Johnson A E, van Waes M A
Department of Medical Biochemistry, Texas A&M University, College Station 77843, USA.
Annu Rev Cell Dev Biol. 1999;15:799-842. doi: 10.1146/annurev.cellbio.15.1.799.
Cotranslational protein translocation across and integration into the membrane of the endoplasmic reticulum (ER) occur at sites termed translocons. Translocons are composed of several ER membrane proteins that associate to form an aqueous pore through which secretory proteins and lumenal domains of membrane proteins pass from the cytoplasm to the ER lumen. These sites are not passive holes in the bilayer, but instead are quite dynamic both structurally and functionally. Translocons cycle between ribosome-bound and ribosome-free states, and convert between translocation and integration modes of operation. These changes in functional state are accompanied by structural rearrangements that alter translocon conformation, composition, and interactions with ligands such as the ribosome and BiP. Recent studies have revealed that the translocon is a complex and sophisticated molecular machine that regulates the movement of polypeptides through the bilayer, apparently in both directions as well as laterally into the bilayer, all while maintaining the membrane permeability barrier.
共翻译的蛋白质跨内质网（ER）膜转运并整合到内质网膜上，发生在称为易位子的位点。易位子由几种内质网膜蛋白组成，这些蛋白相互结合形成一个水相孔道，分泌蛋白和膜蛋白的腔结构域通过该孔道从细胞质进入内质网腔。这些位点并非双层膜上的被动孔洞，而是在结构和功能上都相当动态。易位子在核糖体结合状态和无核糖体状态之间循环，并在转运和整合操作模式之间转换。功能状态的这些变化伴随着结构重排，从而改变易位子的构象、组成以及与核糖体和BiP等配体的相互作用。最近的研究表明，易位子是一个复杂而精密的分子机器，它调节多肽穿过双层膜的运动，显然是双向的，以及横向进入双层膜，同时维持膜的通透性屏障。
