2012年1月10日，PNAS在线发表了中国科学院生物物理研究所朱平研究组程凌鹏副研究员等人的研究论文“Atomic model of a cypovirus built from cryo-EM structure provides insight into the mechanism of mRNA capping”。该研究利用生物成像技术实验室2010年4月调试成功的冷冻电镜平台，用单颗粒图像处理技术获得了呼肠孤病毒科的质型多角体病毒近原子分辨率的三维结构（3.9埃），并独立构建了全原子模型。对研究dsRNA病毒的mRNA加帽机制有重要意义。（详见PNAS：呼肠孤病毒科的质型多角体病毒近原子分辨率的三维结构）
近日，程凌鹏等人利用之前的研究，深入研究了质型多角体病毒在转录过程中的结构，于4月6日再次在PNAS在线发表了一篇名为“Cryo-EM structure of a transcribing cypovirus”的论文。
已知呼肠病毒科家族中的双链RNA病毒能够转录及加帽初期mRNA。有趣的是，整个的转录周期，二十面体病毒的衣壳一直保持完整，但是mRNA转录及加帽过程是被病毒衣壳蛋白协调的机制还未可知。质型多角体病毒属则提供了一个好的模式系统，来研究呼肠病毒科家族的mRNA转录及加帽机制。
研究人员通过冷冻电子显微镜技术得到的一个近乎原子分辨率的密度图，得到了一个转录中的质型多角体病毒的完整主干模型。对比于非转录中的质型多角体病毒属的结构，转录中的质型多角体病毒的主要衣壳蛋白经历了一系列的构象改变，这导致了衣壳的结构改变：一方面是增大的衣壳腔，在紧密包装的衣壳中，这对基因组RNA提供了更多的移灵活性；另一方面是衣壳内扩大的peripentonal通道，研究表明这是初期mRNA的通道。
除此以外，在每一个五倍轴turret蛋白的构象改变导致了一个相对开放的turret。研究还发现，在mRNA加帽期间，一个被转移到5'端去磷酸化mRNA的GMP，被发现在turret蛋白口袋样结构的尿苷转移酶结构域。（生物谷Deepblue编译）
doi: 10.1073/pnas.1200206109
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Cryo-EM structure of a transcribing cypovirus
Chongwen Yanga, Gang Jia, Hongrong Liub, Kai Zhanga, Guangqiao Liua, Fei Suna, Ping Zhua, and Lingpeng Chenga.
Double-stranded RNA viruses in the family Reoviridae are capable of transcribing and capping nascent mRNA within an icosahedral viral capsid that remains intact throughout repeated transcription cycles.However, how the highly coordinated mRNA transcription and capping process is facilitated by viral capsid proteins is still unknown. Cypovirus provides a good model system for studying the mRNA transcription and capping mechanism of viruses in the family Reoviridae.Here, we report a full backbone model of a transcribing cypovirus built from a near-atomic-resolution density map by cryoelectron microscopy.Compared with the structure of a nontranscribing cypovirus, the major capsid proteins of transcribing cypovirus undergo a series of conformational changes, giving rise to structural changes in the capsid shell: (i) an enlarged capsid chamber, which provides genomic RNA with more flexibility to move within the densely packed capsid, and (ii) a widened peripentonal channel in the capsid shell, which we confirmed to be a pathway for nascent mRNA.A rod-like structure attributable to a partially resolved nascent mRNA was observed in this channel.In addition, conformational change in the turret protein results in a relatively open turret at each fivefold axis.A GMP moiety, which is transferred to 5’-diphosphorylated mRNA during the mRNA capping reaction, was identified in the pocket-like guanylyltransferase domain of the turret protein.
