Gram-negative bacterial TonB protein (IPR003538)

Short name: TonB

Overlapping homologous superfamilies


Family relationships



TonB-dependent transporters (TBDT) are bacterial outer membrane (OM) proteins that bind and transport ferric chelates called siderophores. While iron complexes constitute the majority of substrates for TBDTs, others, like vitamin B12, are also transported by this mechanism [PMID: 18539464]. These transporters show high affinity and specificity for siderophores and require energy derived from the proton motive force across the inner membrane to transport them. The energy force is provided through interaction with an inner membrane protein complex consisting of TonB, ExbB, and ExbD [PMID: 20420522]. The source of this energy is the ion electrochemical gradient of the cytoplasmic membrane, harvested by heteromultimeric complexes of ExbB and ExbD proteins, and transduced to the OM high affinity siderophore transporters by the protein TonB [PMID: 17225934].

TonB is composed of three domains. The N-terminal transmembrane helix anchors the protein to the inner membrane and makes contact with ExbB and ExbD to form an energy transducing complex. The C-terminal globular domain directly contacts the transporters in the OM. These two domains are separated by a flexible, unstructured proline-rich domain that resides within the periplasm [PMID: 23527057].

Escherichia coli has only one TonB protein which is shared by different TBDTs involved in in the acquisition of various substrates, but most bacteria have more than one tonB gene [PMID: 17225063].

GO terms

Biological Process

GO:0015891 siderophore transport

Molecular Function

GO:0031992 energy transducer activity
GO:0015343 siderophore transmembrane transporter activity

Cellular Component

GO:0030288 outer membrane-bounded periplasmic space

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.