Harano T, Omura T
J Biochem. 1977 Dec;82(6):1551-7. doi: 10.1093/oxfordjournals.jbchem.a131849.
The direction of discharge of the nascent peptides of NADPH-cytochrome c reductase and cytochrome b5 from bound polyribosomes of rough microsomes was investigated in order to elucidate the mechanism of separation of these membrane proteins from secretory proteins, which are also synthesized by the same class of ribosomes of rough endoplasmic reticulum. The nascent peptides of NADPH-cytochrome c reductase and cytochrome b5 in intact rough microsomes were accessible to externally added 125I-Fab's against these proteins, and were susceptible to trypsin digestion, whereas the nascent peptides of serum albumin were not. The nascent peptides of these two microsomal proteins were released into the cytoplasm by puromycin treatment of intact rough microsomes, while the nascent peptides of serum albumin were retained in the microsomal lumen. These observations suggest that the nascent peptides of microsomal proteins, which are present on the cytoplasmic surface of the endoplasmic reticulum membrane, are exposed on the surface of microsomal vesicles, while those of secretory proteins are enclosed inside the vesicles. Therefore, the topographical separation of microsomal membrane proteins from secretory proteins is accomplished at the step of their synthesis by the bound polyribosomes of rough endoplasmic reticulum.
为了阐明这些膜蛋白与同样由粗面内质网的同一类核糖体合成的分泌蛋白的分离机制，研究了NADPH-细胞色素c还原酶和细胞色素b5新生肽从粗面微粒体结合多核糖体的排出方向。完整粗面微粒体中NADPH-细胞色素c还原酶和细胞色素b5的新生肽可被外源添加的针对这些蛋白的125I-Fab片段识别，且易被胰蛋白酶消化，而血清白蛋白的新生肽则不然。用嘌呤霉素处理完整的粗面微粒体，这两种微粒体蛋白的新生肽被释放到细胞质中，而血清白蛋白的新生肽则保留在微粒体腔中。这些观察结果表明，内质网膜细胞质表面存在的微粒体蛋白新生肽暴露在微粒体小泡表面，而分泌蛋白的新生肽则被包裹在小泡内部。因此，粗面内质网结合多核糖体在合成步骤中实现了微粒体膜蛋白与分泌蛋白的拓扑学分离。
