Bostina Mihnea, Mohsin Brigitte, Kühlbrandt Werner, Collinson Ian
Department of Structural Biology, Max-Planck-Institute of Biophysics, 60439 Frankfurt am Main, Germany.
J Mol Biol. 2005 Oct 7;352(5):1035-43. doi: 10.1016/j.jmb.2005.08.005.
The Sec complex forms the core of a conserved machinery transporting proteins across or into membranes. In Escherichia coli SecYEG is active as an oligomer, but the structure predicts that the protein-conducting channel is formed by the monomer. A homology model of the E.coli complex was built using the atomic structure of Methanococcus jannaschii SecYEbeta. Another structure of the membrane-bound dimer was then determined by fitting the homology model to an 8A map of SecYEG determined by electron microscopy. We found that the substrate-binding site of the dimer has opened slightly and the plug domain moved toward the outside. This new position retains the channel in a closed state. These differences partially reflect the movements that have been proposed to occur during channel gating. Further opening of the substrate-binding pocket to bind and release bound substrate and displacement of the plug during secretion, presumably rely on the action of the partner proteins. The contacts arising at the dimer interface in the environment of the lipid bilayer may have activated the assembly.
Sec复合物构成了一种保守机制的核心，该机制负责将蛋白质转运穿过膜或转运到膜内。在大肠杆菌中，SecYEG以寡聚体形式发挥作用，但结构预测蛋白质传导通道由单体形成。利用詹氏甲烷球菌SecYEβ的原子结构构建了大肠杆菌复合物的同源模型。然后，通过将同源模型拟合到电子显微镜测定的SecYEG的8埃图谱上，确定了膜结合二聚体的另一种结构。我们发现二聚体的底物结合位点略有打开，塞子结构域向外侧移动。这个新位置使通道保持关闭状态。这些差异部分反映了在通道门控过程中可能发生的运动。底物结合口袋的进一步打开以结合和释放结合的底物，以及在分泌过程中塞子的移位，大概依赖于伴侣蛋白的作用。在脂质双层环境中，二聚体界面处产生的接触可能激活了组装。
