Distinguishing functional from structural roles of conserved pore residues during formate translocation by the FocA anion channel.

The formate-specific anion channel FocA of Escherichia coli belongs to the superfamily of homopentameric formate-nitrite transporters (FNT). Minimally nine amino acid residues are conserved in the formate translocation pore of each protomer of the pentamer, including a histidine (H209) and a threonine (T91), both of which are crucial for bidirectional formate translocation through the pore. Information regarding in vivo functional or structural roles for the other seven conserved residues is limited, or nonexistent. Here, we conducted an amino acid-exchange analysis of these seven conserved residues. Using an established formate-responsive lacZ-based assay to monitor changes in intracellular formate levels and anaerobic growth rate due to the inhibitory formate analog hypophosphite, we identified five of the seven residues analyzed to be important for the structural integrity of the pentamer, in particular, two highly conserved asparagine residues, N213 and N262. The remaining two conserved residues, K156 and N172, were essential for formate/hypophosphite translocation. K156 is located on the periplasmic fringe of the pore and aids the attraction of formate to the channel. Here, we show that this residue is also important for formate efflux from the cytoplasm to the periplasm, suggesting a role in formate release from the pore. N172 could be replaced by alanine with retention of low-level bidirectional anion translocation function; however, exchange for threonine abolished anion translocation. N172 is, therefore, crucial for bidirectional formate translocation, possibly through its interaction with the conserved pore residue, T91.