Uptake of siderophores and vitamin B(12) through the outer membrane of Escherichia coli is effected by an active transport system consisting of several outer membrane receptors and a protein complex of the inner membrane. The link between these is TonB, a protein associated with the cytoplasmic membrane, which forms a large periplasmic domain capable of interacting with several outer membrane receptors, e.g. FhuA, FecA, and FepA for siderophores and BtuB for vitamin B(12.) The active transport across the outer membrane is driven by the chemiosmotic gradient of the inner membrane and is mediated by the TonB protein. The receptor-binding domain of TonB appears to be formed by a highly conserved C-terminal amino acid sequence of approximately 100 residues. Crystal structures of two C-terminal TonB fragments composed of 85 (TonB-85) and 77 (TonB-77) amino acid residues, respectively, have been previously determined (Chang, C., Mooser, A., Pluckthun, A., and Wlodawer, A. (2001) J. Biol. Chem. 276, 27535-27540 and Koedding, J., Howard, S. P., Kaufmann, L., Polzer, P., Lustig, A., and Welte, W. (2004) J. Biol. Chem. 279, 9978-9986). In both cases the TonB fragments form dimers in solution and crystallize as dimers consisting of monomers tightly engaged with one another by the exchange of a beta-hairpin and a C-terminal beta-strand. Here we present the crystal structure of a 92-residue fragment of TonB (TonB-92), which is monomeric in solution. The structure, determined at 1.13-A resolution, shows a dimer with considerably reduced intermolecular interaction compared with the other known TonB structures, in particular lacking the beta-hairpin exchange.

中文翻译：

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与较小的TonB片段的结构相比，来自大肠杆菌的TonB的92个残基C端片段的晶体结构显示出显着的构象变化。

通过大肠埃希菌的外膜摄取铁载体和维生素B（12）是由一个主动转运系统实现的，该系统由几个外膜受体和一个内膜蛋白复合物组成。它们之间的联系是TonB，一种与细胞质膜相关的蛋白，它形成一个大的胞质结构域，能够与几种外膜受体相互作用，例如铁载体为FhuA，FecA和FepA，维生素B为BtuB（12。）跨内膜的转运是由内膜的化学渗透梯度驱动的，并由TonB蛋白介导。TonB的受体结合结构域似乎由大约100个残基的高度保守的C末端氨基酸序列形成。先前已经确定了分别由85（TonB-85）和77（TonB-77）个氨基酸残基组成的两个C端TonB片段的晶体结构（Chang，C.，Mooser，A.，Pluckthun，A.，和Wlodawer，A.（2001）J. Biol。Chem。276，27535-27540和Koedding，J.，Howard，SP，Kaufmann，L.，Polzer，P.，Lustig，A.和Welte，W. 2004）J.Biol.Chem.279，9978-9986）。在这两种情况下，TonB片段在溶液中形成二聚体并结晶为由通过交换β-发夹和C末端β链彼此紧密结合的单体组成的二聚体。在这里，我们介绍了TonB（TonB-92）的92个残基片段的晶体结构，该片段在溶液中是单体的。与其他已知的TonB结构相比，以1.13-A分辨率确定的结构显示出二聚体，其分子间相互作用大大降低，