Fig. 4

Residues showing subfamily-specific variations at the central constriction. Residues of FNT channels forming central constriction are shown for (a) FocA (PDB ID: 3KLY, monomer A), (b) NirC-α (UniProt ID: I1XLF9) and (c) YfdC-α (UniProt ID: U1X6D4). The four residues correspond to Phe-75 (TM2a), Phe-202 (TM5a), His-209 (S-loop) and Ala-212 (TM5b) according to the PDB structure 3KCU numbering. His-209 residue in the S-loop is almost invariant in all the FNT channels. The aromatic character of Phe-75 in TM2a shows near absolute conservation. Ala-212 in TM5b is replaced by Val in NirC-α and YfdC-α subfamilies (Table 4). Phe-202 in TM5a is substituted by other hydrophobic residues such as Ile, Val, Met and Leu in YfdC subfamilies