Institute of Structural and Molecular Biology, University College London and Birkbeck College, Malet Street, London WC1E 7HX, UK.
Nature. 2011 Jun 2;474(7349):49-53. doi: 10.1038/nature10109.
Type 1 pili are the archetypal representative of a widespread class of adhesive multisubunit fibres in Gram-negative bacteria. During pilus assembly, subunits dock as chaperone-bound complexes to an usher, which catalyses their polymerization and mediates pilus translocation across the outer membrane. Here we report the crystal structure of the full-length FimD usher bound to the FimC-FimH chaperone-adhesin complex and that of the unbound form of the FimD translocation domain. The FimD-FimC-FimH structure shows FimH inserted inside the FimD 24-stranded β-barrel translocation channel. FimC-FimH is held in place through interactions with the two carboxy-terminal periplasmic domains of FimD, a binding mode confirmed in solution by electron paramagnetic resonance spectroscopy. To accommodate FimH, the usher plug domain is displaced from the barrel lumen to the periplasm, concomitant with a marked conformational change in the β-barrel. The amino-terminal domain of FimD is observed in an ideal position to catalyse incorporation of a newly recruited chaperone-subunit complex. The FimD-FimC-FimH structure provides unique insights into the pilus subunit incorporation cycle, and captures the first view of a protein transporter in the act of secreting its cognate substrate.
I 型菌毛是革兰氏阴性菌中广泛存在的一类黏附性多亚基纤维的典型代表。在菌毛组装过程中，亚基作为伴侣结合复合物与 usher 对接， usher 催化其聚合并介导菌毛穿过外膜的转运。在这里，我们报告了全长 FimD usher 与 FimC-FimH 伴侣-黏附素复合物的晶体结构，以及未结合形式的 FimD 转运结构域的晶体结构。FimD-FimC-FimH 结构显示 FimH 插入 FimD 的 24 股β-桶转运通道内。FimC-FimH 通过与 FimD 的两个羧基末端周质域相互作用而保持在原位，这种结合模式在溶液中通过电子顺磁共振波谱得到证实。为了容纳 FimH，usher 塞结构域从桶腔移位到周质，同时β-桶发生明显的构象变化。FimD 的氨基末端结构域被观察到处于理想位置，以催化新募集的伴侣-亚基复合物的掺入。FimD-FimC-FimH 结构提供了菌毛亚基掺入循环的独特见解，并首次观察到蛋白转运体分泌其同源底物的过程。
