SecB-Mediated Protein Export Need Not Occur via Kinetic Partitioning
SecB-Mediated Protein Export Need Not Occur via Kinetic Partitioning
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DOI：
10.1016/j.jmb.2008.10.094
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发表时间：
2009-01-30
影响因子：
5.6
通讯作者：
Varadarajan, Raghavan
中科院分区：
文献类型：
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作者：
Krishnan, Beena;Kulothungan, S. Rajendra;Varadarajan, Raghavan
在大肠杆菌中，胞质伴侣 SecB 负责前体蛋白的子集选择性进入 Sec 途径。在体外，SecB 与多种未折叠底物结合，没有明显的序列特异性，但不与天然蛋白质结合。因此，有人认为选择性是通过蛋白质 SecB 关联之间底物的动力学分配来实现的。动力学分配的证据基于早期的观察，即 SecB 阻断麦芽糖结合蛋白前体形式 (preMBP)(5) 和慢速折叠麦芽糖结合蛋白 (MBP) 突变体的重折叠，但不能阻断快速折叠的成熟野生型 MBP。为了定量验证动力学分配模型，我们独立测量了 SecB 与重折叠 preMBP（SecB 的生理底物）和成熟 MBP 相互作用所涉及的每个速率常数。测量的速率常数可以正确预测各种 SecB、MBP 和 preMBP 浓度下的底物折叠动力学。数据分析表明，对于许多底物来说，动力学分配不太可能是 SecB 介导的蛋白质输出的原因。相反，SecB 结合底物在与 SecB 结合时继续折叠的能力以及它们与分泌机制的其他组件（如 SecA）相互作用的能力可能是分别抑制和促进蛋白质输出的关键相反决定因素。 (C) 2008 Elsevier Ltd. 保留所有权利。
In Escherichia coli, the cytosolic chaperone SecB is responsible for the selective entry of a subset of precursor proteins into the Sec pathway. In vitro, SecB binds to a variety of unfolded substrates without apparent sequence specificity, but not native proteins. Selectivity has therefore been suggested to occur by kinetic partitioning of substrates between protein SecB association. Evidence for kinetic partitioning is based on earlier observations that SecB blocks the refolding of the precursor form of maltose-binding protein (preMBP)(5) and slow-folding maltose-binding protein (MBP) mutants, but not faster-folding mature wild-type MBP. In order to quantitatively validate the kinetic partitioning model, we have independently measured each of the rate constants involved in the interaction of SecB with refolding preMBP (a physiological substrate of SecB) and mature MBP. The measured rate constants correctly predict substrate folding kinetics over a wide range of SecB, MBP, and preMBP concentrations. Analysis of the data reveals that, for many substrates, kinetic partitioning is unlikely to be responsible for SecB-mediated protein export. Instead, the ability of SecB-bound substrates to continue folding while bound to SecB and their ability to interact with other components of the secretory machinery such as SecA may be key opposing determinants that inhibit and promote protein export, respectively. (C) 2008 Elsevier Ltd. All rights reserved.