Structure of the SecY channel during initiation of protein translocation.
Structure of the SecY channel during initiation of protein translocation.
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许多分泌蛋白通过信号序列靶向由原核 SecY 或真核 Sec61 复合物形成的蛋白传导通道，并在合成过程中跨膜转运。非活性通道的晶体结构表明，异源三聚体复合物的 SecY 亚基由两半组成，形成沙漏形孔，膜中部有一个收缩部，还有一个面向脂质相的侧门。封闭的通道有一个空的细胞质漏斗和一个充满小螺旋结构域（称为塞子）的细胞外漏斗。在易位起始过程中，核糖体-新生链复合物与 SecY/Sec61 复合物结合，导致新生链插入。然而，易位过程中通道开放的机制尚不清楚。在这里，我们通过使用冷冻电子显微镜确定非活性和活性核糖体通道复合物的结构来解决这个问题。源自詹氏甲烷球菌或大肠杆菌的非翻译核糖体-SecY 通道复合物显示该通道处于关闭状态，并表明核糖体结合本身仅引起微小的变化。活性大肠杆菌核糖体通道复合物的结构表明，新生链打开了通道，导致 SecY 的 N 端和 C 端半部大部分发生刚性体运动。在这个早期易位中间体中，多肽作为环插入 SecY 通道，疏水信号序列插入开放的侧门。新生链还在 SecY 的细胞质表面形成环，而不是直接进入通道。
Many secretory proteins are targeted by signal sequences to a protein-conducting channel, formed by prokaryotic SecY- or eukaryotic Sec61-complexes, and are translocated across the membrane during their synthesis. Crystal structures of the inactive channel show that the SecY subunit of the heterotrimeric complex consists of two halves that form an hourglass-shaped pore with a constriction in the middle of the membrane and a lateral gate that faces the lipid phase. The closed channel has an empty cytoplasmic funnel and an extracellular funnel that is filled with a small helical domain, called the plug. During initiation of translocation, a ribosome–nascent chain complex binds to the SecY/Sec61 complex, resulting in insertion of the nascent chain. However, the mechanism of channel opening during translocation is unclear. Here, we have addressed this question by determining structures of inactive and active ribosome–channel complexes with cryo-electron microscopy. Non-translating ribosome–SecY channel complexes derived from Methanococcus jannaschii or Escherichia coli show the channel in its closed state, and indicate that ribosome binding per se causes only minor changes. The structure of an active E. coli ribosome–channel complex demonstrates that the nascent chain opens the channel, causing mostly rigid body movements of the N- and C-terminal halves of SecY. In this early translocation intermediate, the polypeptide inserts as a loop into the SecY channel with the hydrophobic signal sequence intercalated into the open lateral gate. The nascent chain also forms a loop on the cytoplasmic surface of SecY rather than directly entering the channel.