Family 2.A.44 - The Nucleobase Cation Symporter-2 Family
Family ID: 53376
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. The homologous yeast protein
may function as a short chain aliphatic carboxylate H+ symporter,
transporting formate, acetate and propionate, and functioning
primarily as an acetate uptake permease.
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 a-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 putative nitrite uptake permeases (NirC)
of bacteria comprise cluster III, and the single yeast protein,
the putative acetate:H+ symporter alone comprises cluster IV.
The energy
coupling mechanisms for proteins of the FNT family have not been
extensively characterized. HCO2 -, CH3CO2 - 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.
