This large family consists of receptors found in nearly all classes of Gram-negative bacteria and Cyanobacteria. These outer membrane proteins act as receptors for organic iron chelators (siderophores) and vitamins necessary for bacterial life. Members of this family are also termed TonB-dependent receptors because their action is mediated by the periplasmic domain of TonB protein. A complex of three membrane proteins (TonB, ExbB and ExbD) couples the chemiosmotic potential of the cytoplasmic membrane with siderophore uptake across the outer membrane. The crystallographic structures of several TonB-dependent Receptors (FhuA, FepA, FecA, BtuB, FusA) have recently been determined. These outer membrane receptors show a novel fold consisting of two domains. A 22-stranded antiparallel &ß-barrel traverses the outer membrane with the adjacent ß-strands being connected by large extracellular loops and periplasmic turns. At the N-terminal there is the Plug domain (of approximately 140 residues in length), located inside the &ß-barrel, which is composed primarily of a mixed 4-stranded ß sheets and a series of interspersed alpha helices. The Plug domain closes the channel pore and has been shown to interact with both the ligand and the periplasmic components. Ligand binding induces distinct local and allosteric transitions that establish the structural basis of signal transduction across the outer membrane.

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