Blobel G, Dobberstein B
J Cell Biol. 1975 Dec;67(3):835-51. doi: 10.1083/jcb.67.3.835.
Fractionation of MOPC 41 DL-1 tumors revealed that the mRNA for the light chain of immunoglobulin is localized exclusively in membrane-bound ribosomes. It was shown that the translation product of isolated light chain mRNA in a heterologous protein-synthesizing system in vitro is larger than the authentic secreted light chain; this confirms similar results from several laboratories. The synthesis in vitro of a precursor protein of the light chain is not an artifact of translation in a heterologous system, because it was shown that detached polysomes, isolated from detergent-treated rough microsomes, not only contain nascent light chains which have already been proteolytically processed in vivo but also contain unprocessed nascent light chains. In vitro completion of these nascent light chains thus resulted in the synthesis of some chains having the same mol wt as the authentic secreted light chains, because of completion of in vivo proteolytically processed chains and of other chains which, due to the completion of unprocessed chains, have the same mol wt as the precursor of the light chain. In contrast, completion of the nascent light chains contained in rough microsomes resulted in the synthesis of only processed light chains. Taken together, these results indicate that the processing activity is present in isolated rough microsomes, that it is localized in the membrane moiety of rough microsomes, and, therefore, that it was most likely solubilized during detergent treatment used for the isolation of detached polysomes. Furthermore, these results established that processing in vivo takes place before completion of the nascent chain. The data also indicate that in vitro processing of nascent chains by rough microsomes is dependent on ribosome binding to the membrane. If the latter process is interfered with by aurintricarboxylic acid, rough microsomes also synthesize some unprocessed chains. The data presented in this paper have been interpreted in the light of a recently proposed hypothesis. This hypothesis, referred to as the signal hypothesis, is described in greater detail in the Discussion section.
对MOPC 41 DL-1肿瘤进行分级分离发现，免疫球蛋白轻链的mRNA仅定位在膜结合核糖体中。结果表明，在体外异源蛋白质合成系统中，分离出的轻链mRNA的翻译产物比真正分泌的轻链大；这证实了几个实验室的类似结果。轻链前体蛋白的体外合成不是异源系统中翻译的假象，因为已表明，从经去污剂处理的糙面微粒体中分离出的游离多核糖体，不仅含有在体内已进行蛋白水解加工的新生轻链，还含有未加工的新生轻链。因此，这些新生轻链的体外完成导致合成了一些与真正分泌的轻链具有相同分子量的链，这是由于体内蛋白水解加工链的完成以及其他链的完成，这些未加工链的完成使得它们与轻链前体具有相同的分子量。相反，糙面微粒体中所含新生轻链的完成仅导致加工后轻链的合成。综上所述，这些结果表明，加工活性存在于分离出的糙面微粒体中，它定位于糙面微粒体的膜部分，因此，很可能在用于分离游离多核糖体的去污剂处理过程中被溶解。此外，这些结果表明，体内加工在新生链完成之前发生。数据还表明，糙面微粒体对新生链的体外加工依赖于核糖体与膜的结合。如果后一过程受到金精三羧酸的干扰，糙面微粒体也会合成一些未加工的链。本文所呈现的数据已根据最近提出的一个假说来进行解释。这个假说被称为信号假说，将在讨论部分更详细地描述。
