Substrate-induced transmembrane signaling in the cobalamin transporter BtuB
Substrate-induced transmembrane signaling in the cobalamin transporter BtuB
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DOI：
10.1038/nsb914
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发表时间：
2003-05-01
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影响因子：
--
通讯作者：
Wiener, MC
中科院分区：
文献类型：
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作者：
Chimento, DP;Mohanty, AK;Wiener, MC
革兰氏阴性菌的外膜具有摄取稀缺营养所必需的转运蛋白。在TonB依赖性转运蛋白中，七个残基的保守序列，Ton盒，面对周质并与内膜Tong蛋白相互作用以激活主动转运循环。一个关键的机制步骤是在底物结合时转运蛋白的Ton盒中的结构变化;这种必要的跨膜信号传导事件增加了转运蛋白对Tong的亲和力，并使主动转运得以进行。我们已经解决了晶体结构的Btu B，外膜钴胺素转运大肠杆菌，在存在和不存在的维生素B-12。在这些结构中，Ton盒是有序的，并且在结合底物的存在下经历构象变化。钙被认为是氰钴胺素与BtuB高亲和力结合（K-d类似于0.3 nM）的必要因素。我们观察到两个绑定的钙离子，命令三个细胞外环的BtuB，从而提供了一个直接的（和不寻常的）结构作用的钙。
The outer membranes of Gram-negative bacteria possess transport proteins essential for uptake of scarce nutrients. In TonB-dependent transporters, a conserved sequence of seven residues, the Ton box, faces the periplasm and interacts with the inner membrane Tong protein to energize an active transport cycle. A critical mechanistic step is the structural change in the Ton box of the transporter upon substrate binding; this essential transmembrane signaling event increases the affinity of the transporter for Tong and enables active transport to proceed. We have solved crystal structures of BtuB, the outer membrane cobalamin transporter from Escherichia coli, in the absence and presence of cyanocobalamin (vitamin B-12). In these structures, the Ton box is ordered and undergoes a conformational change in the presence of bound substrate. Calcium has been implicated as a necessary factor for the high-affinity binding (K-d similar to0.3 nM) of cyanocobalamin to BtuB. We observe two bound calcium ions that order three extracellular loops of BtuB, thus providing a direct (and unusual) structural role for calcium.