TonB-dependent receptors-structural perspectives.

Plants, bacteria, fungi, and yeast utilize organic iron chelators (siderophores) to establish commensal and pathogenic relationships with hosts and to survive as free-living organisms. In Gram-negative bacteria, transport of siderophores into the periplasm is mediated by TonB-dependent receptors. A complex of three membrane-spanning proteins TonB, ExbB and ExbD couples the chemiosmotic potential of the cytoplasmic membrane with siderophore uptake across the outer membrane. The crystallographic structures of two TonB-dependent receptors (FhuA and FepA) have recently been determined. These outer membrane transporters show a novel fold consisting of two domains. A 22-stranded antiparallel beta-barrel traverses the outer membrane and adjacent beta-strands are connected by extracellular loops and periplasmic turns. Located inside the beta-barrel is the plug domain, composed primarily of a mixed four-stranded beta-sheet and a series of interspersed alpha-helices. Siderophore binding induces distinct local and allosteric transitions that establish the structural basis of signal transduction across the outer membrane and suggest a transport mechanism.