Structure
TonB spans the periplasm [1], with the C-terminus of TonB spanning from residues ~150 to 239[2]. For more details on the Ton system see Ton.
Function
TonB is involved in the uptake of iron as a component of the TonB/ExbB/ExbD complex of the Ton system. It's activity is determined by the presence of ExbB and ExbD[3]. The C-terminus of TonB interacts with outer membrane transporters, known as TonB-dependent outer membrane transporters (TBDTs)[4], allowing the translocation of biochemical molecules between the inner and outer membrane[2].
Interaction with ExbB/ExbD
TonB and ExbD bind to ExbB in a protein complex that prevents the degradation of the TonB protein. TonB and ExbD are anchored to the cytoplasmic membrane via their N-terminal hydrophobic sequences, with the rest of the protein complex extending into the periplasmic space, allowing for interaction with outer membrane proteins.[3]
TonB takes on the role of an energy transducer, while ExbB and ExbD are involved in the transport of biochemical molecules. In order for sucessful energy transduction to occur, there are processes that must occur[5]:
- TonB must couple to the proton gradient of the cytoplasmic membrane
- TonB must convert its association with the outer membrane, for which it has high affinity, to that with the cytoplasmic membrane, which is also thought to be high affinity.
BtuB-TonB Complex
| As shown in the 3D structure to the right (2GSK), TonB complexes with BtuB in order to aid the transport of nutrients such as cobalamins[6] across the outer membrane by incorporating the proton-motive force into the outer membrane.[7] TonB attaches to BtuB on the periplasmic side of the 614 amino acid BtuB protein[7].
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