In Escherichia coli the tonB protein interacts with outer membrane receptor proteins that carry out high-affinity binding and energy-dependent uptake of specific substrates into the periplasmic space. These substrates are either poorly permeable through the 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. The tonB protein also interacts with some colicins. The proteins that are currently known or presumed to interact with tonB are:

  Gene   Function or name
  ----   -------------------------------------------------------------------
  btuB   Receptor for cobalamin.
  cirA   Receptor for colicin I (exact substrate not known).
  fatA   Receptor for ferric anguibactin (Vibrio anguillarum).
  fcuA   Receptor for ferrichrome (Yersinia enterocolitica).
  fecA   Receptor for iron dicitrate.
  fepA   Receptor for ferrienterobactin (ferric enterochelin).
  fhuA   Receptor for ferrichrome-iron.
  fhuE   Receptor for ferric coprogen, ferrioxamine B, and rhodotrulic acid.
  foxA   Receptor for ferrioxamine (Yersinia enterocolitica).
  fptA   Receptor for Fe(III)-pyochelin (Pseudomonas aeruginosa).
  fpvA   Receptor for ferripyoverdine (Pseudomonas aeruginosa).
  fyuA   Receptor for pesticin and yersiniabactin (Yersinia).
  hemR   Receptor for hemin (Yersinia enterocolitica).
  hxuC   Receptor for heme/hemopexin (Haemophilus influenzae).
  irgA   Receptor for ferric vibriobactin (Vibrio cholerae).
  iroA   Receptor for a siderophore (Neisseria meningitidis).
  iutA   Receptor for ferric aerobactin.
  pbuA   Receptor for ferric pseudobactin M114 (Pseudomonas strain M114).
  pfeA   Receptor for ferric enterobactin (Pseudomonas aeruginosa).
  pupA   Receptor for ferric pseudobactin 358 (Pseudomonas putida).
  pupB   Receptor for ferric pseudobactin BN7 and BN8 (Pseudomonas putida).
  tbp1   Receptor for transferrin (Neisseria and Haemophilus).

  Haemophilus influenzae hypothetical proteins HI0262, HI0661, HI0635, HI0712,
  HI1217, and HI1567.

  cba    Colicin B, a channel forming colicin.
  cda    Colicin D.
  cma    Colicin M, inhibitor of murein biosynthesis.

Most of these proteins contain, at their N-terminus, a short conserved region [1,2,3], called the tonB-box, involved in the interaction of the protein with the tonB protein [4]. With the exception of the colicin B, D and M receptors they all also contain, in their C-terminal extremity a short conserved domain that contains two invariant residues.