Ködding Jiri, Killig Frank, Polzer Patrick, Howard S Peter, Diederichs Kay, Welte Wolfram
Department of Biology, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany.
J Biol Chem. 2005 Jan 28;280(4):3022-8. doi: 10.1074/jbc.M411155200. Epub 2004 Nov 2.
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.
铁载体和维生素B12通过大肠杆菌外膜的摄取是由一个主动运输系统实现的，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质，它形成一个大的周质结构域，能够与几个外膜受体相互作用，例如用于铁载体的FhuA、FecA和FepA以及用于维生素B12的BtuB。跨外膜的主动运输由内膜的化学渗透梯度驱动，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个（TonB-85）和77个（TonB-77）氨基酸残基组成的两个C末端TonB片段的晶体结构（Chang, C., Mooser, A., Pluckthun, A., and Wlodawer, A. (2001) J. Biol. Chem. 276, 27535 - 27540和Koedding, J., Howard, S. P., Kaufmann, L., Polzer, P., Lustig, A., and 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，并以由单体通过β-发夹和C末端β-链的交换紧密结合而成的二聚体形式结晶。在这里，我们展示了TonB的一个92个残基片段（TonB-92）的晶体结构，它在溶液中是单体形式。该结构在1.13埃分辨率下确定，显示出一个二聚体，与其他已知的TonB结构相比，分子间相互作用显著降低，特别是缺少β-发夹交换。
