Hussain M M, Bakillah A, Jamil H
Department of Pathology, Allegheny University of the Health Sciences, Philadelphia, Pennsylvania 19129, USA.
Biochemistry. 1997 Oct 21;36(42):13060-7. doi: 10.1021/bi971395a.
Microsomal triglyceride transfer protein (MTP), a heterodimer of 97 kDa and protein disulfide isomerase, is required for the assembly of apolipoprotein B (apoB)-containing triglyceride-rich lipoproteins. These proteins have been shown to interact with each other during early stages of lipoprotein biosynthesis. Our studies indicated that binding between apoB and heterodimeric MTP was of high affinity (Kd 10-30 nM) due to ionic interactions. In contrast to MTP, protein disulfide isomerase alone interacted very poorly with lipoproteins, indicating the importance of the heterodimer in these bindings. Preincubation of lipoproteins with detergents enhanced their interaction with MTP. Native VLDL bound poorly to MTP, but its preincubation with Tween-20 resulted in significantly increased binding to MTP. Furthermore, binding of LDL was enhanced by preincubation with taurocholate, indicating that partial delipidation of apoB-containing lipoproteins results in increased binding to MTP. Subsequently, attempts were made to study interactions between C-terminally truncated apoB polypeptides and MTP. Binding of all the polypeptides to MTP was enhanced in the presence of taurocholate. Comparisons revealed that the binding of different apoB polypeptides to MTP was in the order of apoB18 > apoB28 > apoB42 > apoB100. These studies indicated that optimum interactions occur between apoB18 and MTP, and that the increase in apoB length beyond apoB18 has a negative effect on these interactions. Since apoB18 does not assemble triglyceride-rich lipoproteins, these studies suggest that apoB may interact with MTP before its lipidation. It is proposed that steps in lipoprotein biosynthesis may be dictated by the sequential display of different functional domains on the apoB polypeptide.
微粒体甘油三酯转运蛋白（MTP）是一种由97 kDa亚基和蛋白二硫键异构酶组成的异二聚体，是含载脂蛋白B（apoB）的富含甘油三酯脂蛋白组装所必需的。这些蛋白已被证明在脂蛋白生物合成的早期阶段相互作用。我们的研究表明，由于离子相互作用，apoB与异二聚体MTP之间的结合具有高亲和力（解离常数Kd为10 - 30 nM）。与MTP相反，单独的蛋白二硫键异构酶与脂蛋白的相互作用非常弱，这表明异二聚体在这些结合中很重要。脂蛋白与去污剂预孵育可增强它们与MTP的相互作用。天然极低密度脂蛋白（VLDL）与MTP的结合较差，但与吐温-20预孵育后，其与MTP的结合显著增加。此外，与牛磺胆酸盐预孵育可增强低密度脂蛋白（LDL）的结合，这表明含apoB脂蛋白的部分脱脂导致与MTP的结合增加。随后，尝试研究C末端截短的apoB多肽与MTP之间的相互作用。在牛磺胆酸盐存在下，所有多肽与MTP的结合均增强。比较发现，不同apoB多肽与MTP的结合顺序为apoB18 > apoB28 > apoB42 > apoB100。这些研究表明，apoB18与MTP之间发生最佳相互作用，并且apoB长度超过apoB18后增加对这些相互作用有负面影响。由于apoB18不能组装富含甘油三酯的脂蛋白，这些研究表明apoB在其脂化之前可能与MTP相互作用。有人提出，脂蛋白生物合成步骤可能由apoB多肽上不同功能域的顺序展示所决定。
