Nunn R S, Marković-Housley Z, Génovésio-Taverne J C, Flükiger K, Rizkallah P J, Jansonius J N, Schirmer T, Erni B
Biozentrum, Abteilung Strukturbiologie, University of Basel, Switzerland.
J Mol Biol. 1996 Jun 14;259(3):502-11. doi: 10.1006/jmbi.1996.0335.
The mannose transporter from Escherichia coli is a member of the phosphoenolpyruvate-dependent phosphotransferase system. The multi-subunit complex couples translocation across the bacterial inner membrane with phosphorylation of the solute. A functional fragment (IIA(Man), residues 2 to 133) of the membrane-associated IIAB(Man) subunit of the mannose transporter was expressed as a selenomethionine protein, and the unphosphorylated molecule was crystallized and its structure solved by X-ray crystallography. The protein consists of a central five-stranded beta-sheet covered by helices on either face. The order of the secondary structure elements is (beta alpha)4, alpha beta. Four beta-strands are arranged in a parallel manner with strand order 2134 and are linked by helices forming right-handed cross-over connections. The fifth strand that forms one edge of the sheet and runs antiparallel to the others is swapped between the subunits of the dimeric structure. Helices D and E form a helical hairpin. Histidine 10, which is transiently phosphorylated during catalysis, is located at the topological switch-point of the structure, close to the subunit interface. Its imidazole ring is hydrogen bonded to the buried side-chain of Asp67. It is likely that Asp67 acts as a general base and thus increases the nucleophilicity of the histidine. Modeling suggests that the covalently bound phosphoryl group would be stabilized by the macrodipole of helix C. Putative interactions between IIA(Man) and the histidine-containing phosphocarrier protein are discussed.
来自大肠杆菌的甘露糖转运蛋白是磷酸烯醇丙酮酸依赖性磷酸转移酶系统的成员。这个多亚基复合物将跨细菌内膜的转运与溶质的磷酸化偶联起来。甘露糖转运蛋白的膜相关IIAB（Man）亚基的一个功能片段（IIA（Man），第2至133位残基）被表达为硒代甲硫氨酸蛋白，未磷酸化的分子被结晶，其结构通过X射线晶体学解析。该蛋白由一个中央的五链β-折叠组成，两面都被螺旋覆盖。二级结构元件的顺序是（β-α）4，α-β。四条β-链以平行方式排列，链的顺序为2134，并通过形成右手交叉连接的螺旋相连。形成折叠一侧边缘且与其他链反平行的第五条链在二聚体结构的亚基之间进行了交换。螺旋D和E形成一个螺旋发夹。在催化过程中短暂磷酸化的组氨酸10位于结构的拓扑转换点，靠近亚基界面。其咪唑环与Asp67的埋藏侧链形成氢键。Asp67可能作为一个通用碱，从而增加组氨酸的亲核性。模型表明，共价结合的磷酰基将通过螺旋C的大偶极得到稳定。文中还讨论了IIA（Man）与含组氨酸的磷酸载体蛋白之间的假定相互作用。
