Department of Biomedical Sciences, University of Ulsan College of Medicine, Seoul 05505, Korea.
Asian Medical Institute of Convergence Science and Technology, Asan Medical Center, Seoul 05505, Korea.
Cells. 2020 Feb 24;9(2):518. doi: 10.3390/cells9020518.
Protein import across the endoplasmic reticulum membrane is physiologically regulated in a substrate-selective manner to ensure the protection of stressed ER from the overload of misfolded proteins. However, it is poorly understood how different types of substrates are accurately distinguished and disqualified during translocational regulation. In this study, we found poorly assembled translocon-associated protein (TRAP) complexes in stressed ER. Immunoaffinity purification identified calnexin in the TRAP complex in which poor assembly inhibited membrane insertion of the prion protein (PrP) in a transmembrane sequence-selective manner, through translocational regulation. This reaction was induced selectively by redox perturbation, rather than calcium depletion, in the ER. The liberation of ERp57 from calnexin appeared to be the reason for the redox sensitivity. Stress-independent disruption of the TRAP complex prevented a pathogenic transmembrane form of PrP (ctmPrP) from accumulating in the ER. This study uncovered a previously unappreciated role for calnexin in assisting the redox-sensitive function of the TRAP complex and provided insights into the ER stress-induced reassembly of translocon auxiliary components as a key mechanism by which protein translocation acquires substrate selectivity.
内质网膜上的蛋白输入以底物选择性的方式进行生理调节，以确保受应激的内质网免受错误折叠蛋白的过载的保护。然而，人们对不同类型的底物在翻译调节过程中如何被准确区分和排除知之甚少。在这项研究中，我们在内质网应激时发现了组装不良的易位子相关蛋白（TRAP）复合物。免疫亲和纯化鉴定了 TRAP 复合物中的钙连蛋白，其中组装不良以跨膜序列选择性的方式抑制了朊病毒蛋白（PrP）的膜插入，这是通过翻译调节实现的。这种反应是由内质网中的氧化还原扰动而不是钙耗竭选择性诱导的。ERp57 从钙连蛋白中的释放似乎是氧化还原敏感性的原因。应激非依赖性的 TRAP 复合物的破坏阻止了致病性跨膜形式的 PrP（ctmPrP）在 ER 中的积累。这项研究揭示了钙连蛋白在协助 TRAP 复合物的氧化还原敏感功能方面的先前未被认识的作用，并深入了解了内质网应激诱导的易位子辅助成分的重新组装，这是蛋白质易位获得底物选择性的关键机制。
