新和成的蛋白被定向到SecY/Sec61蛋白传导通道上，以便穿过细胞膜来转位。 本期Nature上两篇论文采用低温电子显微镜来了解这一重要蛋白转位过程的机制。Eunyong Park等人发表了不活跃的和活跃的细菌核糖体-通道复合物的结构；Marko Gogala等人发表了一个哺乳动物核糖体-通道复合物在闭合和半开启构形下的结构。这些结构显示，核糖体结合并不会造成该通道发生很大结构变化。相反，新和成蛋白的一个憎水区域的膜插入似乎会帮助该通道的一部分开启，同时确保该通道对于离子流动来说保持关闭。
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 (or 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 Methanocaldococcus 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 amino- and carboxy-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 entering the channel directly.