Nakatsu Sumiho, Sagara Hiroshi, Sakai-Tagawa Yuko, Sugaya Norio, Noda Takeshi, Kawaoka Yoshihiro
Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo, Japan.
Medical Proteomics Laboratory, Institute of Medical Science, University of Tokyo, Tokyo, Japan.
mBio. 2016 Sep 6;7(5):e01248-16. doi: 10.1128/mBio.01248-16.
The genomes of influenza A and B viruses comprise eight segmented, single-stranded, negative-sense viral RNAs (vRNAs). Although segmentation of the virus genome complicates the packaging of infectious progeny into virions, it provides an evolutionary benefit in that it allows viruses to exchange vRNAs with other strains. Influenza A viruses are believed to package their eight different vRNAs in a specific manner. However, several studies have shown that many viruses are noninfectious and fail to package at least one vRNA. Therefore, the genome-packaging mechanism is not fully understood. In this study, we used electron microscopy to count the number of ribonucleoproteins (RNPs) inside the virions of different influenza A and B virus strains. All eight strains examined displayed eight RNPs arranged in a "7+1" configuration in which a central RNP was surrounded by seven RNPs. Three-dimensional analysis of the virions showed that at least 80% of the virions packaged all eight RNPs; however, some virions packaged only five to seven RNPs, with the exact proportion depending on the strain examined. These results directly demonstrate that most viruses package eight RNPs, but some do indeed package fewer. Our findings support the selective genome-packaging model and demonstrate the variability in the number of RNPs incorporated by virions, suggesting that the genome-packaging mechanism of influenza viruses is more flexible than previously thought.
The genomes of influenza A and B viruses contain segmented RNAs, which complicates genome packaging but provides the evolutionary advantage of allowing the exchange of individual genome segments with those of other strains. Some studies have shown that influenza A viruses package all eight genome segments in a specific manner, whereas others have shown that many virions are noninfectious and fail to package at least one genome segment. However, such viruses have never been directly observed. Here, we used electron microscopy to provide the first direct visual evidence of virions packaging an incomplete set of ribonucleoproteins. The percentage of these noninfectious virions varied from 0 to 20, depending on the virus strain, indicating that most virions package all eight genome segments. These results extend our knowledge about how infectious and noninfectious virions coordinate for successful virus infection.
甲型和乙型流感病毒的基因组由八个分段的单链负义病毒RNA（vRNA）组成。虽然病毒基因组的分段使感染性后代包装到病毒粒子中变得复杂，但它提供了一种进化优势，即允许病毒与其他毒株交换vRNA。据信甲型流感病毒以特定方式包装其八种不同的vRNA。然而，几项研究表明，许多病毒没有感染性，并且至少未能包装一种vRNA。因此，基因组包装机制尚未完全了解。在本研究中，我们使用电子显微镜来计数不同甲型和乙型流感病毒毒株的病毒粒子内核糖核蛋白（RNP）的数量。所检查的所有八种毒株均显示八个RNP以“7 + 1”构型排列，其中一个中央RNP被七个RNP包围。对病毒粒子的三维分析表明，至少80%的病毒粒子包装了所有八个RNP；然而，一些病毒粒子仅包装了五到七个RNP，具体比例取决于所检查的毒株。这些结果直接证明大多数病毒包装八个RNP，但有些确实包装得更少。我们的发现支持选择性基因组包装模型，并证明了病毒粒子掺入的RNP数量的变异性，表明流感病毒的基因组包装机制比以前认为的更灵活。
甲型和乙型流感病毒的基因组包含分段RNA，这使基因组包装变得复杂，但提供了允许与其他毒株的单个基因组片段交换的进化优势。一些研究表明，甲型流感病毒以特定方式包装所有八个基因组片段，而其他研究表明，许多病毒粒子没有感染性，并且至少未能包装一个基因组片段。然而，从未直接观察到此类病毒。在这里，我们使用电子显微镜提供了病毒粒子包装不完整核糖核蛋白集的首个直接视觉证据。这些无感染性病毒粒子的百分比从0到20不等，具体取决于病毒毒株，表明大多数病毒粒子包装了所有八个基因组片段。这些结果扩展了我们对感染性和无感染性病毒粒子如何协同实现成功病毒感染的认识。
