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Conformational transition of Sec machinery inferred from bacterial SecYE structures

机译：从细菌SecYE结构推断出Sec机械的构象转变

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摘要

Over 30% of proteins are secreted across or integrated into membranes. Their newly synthesized forms contain either cleavable signal sequences or non-cleavable membrane anchor sequences, which direct them to the evolutionarily conserved Sec translo-con (SecYEG in prokaryotes and Sec61, comprising α-, γ- and β-subunits, in eukaryotes). The translocon then functions as a protein-conducting channel. These processes of protein localization occur either at or after translation. In bacteria, the SecA ATPase drives post-translational translocation. The only high-resolution structure of a translocon available so far is that for SecYEβ from the archaeon Methanococcus jannaschii, which lacks SecA. Here we present the 3.2-A-resolution crystal structure of the SecYE translocon from a SecA-containing organism, Thermits thermophilus. The structure, solved as a complex with an anti-SecY Fab fragment, revealed a 'pre-open' state of SecYE, in which several transmem-brane helices are shifted, as compared to the previous SecYEβ structure, to create a hydrophobic crack open to the cytoplasm. Fab and SecA bind to a common site at the tip of the cytoplasmic domain of SecY. Molecular dynamics and disulphide mapping analyses suggest that the pre-open state might represent a SecYE conformational transition that is inducible by SecA binding. Moreover, we identified a SecA-SecYE interface that comprises SecA residues originally buried inside the protein, indicating that both the channel and the motor components of the Sec machinery undergo cooperative conformational changes on formation of the functional complex.
机译：超过30％的蛋白质跨膜分泌或整合到膜中。它们的新合成形式包含可裂解的信号序列或不可裂解的膜锚序列，将它们引导至进化保守的Sec反式con-con（原核生物中为SecYEG，真核生物中为α-，γ-和β-亚基的Sec61）。然后，translocon充当蛋白质传导通道。这些蛋白质定位过程在翻译时或翻译后发生。在细菌中，SecA ATPase驱动翻译后易位。迄今为止，仅有的转座子的高分辨率结构是来自詹氏甲烷球菌的古细菌SecYEβ，它缺乏SecA。在这里，我们介绍了一个来自SecA的有机体Thermits thermophilus的SecYE translocon的3.2-A分辨率晶体结构。该结构被解析为具有抗SecY Fab片段的复合物，揭示了SecYE的“预开放”状态，与先前的SecYEβ结构相比，其中多个跨膜螺旋移位，从而形成了疏水性裂缝到细胞质Fab和SecA结合到SecY的胞质结构域末端的共同位点。分子动力学和二硫化物作图分析表明，开放前状态可能代表SecA结合诱导的SecYE构象转变。此外，我们确定了一个SecA-SecYE界面，该界面包含最初埋在蛋白质内部的SecA残基，表明Sec机械的通道和运动组件在功能复合物形成时均发生协同构象变化。

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