Li Jade, Leung Adelaine K, Kondo Yasushi, Oubridge Chris, Nagai Kiyoshi
Medical Research Council Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, England.
Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Saskatchewan S7N 5B4, Canada.
Acta Crystallogr D Struct Biol. 2016 Jan;72(Pt 1):131-46. doi: 10.1107/S2059798315022111. Epub 2016 Jan 1.
The core domain of small nuclear ribonucleoprotein (snRNP), comprised of a ring of seven paralogous proteins bound around a single-stranded RNA sequence, functions as the assembly nucleus in the maturation of U1, U2, U4 and U5 spliceosomal snRNPs. The structure of the human U4 snRNP core domain was initially solved at 3.6 Å resolution by experimental phasing using data with tetartohedral twinning. Molecular replacement from this model followed by density modification using untwinned data recently led to a structure of the minimal U1 snRNP at 3.3 Å resolution. With the latter structure providing a search model for molecular replacement, the U4 core-domain structure has now been re-refined. The U4 Sm site-sequence AAUUUUU has been shown to bind to the seven Sm proteins SmF-SmE-SmG-SmD3-SmB-SmD1-SmD2 in an identical manner as the U1 Sm-site sequence AAUUUGU, except in SmD1 where the bound U replaces G. The progression from the initial to the re-refined structure exemplifies a tortuous route to accuracy: where well diffracting crystals of complex assemblies are initially unavailable, the early model errors are rectified by exploiting preliminary interpretations in further experiments involving homologous structures. New insights are obtained from the more accurate model.
小核核糖核蛋白（snRNP）的核心结构域由围绕单链RNA序列结合的七个同源蛋白环组成，在U1、U2、U4和U5剪接体snRNP的成熟过程中作为组装核发挥作用。人类U4 snRNP核心结构域的结构最初通过使用具有四面体孪晶的数据进行实验相位解析，分辨率为3.6 Å。基于该模型进行分子置换，随后使用非孪晶数据进行密度修正，最近得到了分辨率为3.3 Å的最小U1 snRNP结构。利用后者的结构作为分子置换的搜索模型，U4核心结构域的结构现已重新精制。已证明U4 Sm位点序列AAUUUUU与七个Sm蛋白SmF-SmE-SmG-SmD3-SmB-SmD1-SmD2的结合方式与U1 Sm位点序列AAUUUGU相同，只是在SmD1中结合的U取代了G。从最初的结构到重新精制的结构的进展体现了一条通向精确性的曲折道路：在最初无法获得复杂组装体的良好衍射晶体的情况下，通过在涉及同源结构的进一步实验中利用初步解释来纠正早期模型误差。从更精确的模型中获得了新的见解。
