NMR structure of the C-terminal domain of TonB protein from .

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 TonB have been previously reported. Here we determined the solution NMR structure of a 96-residue fragment of TonB (TonB-96). The structure shows a monomeric structure with the flexible C-terminal region (residues 338-342), different from the NMR structure of TonB (TonB-137). The extended and flexible C-terminal residues are confirmed by N relaxation analysis and molecular dynamics simulation. We created models for the TonB-96/TonB box interaction and propose that the internal fluctuations of 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.