Protein with a TonB box. 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. TonB also interacts with some colicins and is involved in the energy dependent, irreversible steps of bacteriophages phi-80 and T1 infection.