<p>This domain is found at the C terminus of outer membrane receptors which interact with the TonB [<cite idref="PUB00002093"/>]. In <taxon tax_id="562">Escherichia coli</taxon> the TonB 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 substratesbut do not carry out active transport. The TonB protein also interacts withsome colicins. </p>