FIG 1.

(A) TonB-dependent energy transduction. The iron-bearing siderophore enterochelin (triangles) binds to the TBDT FepA with subnanomolar affinity (71). This high-affinity necessitates a mechanism to release the siderophore into the periplasmic space. In the presence of cytoplasmic membrane proteins TonB, ExbB, ExbD, and the cytoplasmic membrane proton motive force (PMF), the Fe-enterochelin is actively transported across the outer membrane through FepA. Numbers indicate the ratio of the proteins in E. coli under iron-limiting conditions (13). Because TonB is limiting, there must be a cycle during which TonB binds and releases TBDTs. The stages in the cycle have been studied, resulting in a model where ExbB acts as a scaffold to manage an essential PMF-dependent interaction between TonB and ExbD that subsequently positions TonB for correct interaction with FepA. The red box around ExbD identifies the ExbD disordered region which is the topic of this study. Subsequent transport of enterochelin across the cytoplasmic membrane does not require the TonB system (adapted from the work of Gresock et al. [24]). (B) NMR solution structure of ExbD periplasmic domain residues 44 to 141 (PDB ID 2PFU) (37). The alpha helices and beta strands are colored in blue and green, respectively. The ExbD disordered region is boxed. The first residue following the ExbD TMD, D44, and last residue, P63, of the ExbD disordered region are labeled with the side chain displayed. This image was generated by using Swiss-PdbViewer (https://spdbv.vital-it.ch/) (37).