| Description | Proteins in this family are predominantly bacterial TonB-dependent transporters that have a N-terminal plug domain which acts as the channel gate, blocking the pore until the channel is bound by a ligand when it undergoes conformational changes and opens the channel; and a C-terminal β-barrel domain.Members of this entry are multipass transmembrane proteins. 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-dependent regulatory systems consist of six components: a specialised outer membrane-localized TonB-dependent receptor (TonB-dependent transducer) that interacts with its energizing TonB-ExbBD protein complex, a cytoplasmic membrane-localized anti-sigma factor and an extracytoplasmic function (ECF)-subfamily sigma factor []. The TonB complex senses signals from outside the bacterial cell and transmits them via two membranes into the cytoplasm, leading to transcriptional activation of target genes. Energy for active transport is again obtained through physical interaction with TonB []. The proteins that are currently known or presumed to interact with TonB include BtuB, CirA, FatA, FcuT, FecA [], FhuA [], FhuE, FepA which is important for iron uptake and allows the bacterium to extract iron from the environment [], FptA, HemR, IrgA, IutA, PfeA, PupA and Tbp1. The TonB protein also interacts with some colicins.The family includes the vitamin B12 transporter BtuB from Escherichia coli, which translocates vitamin B12 (cyanocobalamin) across the outer membrane to the periplasmic space. A conserved sequence of seven residues, the Ton box, faces the periplasm and interacts with the inner membrane TonB protein to energize this active transport cycle [ ]. | Name | TonB-dependent receptor-like |
| Short Name | TonB-dep_rcpt-like | Type | Family |
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