Role of the Native Outer-Membrane Environment on the Transporter BtuB
Role of the Native Outer-Membrane Environment on the Transporter BtuB
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DOI：
10.1016/j.bpj.2016.08.033
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发表时间：
2016-10-04
影响因子：
3.4
通讯作者：
Gumbart, James C.
中科院分区：
文献类型：
--
作者：
Balusek, Curtis;Gumbart, James C.
Btu B是一种TonB依赖性转运蛋白，允许钴胺素（CBL）或维生素B-12高亲和力结合和转运穿过革兰氏阴性细菌的不对称外膜（OM）。已经表明，Ca 2+结合是CBL与BtuB的高亲和力结合所必需的，并且早期的模拟表明，钙离子用于稳定关键的底物结合细胞外环。然而，这些模拟没有考虑OM中的脂多糖。为了阐明Ca 2+和脂多糖在蛋白质功能中的作用，我们在对称和不对称双层中模拟载脂蛋白和Ca 2+负载的BtuB。模拟结果表明，LPS的寡糖在一定程度上稳定了细胞外环，显然避免了对Ca 2+的需要。然而，它表明，Ca 2+离子稳定一个关键的底物结合环，甚至更大的程度，以及重新定位特定的CBL结合残基，使它们更接近组织中发现的CBL结合结构。这些结果表明，模拟外膜蛋白时，包括现实的膜模型的重要性。
BtuB is a TonB-dependent transporter that permits the high-affinity binding and transport of cobalamin (CBL), or vitamin B-12, across the asymmetric outer membrane (OM) of Gram-negative bacteria. It has been shown that Ca2+ binding is necessary for high-affinity binding of CBL to BtuB, and earlier simulations suggested that calcium ions serve to stabilize key substrate-binding extracellular loops. However, those simulations did not account for the lipopolysaccharides in the OM. To illuminate the roles of both Ca2+ and lipopolysaccharides in protein functionality, we performed simulations of apo and Ca2+-loaded BtuB in symmetric and asymmetric bilayers. The simulations reveal that the oligosaccharides of LPS stabilize the extracellular loops to some degree, apparently obviating the need for Ca2+. However, it is shown that Ca2+ ions stabilize a key substrate-binding loop to an even greater degree, as well as reposition specific CBL-binding residues, bringing them closer to the organization found in the CBL-bound structure. These results indicate the importance of including realistic membrane models when simulating outer-membrane proteins.