Structural evidence for iron-free citrate and ferric citrate binding to the TonB-dependent outer membrane transporter FecA.
Structural evidence for iron-free citrate and ferric citrate binding to the TonB-dependent outer membrane transporter FecA.
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DOI：
10.1016/s0022-2836(03)00855-6
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发表时间：
2003-09
影响因子：
5.6
通讯作者：
W. Yue;S. Grizot;S. Buchanan
中科院分区：
文献类型：
--
作者：
W. Yue;S. Grizot;S. Buchanan
大肠杆菌具有TonB依赖性转运系统，该系统利用柠檬酸盐的铁结合能力及其天然丰度。在这里，我们描述了三种结构的外膜柠檬酸铁转运体FecA：unliganded和复合无铁或二铁柠檬酸。我们展示了无铁和铁铁载体之间的歧视的结构机制：二柠檬酸二铁的结合，但不单独无铁二柠檬酸，导致主要的构象重排的转运。与无铁二柠檬酸盐结合的FecA的结构代表与无铁铁载体结合的TonB依赖性转运蛋白的第一结构。与无铁二柠檬酸盐的结合相比，二柠檬酸铁与FecA的结合导致两个柠檬酸盐离子相对于彼此的取向以及它们与FecA的相互作用的变化。配体结合的变化伴随着FecA的三个区域的构象变化：两个细胞外环，一个塞子结构域环和周质TonB盒基序。铁载体和转运蛋白的位置和构象变化启动了两个独立的事件：柠檬酸铁转运到周质和fecABCDE转运基因的转录诱导。从这些数据中，我们提出了一个两步的配体识别事件：FecA结合无铁二柠檬酸在非生产状态或第一步，其次是铁载体置换，形成运输能力，二铁二柠檬酸结合状态的第二步。
Escherichia coli possesses a TonB-dependent transport system, which exploits the iron-binding capacity of citrate and its natural abundance. Here, we describe three structures of the outer membrane ferric citrate transporter FecA: unliganded and complexed with iron-free or diferric dicitrate. We show the structural mechanism for discrimination between the iron-free and ferric siderophore: the binding of diferric dicitrate, but not iron-free dicitrate alone, causes major conformational rearrangements in the transporter. The structure of FecA bound with iron-free dicitrate represents the first structure of a TonB-dependent transporter bound with an iron-free siderophore. Binding of diferric dicitrate to FecA results in changes in the orientation of the two citrate ions relative to each other and in their interactions with FecA, compared to the binding of iron-free dicitrate. The changes in ligand binding are accompanied by conformational changes in three areas of FecA: two extracellular loops, one plug domain loop and the periplasmic TonB-box motif. The positional and conformational changes in the siderophore and transporter initiate two independent events: ferric citrate transport into the periplasm and transcription induction of the fecABCDE transport genes. From these data, we propose a two-step ligand recognition event: FecA binds iron-free dicitrate in the non-productive state or first step, followed by siderophore displacement to form the transport-competent, diferric dicitrate-bound state in the second step.