TonB complexes sense signals from the outside of bacterial cells and transmit them into the cytoplasm, leading to transcriptional activation of target genes.

TonB-dependent receptors in gram-negative bacteria are associated with the uptake and transport of large substrates such as iron siderophore complexes and vitamin B12.

[3] These substrates are either poorly transported through non-specific porin channels or are encountered at very low concentrations.

In the absence of TonB, these receptors bind their substrates but do not carry out active transport.

[4] The proteins that are currently known or presumed to interact with TonB include BtuB,[5] CirA, FatA, FcuT, FecA,[6] FhuA,[7] FhuE, FepA,[8] FptA, HemR, IrgA, IutA, PfeA, PupA, LbpA and TbpA.