MRC Laboratory of Molecular Biology, Cambridge, United Kingdom.
Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, United States.
Elife. 2024 Jun 18;13:RP95814. doi: 10.7554/eLife.95814.
The protein translocon at the endoplasmic reticulum comprises the Sec61 translocation channel and numerous accessory factors that collectively facilitate the biogenesis of secretory and membrane proteins. Here, we leveraged recent advances in cryo-electron microscopy (cryo-EM) and structure prediction to derive insights into several novel configurations of the ribosome-translocon complex. We show how a transmembrane domain (TMD) in a looped configuration passes through the Sec61 lateral gate during membrane insertion; how a nascent chain can bind and constrain the conformation of ribosomal protein uL22; and how the translocon-associated protein (TRAP) complex can adjust its position during different stages of protein biogenesis. Most unexpectedly, we find that a large proportion of translocon complexes contains RAMP4 intercalated into Sec61's lateral gate, widening Sec61's central pore and contributing to its hydrophilic interior. These structures lead to mechanistic hypotheses for translocon function and highlight a remarkably plastic machinery whose conformations and composition adjust dynamically to its diverse range of substrates.
内质网上的蛋白转位通道由 Sec61 移位通道和许多辅助因子组成，它们共同促进分泌蛋白和膜蛋白的生物发生。在这里，我们利用冷冻电子显微镜（cryo-EM）和结构预测的最新进展，深入了解核糖体-转位通道复合物的几种新构象。我们展示了一个环构象中的跨膜结构域（TMD）如何在膜插入过程中穿过 Sec61 的侧向门；新生链如何结合并限制核糖体蛋白 uL22 的构象；以及转位通道相关蛋白（TRAP）复合物如何在蛋白质生物发生的不同阶段调整其位置。最令人惊讶的是，我们发现大量的转位通道复合物含有 RAMP4 插入 Sec61 的侧向门，从而扩大了 Sec61 的中心孔并有助于其亲水内部。这些结构为转位通道的功能提供了机制假设，并突出了一种非常灵活的机制，其构象和组成动态调整以适应其各种底物。
