Wolin S L, Walter P
Department of Biochemistry and Biophysics, University of California Medical School, San Francisco, California 94143-0448.
J Cell Biol. 1993 Jun;121(6):1211-9. doi: 10.1083/jcb.121.6.1211.
Ribosomes synthesizing nascent secretory proteins are targeted to the membrane by the signal recognition particle (SRP), a small ribonucleoprotein that binds to the signal peptide as it emerges from the ribosome. SRP arrests further elongation, causing ribosomes to stack behind the arrested ribosome. Upon interaction of SRP with its receptor on the ER membrane, the translation arrest is released and the ribosome becomes bound to the ER membrane. We have examined the distribution of unattached and membrane-bound ribosomes during the translation of mRNAs encoding two secretory proteins, bovine preprolactin and rat preproinsulin I. We find that the enhancement of ribosome stacking that occurs when SRP arrests translation of these proteins is relaxed in the presence of microsomal membranes. We also demonstrate that two previously described populations of membrane-associated ribosomes, distinguished by their sensitivity to high salt or EDTA extraction, correspond to ribosomes that have synthesized differing lengths of the nascent polypeptide. This analysis has revealed that nascent chain insertion into the membrane begins at distinct points for different presecretory proteins.
正在合成新生分泌蛋白的核糖体通过信号识别颗粒（SRP）被靶向到膜上，SRP是一种小核糖核蛋白，当信号肽从核糖体出现时它会与之结合。SRP阻止进一步延伸，导致核糖体在被阻止的核糖体后面堆积。当SRP与其在内质网膜上的受体相互作用时，翻译阻止被解除，核糖体与内质网膜结合。我们研究了在编码两种分泌蛋白——牛前催乳素和大鼠前胰岛素I的mRNA翻译过程中游离核糖体和膜结合核糖体的分布。我们发现，当SRP阻止这些蛋白的翻译时发生的核糖体堆积增强在微粒体膜存在的情况下会松弛。我们还证明，之前描述的两类膜相关核糖体群体，根据它们对高盐或EDTA提取的敏感性来区分，对应于已经合成了不同长度新生多肽的核糖体。该分析表明，新生链插入膜的过程对于不同的前分泌蛋白始于不同的点。
