Itakura Eisuke, Zavodszky Eszter, Shao Sichen, Wohlever Matthew L, Keenan Robert J, Hegde Ramanujan S
MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK; Department of Biology, Faculty of Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan.
MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK.
Mol Cell. 2016 Jul 7;63(1):21-33. doi: 10.1016/j.molcel.2016.05.020. Epub 2016 Jun 23.
We investigated how mitochondrial membrane proteins remain soluble in the cytosol until their delivery to mitochondria or degradation at the proteasome. We show that Ubiquilin family proteins bind transmembrane domains in the cytosol to prevent aggregation and temporarily allow opportunities for membrane targeting. Over time, Ubiquilins recruit an E3 ligase to ubiquitinate bound clients. The attached ubiquitin engages Ubiquilin's UBA domain, normally bound to an intramolecular UBL domain, and stabilizes the Ubiquilin-client complex. This conformational change precludes additional chances at membrane targeting for the client, while simultaneously freeing Ubiquilin's UBL domain for targeting to the proteasome. Loss of Ubiquilins by genetic ablation or sequestration in polyglutamine aggregates leads to accumulation of non-inserted mitochondrial membrane protein precursors. These findings define Ubiquilins as a family of chaperones for cytosolically exposed transmembrane domains and explain how they use ubiquitin to triage clients for degradation via coordinated intra- and intermolecular interactions.
我们研究了线粒体膜蛋白如何在细胞溶质中保持可溶状态，直至被转运到线粒体或在蛋白酶体中降解。我们发现泛素连接蛋白家族蛋白在细胞溶质中与跨膜结构域结合，以防止聚集，并暂时为膜靶向提供机会。随着时间的推移，泛素连接蛋白招募一种E3连接酶对结合的底物进行泛素化。附着的泛素与泛素连接蛋白的UBA结构域结合，该结构域通常与分子内的UBL结构域结合，并稳定泛素连接蛋白-底物复合物。这种构象变化排除了底物再次进行膜靶向的机会，同时释放泛素连接蛋白的UBL结构域以靶向蛋白酶体。通过基因敲除或在多聚谷氨酰胺聚集体中隔离泛素连接蛋白会导致未插入的线粒体膜蛋白前体积累。这些发现将泛素连接蛋白定义为一类针对细胞溶质中暴露的跨膜结构域的伴侣蛋白，并解释了它们如何通过协调的分子内和分子间相互作用利用泛素来分类底物进行降解。
