been extensively characterized. HCO₂^- and NO₂^- uptakes are probably coupled to H⁺ symport. HCO₂^- efflux may be driven by the membrane potential by a uniport mechanism or by H⁺ antiport. FocA of E. coli catalyzes bidirectional formate transport, has a pentameric quaternary structure and may function by a channel-type mechanism (Falke et al., 2009).

FocA is a representative member of the formate-nitrite transporter family, which transports short-chain acids in bacteria, archaea, fungi, algae and parasites. Wang et al. (2009) reported the crystal structure of the E. coli FocA at 2.25 A resolution. FocA forms a symmetric pentamer, with each protomer consisting of six transmembrane segments. Despite a lack of sequence homology, the overall structure of the FocA protomer closely resembles that of aquaporin and strongly argues that FocA is a channel, rather than a transporter. Structural analysis identifies potentially important channel residues, defines the channel path and reveals two constriction sites. Unlike aquaporin, FocA is impermeable to water but allows the passage of formate.

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

(1) RCO₂^- or NO₂^- (out) Æ RCO₂^- or NO₂^- (in)

and

(2) HCO₂^- (in) HCO₂^- (out).