NMR structure of the C-terminal domain of TonB protein from Pseudomonas aeruginosa

The TonB protein plays an essential role in the energy transduction system to drive active transport across the outer membrane (OM) using the proton-motive force of the cytoplasmic membrane of Gram-negative bacteria. The C-terminal domain (CTD) of TonB protein is known to interact with the conserved TonB box motif of TonB-dependent OM transporters, which likely induces structural changes in the OM transporters. Several distinct conformations of differently dissected CTDs of Escherichia coli TonB have been previously reported. Here we determined the solution NMR structure of a 96-residue fragment of Pseudomonas aeruginosa TonB ( Pa TonB-96). The structure shows a monomeric structure with the flexible C-terminal region (residues 338–342), different from the NMR structure of E. coli TonB ( Ec TonB-137). The extended and flexible C-terminal residues are confirmed by 15 N relaxation analysis and molecular dynamics simulation. We created models for the Pa TonB-96/TonB box interaction and propose that the internal fluctuations of Pa TonB-96 makes it more accessible for the interactions with the TonB box and possibly plays a role in disrupting the plug domain of the TonB-dependent OM transporters.