Instituto de Biología Funcional y Genómica, Consejo Superior de Investigaciones Científicas/Universidad de Salamanca, Edificio Departamental, Avenida del Campo Charro, Salamanca 37007, Spain.
Proc Natl Acad Sci U S A. 2011 Oct 25;108(43):17667-71. doi: 10.1073/pnas.1111900108. Epub 2011 Oct 10.
The 5' cap structure (m(7)GpppX-) is an essential feature of eukaryotic mRNA required for mRNA stability and efficient translation. Influenza virus furnishes its mRNA with this structure by a cap-snatching mechanism, in which the viral polymerase cleaves host mRNA endonucleolytically 10-13 nucleotides from the 5' end and utilizes the capped fragment as a primer to synthesize viral transcripts. Here we report a unique cap-snatching mechanism by which the yeast double-stranded RNA totivirus L-A furnishes its transcript with a cap structure derived from mRNA. Unlike influenza virus, L-A transfers only m(7)Gp from the cap donor to the 5' end of the viral transcript, thus preserving the 5' α- and β-phosphates of the transcript in the triphosphate linkage of the final product. This in vitro capping reaction requires His154 of the coat protein Gag, a residue essential for decapping of host mRNA and known to form m(7)Gp-His adduct. Furthermore, the synthesis of capped viral transcripts in vivo and their expression were greatly compromised by the Arg154 mutation, indicating the involvement of Gag in the cap-snatching reaction. The overall reaction and the structure around the catalytic site in Gag resemble those of guanylyltransferase, a key enzyme of cellular mRNA capping, suggesting convergent evolution. Given that Pol of L-A is confined inside the virion and unable to access host mRNA in the cytoplasm, the structural protein Gag rather than Pol catalyzing this unique cap-snatching reaction exemplifies the versatility as well as the adaptability of eukaryotic RNA viruses.
5' 帽结构 (m(7)GpppX-) 是真核 mRNA 的必需特征，对于 mRNA 的稳定性和翻译效率至关重要。流感病毒通过帽抢夺机制为其 mRNA 提供这种结构，在该机制中，病毒聚合酶在距 5' 端 10-13 个核苷酸处对内切切割宿主 mRNA，并利用加帽片段作为引物合成病毒转录物。在这里，我们报告了一种独特的帽抢夺机制，酵母双链 RNA 病毒 L-A 利用该机制为其转录物提供源自 mRNA 的帽结构。与流感病毒不同，L-A 仅将 m(7)Gp 从帽供体转移到病毒转录物的 5' 端，从而在最终产物的三磷酸连接中保留转录物的 5'α-和β-磷酸。这种体外加帽反应需要衣壳蛋白 Gag 的 His154，这是宿主 mRNA 脱帽所必需的残基，并且已知它形成 m(7)Gp-His 加合物。此外，Arg154 突变极大地损害了体内有帽病毒转录物的合成及其表达，表明 Gag 参与了帽抢夺反应。总体反应和 Gag 中催化位点周围的结构类似于细胞 mRNA 加帽的关键酶鸟苷转移酶，表明趋同进化。鉴于 L-A 的 Pol 被限制在病毒粒子内，并且无法在细胞质中访问宿主 mRNA，结构蛋白 Gag 而不是 Pol 催化这种独特的帽抢夺反应，这体现了真核 RNA 病毒的多功能性和适应性。
