Kivlen M H, Dorsey C A, Lingappa V R, Hegde R S
Department of Physiology, University of California, San Francisco 94143, USA.
J Lipid Res. 1997 Jun;38(6):1149-62.
Lipoprotein assembly requires a complex and regulated set of events that includes apolipoprotein B (apoB) translocation across the endoplasmic reticulum (ER) membrane, folding, and association with lipids. Unlike simple secretory proteins which are cotranslationally translocated directly into the ER lumen, nascent apoB contains pause transfer (PT) sequences that direct the transient stopping and subsequent restarting of its translocation, a phenomenon termed translocational pausing. During one particular translocational pause in apoB, the ribosome-membrane junction and ER translocation channel have been shown to be altered in such a way as to expose the nascent polypeptide to the cytosol and direct a change in the proteins neighboring the nascent chain. In this study, we have experimentally identified the location and distribution of the translocational pauses that are present throughout apoB-100. We find that pause transfer sequences are distributed asymmetrically, clustering in three distinct domains: a) nine functional PT sequences appear in the amino terminal 20% of apoB, b) four more PT sequences occur just before the end of apoB-48, and c) an additional ten PT sequences are found between apoB-65-95. These clusters are interrupted by two lipid binding regions of approximately 100 kD each in which no PT sequences occur. The implications of this asymmetric distribution of PT sequences, and their correlation with previously hypothesized structural and functional domains of apoB, are discussed.
脂蛋白组装需要一系列复杂且受调控的事件，其中包括载脂蛋白B（apoB）跨内质网（ER）膜的转运、折叠以及与脂质的结合。与简单的分泌蛋白不同，简单分泌蛋白在共翻译过程中直接转运到内质网腔中，新生的apoB含有暂停转移（PT）序列，这些序列会导致其转运暂时停止并随后重新开始，这种现象称为转运暂停。在apoB的一次特定转运暂停期间，核糖体 - 膜连接点和内质网转运通道已被证明发生了改变，使得新生多肽暴露于细胞质中，并导致新生链附近蛋白质发生变化。在本研究中，我们通过实验确定了apoB - 100中存在的转运暂停的位置和分布。我们发现暂停转移序列分布不对称，聚集在三个不同的区域：a）九个功能性PT序列出现在apoB氨基末端的20%区域；b）在apoB - 48末端之前还有四个PT序列；c）在apoB - 65 - 95之间还发现了另外十个PT序列。这些簇被两个各自约100 kD的脂质结合区域打断，在这些区域中没有PT序列。本文讨论了PT序列这种不对称分布的意义，以及它们与先前假设的apoB结构和功能域的相关性。
