TonB is an inner (cytoplasmic) membrane-anchored, periplasm-spanning energy-transducing protein of Gram-negative bacteria. Together with its partner proteins ExbB and ExbD, it forms the TonB-ExbB-ExbD complex that harnesses the proton-motive force of the inner membrane and transmits this energy across the periplasm to TonB-dependent transporters (TBDTs) in the outer membrane. By contacting the conserved "TonB box" of a TBDT through its periplasmic C-terminal domain, TonB drives conformational changes in the receptor plug that enable active, high-affinity uptake of scarce nutrients - principally ferric-siderophore complexes (e.g. ferric-pyoverdine) and other iron sources, but also vitamin B12 and certain other substrates that cannot cross the outer membrane by passive diffusion. The protein has an N-terminal single-pass transmembrane anchor, a long proline-rich/disordered periplasm-spanning segment, and a C-terminal TonB domain that engages outer-membrane receptors. In P. putida KT2440, which encodes a large repertoire of TonB-dependent receptors and produces the siderophore pyoverdine, TonB is central to iron acquisition and to fitness under iron-limited conditions.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: TonB is anchored in the inner (plasma) membrane by an N-terminal single-pass transmembrane helix. Reason: Consistent with the UniProt subcellular location (cell inner membrane, single-pass, periplasmic side) and with the well-established topology of TonB-family proteins, which are anchored in the cytoplasmic membrane. In bacteria the plasma membrane is the inner membrane, so this localization is correct. |
| GO:0015031 protein transport | IEA GO_REF:0000104 | MARK AS OVER ANNOTATED | Summary: TonB energizes outer-membrane uptake of iron-siderophore complexes and other small nutrients, not protein transport. Reason: This term arises from an electronic UniRule transfer and from the generic "Protein transport" keyword, but it does not reflect TonB function. TonB does not mediate protein translocation; it couples the proton-motive force to TonB-dependent outer-membrane transporters that import ferric-siderophores, iron, and vitamins. The biologically appropriate process is transmembrane transport / siderophore-iron import, already captured by other annotations, so "protein transport" is an over-annotation. |
| GO:0030288 outer membrane-bounded periplasmic space | IEA GO_REF:0000104 | ACCEPT | Summary: TonB spans the periplasm; its periplasm-spanning region and C-terminal domain reside in and act within the periplasmic space to contact outer-membrane receptors. Reason: TonB is anchored in the inner membrane but its long proline-rich segment and C-terminal TonB domain extend across the periplasm to engage the TonB box of outer-membrane transporters. Localization of the functional portion to the periplasmic space is consistent with established TonB topology and mechanism. |
| GO:0031992 energy transducer activity | IEA GO_REF:0000104 | ACCEPT | Summary: TonB is the prototypical energy transducer, transmitting proton-motive-force energy from the inner membrane (via ExbB/ExbD) to outer-membrane transporters. Reason: This is the core molecular function of TonB. The TonB/ExbB/ExbD complex transduces the inner-membrane proton-motive force into mechanical work delivered to TonB-dependent transporters, energizing their active transport cycle. Well supported by conserved TonB-family mechanism. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: TonB is required for active transmembrane (outer-membrane) transport of siderophore-iron complexes and related nutrients. Reason: TonB participates in transmembrane transport by energizing outer-membrane TBDTs, so the broad term is biologically correct. However, it is general; the more specific and informative process is siderophore-dependent iron import across the outer membrane. Retained as a correct but non-core, high-level process term. |
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