2009年8月5日，北京生命科学研究所叶克穷实验室在《美国科学院院报》在线发表题为“Structural organization of box C/D RNA-guided RNA methyltransferase”的文章，该研究解析了C/D RNA蛋白质复合物的晶体结构，促进了对该复合物的组合方式和其修饰RNA甲基化机理的了解。
RNA分子不仅仅作为编码蛋白质的模板，许多不编码蛋白质的RNA在生命活动也有重要的功能。C/D RNA就是一类广泛存在于古细菌和真核生物中的非编码RNA，他们因为含有共同的保守序列模块C和D而得名。C/D RNA主要功能是指导RNA底物——如rRNA、snRNA和tRNA——发生特定位点上核糖的甲基化。C/D RNA含有一段向导序列，这段序列通过和修饰位点两边的碱基序列互补配对而实现对底物的特异性选择。在古细菌中，C/D RNA与RNA结合蛋白L7Ae、骨架蛋白Nop5和甲基化酶fibrillarin三个蛋白质亚基组成RNA-蛋白质复合体，该复合物能对底物进行特异位点的甲基化修饰。虽然科学家对该复合体的组装和功能研究已经持续了十多年时间，但目前还不了解复合物的整体结构。该研究通过结构生物学手段，使用X射线衍射技术解析了一个4埃分辨率的C/D RNP复合物晶体结构。该结构包含了全部三个蛋白亚基和半个C/D RNA，也是目前获得的最完整的C/D RNP复合物结构。该结构加深了对复合物组装方式的认识，发现C/D RNA是通过和L7Ae和Nop5的碳端结构域共同结合装配到复合物，进一步证实催化亚基fibrillarin和Nop5氮端结构域结合。作者发现不同结构中催化亚基占据了不同的位置，显示了该复合物结构内部的活动性，并推测催化亚基的运动可能是其结合底物过程中的必要步骤。另外结构提示Nop5碳端结构域上的一个结构突起可能影响C/D RNA的特异装配，以及C/D RNA和底物的结合。新的结构模型促进了对C/D RNP的组合方式和工作机理更深入的理解。（生物谷Bioon.com）
生物谷推荐原始出处：
PNAS August 5, 2009, doi: 10.1073/pnas.0905128106
Structural organization of box C/D RNA-guided RNA methyltransferase
Keqiong Yea,1, Ru Jiaa, Jinzhong Lina, Minghua Jua, Jin Penga, Anbi Xua and Liman Zhanga,b
aNational Institute of Biological Sciences, Beijing 102206, China; and
bGraduate Program at Chinese Academy of Medical Sciences and Peking union Medical College, Beijing 100730, China
Box C/D guide RNAs are abundant noncoding RNAs that primarily function to direct the 2′-O-methylation of specific nucleotides by base-pairing with substrate RNAs. In archaea, a bipartite C/D RNA assembles with L7Ae, Nop5, and the methyltransferase fibrillarin into a modification enzyme with unique substrate specificity. Here, we determined the crystal structure of an archaeal C/D RNA–protein complex (RNP) composed of all 3 core proteins and an engineered half-guide RNA at 4 ? resolution, as well as 2 protein substructures at higher resolution. The RNP structure reveals that the C-terminal domains of Nop5 in the dimeric complex provide symmetric anchoring sites for 2 L7Ae-associated kink-turn motifs of the C/D RNA. A prominent protrusion in Nop5 seems to be important for guide RNA organization and function and for discriminating the structurally related U4 snRNA. Multiple conformations of the N-terminal domain of Nop5 and its associated fibrillarin in different structures indicate the inherent flexibility of the catalytic module, suggesting that a swinging motion of the catalytic module is part of the enzyme mechanism. We also built a model of a native C/D RNP with substrate and fibrillarin in an active conformation. Our results provide insight into the overall organization and mechanism of action of C/D RNA–guided RNA methyltransferases.
