Protein Domain : TonB-dependent siderophore receptor IPR010105

Type	Family
Description	The outer membrane is an essential component of Gram-negative bacteria, providing them with increased resistance to antibiotics, digestive enzymes, detergents and immune surveillance [ ]. The outer membrane is permeable to small hydrophilic molecules because of the presence of aqueous diffusion channels (e.g. porins). Small molecules present at high concentration diffuse down their concentration gradients into the periplasmic space. Porins are inadequate for the efficient acquisition of iron siderophores, cobalamins (Cbl), and other molecules that are present at very low concentrations and that are too bulky to pass through the lumen of the porin. Another class of outer membrane proteins binds these substrates with high specificity and carries out active transport across the outer membrane. This active transport process requires an energy source and a second protein, TonB. Outer membrane active-transport proteins interact with the transperiplasmic protein TonB through a conserved sequence, the "Ton-box". Interaction with TonB couples the energy of the proton motive force of the inner membrane to drive an outer membrane transport cycle [ ]. To date, crystal structures of four TonB-dependent transporters have been solved. Three of these are iron-siderophore transporters: ferrichrome transporter FhuA [ , ]; ferric enterobactin transporter FepA []; and ferric dicitrate transporter FecA []. The fourth structure is the cobalamin transporter BtuB []. All of these structures are composed of two domains, a conserved N-terminal globular domain (hatch) and a 22-stranded β-barrel (barrel). The hatch domain resides within the barrel and occludes the large pore of the large barrel domain. The hatch domains are composed of a central core of four β-strands connected by loops. The conserved Ton-box is located near the periplasmic opening of the barrel and precedes the conserved hatch core. The barrels have large extracellular loops and short periplasmic turns connecting the β-strands. TonB-dependent transporters bind their cognate substrates using residues from hatch loops, from the interior surfaces of β-strands in the barrel wall, and from extracellular loops of the barrel [ ]. This group of sequences include a variety of TonB-dependent outer membrane siderophore receptors. It has no overlap with TonB receptors known to transport other substances.
Short Name	TonB_sidphr_rcpt