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标题：Metal dependence and branched RNA cocrystal structures of the RNA lariat debranching enzyme Dbr1
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作者：Nathaniel E. Clark ; Adam Katolik ; Kenneth M. Roberts 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：51
页码：14727-14732
DOI：10.1073/pnas.1612729114
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe RNA lariat debranching enzyme Dbr1 cleaves the 2',5'-phosphodiester linkages in intron lariats generated during pre-mRNA splicing. The enzyme is central to RNA metabolism because its activity is required for intron turnover and for the production of small nucleolar RNAs and microRNAs encoded in intronic RNA. Here, the kinetics of Dbr1-mediated debranching of a synthetic RNA substrate are measured by using apoenzyme reconstituted with various divalent cations. The results suggest Fe and Zn are preferred cofactors. Structures of a binuclear catalytic mutant in complex with bona fide branched RNAs reveal a metal-bridging hydroxide positioned to attack the scissile phosphate. The results clarify structure/function relationships in Dbr1 enzymes and are guiding the search for inhibitors that hold promise as therapies for retroviral infections and neurodegenerative disease. Intron lariats are circular, branched RNAs (bRNAs) produced during pre-mRNA splicing. Their unusual chemical and topological properties arise from branch-point nucleotides harboring vicinal 2',5'- and 3',5'-phosphodiester linkages. The 2',5'-bonds must be hydrolyzed by the RNA debranching enzyme Dbr1 before spliced introns can be degraded or processed into small nucleolar RNA and microRNA derived from intronic RNA. Here, we measure the activity of Dbr1 from Entamoeba histolytica by using a synthetic, dark-quenched bRNA substrate that fluoresces upon hydrolysis. Purified enzyme contains nearly stoichiometric equivalents of Fe and Zn per polypeptide and demonstrates turnover rates of [~]3 s-1. Similar rates are observed when apo-Dbr1 is reconstituted with Fe(II)+Zn(II) under aerobic conditions. Under anaerobic conditions, a rate of [~]4.0 s-1 is observed when apoenzyme is reconstituted with Fe(II). In contrast, apo-Dbr1 reconstituted with Mn(II) or Fe(II) under aerobic conditions is inactive. Diffraction data from crystals of purified enzyme using X-rays tuned to the Fe absorption edge show Fe partitions primarily to the {beta}-pocket and Zn to the -pocket. Structures of the catalytic mutant H91A in complex with 7-mer and 16-mer synthetic bRNAs reveal bona fide RNA branchpoints in the Dbr1 active site. A bridging hydroxide is in optimal position for nucleophilic attack of the scissile phosphate. The results clarify uncertainties regarding structure/function relationships in Dbr1 enzymes, and the fluorogenic probe permits high-throughput screening for inhibitors that may hold promise as treatments for retroviral infections and neurodegenerative disease.
关键词：RNA debranching ; intron lariat ; enzyme kinetics ; X-ray crystallography ; Dbr1