Fons Ryen D, Bogert Brigitte A, Hegde Ramanujan S
Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
J Cell Biol. 2003 Feb 17;160(4):529-39. doi: 10.1083/jcb.200210095. Epub 2003 Feb 10.
Although the transport of model proteins across the mammalian ER can be reconstituted with purified Sec61p complex, TRAM, and signal recognition particle receptor, some substrates, such as the prion protein (PrP), are inefficiently or improperly translocated using only these components. Here, we purify a factor needed for proper translocation of PrP and identify it as the translocon-associated protein (TRAP) complex. Surprisingly, TRAP also stimulates vectorial transport of many, but not all, other substrates in a manner influenced by their signal sequences. Comparative analyses of several natural signal sequences suggest that a dependence on TRAP for translocation is not due to any single physical parameter, such as hydrophobicity of the signal sequence. Instead, a functional property of the signal, efficiency of its post-targeting role in initiating substrate translocation, correlates inversely with TRAP dependence. Thus, maximal translocation independent of TRAP can only be achieved with a signal sequence, such as the one from prolactin, whose strong interaction with the translocon mediates translocon gating shortly after targeting. These results identify the TRAP complex as a functional component of the translocon and demonstrate that it acts in a substrate-specific manner to facilitate the initiation of protein translocation.
尽管模型蛋白跨哺乳动物内质网（ER）的转运可以通过纯化的Sec61p复合物、转运体相关膜蛋白（TRAM）和信号识别颗粒受体来重建，但某些底物，如朊病毒蛋白（PrP），仅使用这些组分进行转运时效率低下或转运不当。在这里，我们纯化了PrP正确转运所需的一个因子，并将其鉴定为转运体相关蛋白（TRAP）复合物。令人惊讶的是，TRAP还以受其信号序列影响的方式刺激许多（但不是全部）其他底物的向量转运。对几种天然信号序列的比较分析表明，对TRAP转运的依赖性并非由于任何单一物理参数，如信号序列的疏水性。相反，信号的一种功能特性，即其在启动底物转运中靶向作用后的效率，与对TRAP的依赖性呈负相关。因此，只有使用催乳素的信号序列才能实现不依赖TRAP的最大转运，该信号序列与转运体的强相互作用在靶向作用后不久介导转运体门控。这些结果将TRAP复合物鉴定为转运体的功能组分，并证明它以底物特异性方式促进蛋白质转运的起始。
