TonB box, conserved site <p>This entry describes a short conserved region at the N terminus called the tonB-box [<cite idref="PUB00002420"/>, <cite idref="PUB00002063"/>, <cite idref="PUB00002072"/>], which is involved in the interaction of the protein with the tonB protein [<cite idref="PUB00002093"/>]. In <taxon tax_id="562">Escherichia coli</taxon> 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. </p>