Garcia Ivelitza, Weeks Kevin M
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA.
Biochemistry. 2004 Dec 7;43(48):15179-86. doi: 10.1021/bi048626f.
Prior to folding to a native functional structure, many large RNAs form conformationally collapsed states. Formation of the near-native collapsed state for the bI5 group I intron RNA plays an obligatory role in self-chaperoning assembly with its CBP2 protein cofactor by preventing formation of stable, misassembled complexes. We show that the collapsed state is essential because CBP2 assembles indiscriminately with the bI5 RNA in any folding state to form long-lived complexes. The most stable protein interaction site in the expanded state-CBP2 complex overlaps, but is not identical to, the native site. Folding to the collapsed state circumvents two distinct misassembly events: inhibitory binding by multiple equivalents of CBP2 and formation of bridged complexes in which CBP2 straddles cognate and noncognate RNAs. Strikingly, protein-bound sites in the expanded state RNA complex are almost the inverse of native RNA-RNA and RNA-protein interactions, indicating that folding to the collapsed state significantly reduces the fraction of RNA surfaces accessible for misassembly. The self-chaperoning function for the bI5 collapsed state is likely to be conserved in other ribonucleoproteins where a protein cofactor binds tightly at a simple RNA substructure or has an RNA binding surface composed of multiple functional sites.
在折叠成天然功能结构之前，许多大型RNA会形成构象坍缩状态。I组内含子bI5 RNA形成近天然坍缩状态在与CBP2蛋白辅因子进行自伴侣组装过程中起着不可或缺的作用，它能防止形成稳定的错误组装复合物。我们发现坍缩状态至关重要，因为CBP2会与处于任何折叠状态的bI5 RNA随意组装形成长寿命复合物。扩展状态的CBP2复合物中最稳定的蛋白质相互作用位点与天然位点重叠，但并不相同。折叠成坍缩状态可避免两种不同的错误组装事件：多个当量的CBP2的抑制性结合以及形成桥接复合物，即CBP2跨接同源和非同源RNA。引人注目的是，扩展状态RNA复合物中蛋白质结合位点几乎与天然RNA-RNA和RNA-蛋白质相互作用相反，这表明折叠成坍缩状态会显著减少可用于错误组装的RNA表面比例。bI5坍缩状态的自伴侣功能可能在其他核糖核蛋白中保守，在这些核糖核蛋白中，蛋白质辅因子在简单的RNA亚结构处紧密结合或具有由多个功能位点组成的RNA结合表面。
