Chuck S L, Lingappa V R
Department of Microbiology and Immunology, Physiology, University of California, San Francisco 94143-0444.
J Biol Chem. 1993 Oct 25;268(30):22794-801.
In contrast to typical secretory proteins, apolipoprotein B pauses at distinct points along the nascent chain during its translocation into the lumen of the endoplasmic reticulum. Specific pause transfer sequences mediate such discrete pauses in the translocation of apolipoprotein B. These sequences have been shown to confer this translocational behavior to heterologous chimeric proteins. To investigate the function of pause transfer sequences, we: (i) examine whether the multiple pause transfer sequences of apolipoprotein B act independently or are dependent upon the action of upstream pause transfer sequences, (ii) identify residues of the prototypical B' pause transfer sequence that are involved in pausing, and (iii) determine whether the stopping step of a translocational pause is a consequence of translational pausing, as has been suggested by other investigators. We conclude that pause transfer sequences act independently of each other and of translation; translocational pausing occurs even in the absence of ongoing protein synthesis. Furthermore, like other topogenic sequences such as signal sequences, pause transfer sequences are degenerate in structure yet have distinctive features necessary for their action. This characterization of the B' pause transfer sequence may aid in the identification of such sequences elsewhere in apolipoprotein B and in other proteins and has implications for the mechanism of translocational pausing.
与典型的分泌蛋白不同，载脂蛋白B在转运至内质网腔的过程中，会在新生肽链的不同位点暂停。特定的暂停转移序列介导了载脂蛋白B转运过程中的这种离散暂停。这些序列已被证明能赋予异源嵌合蛋白这种转运行为。为了研究暂停转移序列的功能，我们：（i）检查载脂蛋白B的多个暂停转移序列是独立起作用还是依赖于上游暂停转移序列的作用，（ii）确定原型B'暂停转移序列中参与暂停的残基，以及（iii）确定转运暂停的终止步骤是否如其他研究者所提出的那样是翻译暂停的结果。我们得出结论，暂停转移序列彼此独立且与翻译无关；即使在没有正在进行的蛋白质合成的情况下也会发生转运暂停。此外，与其他拓扑序列如信号序列一样，暂停转移序列在结构上是简并的，但具有其作用所必需的独特特征。对B'暂停转移序列的这种表征可能有助于在载脂蛋白B和其他蛋白质的其他部位鉴定此类序列，并对转运暂停的机制具有启示意义。
