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Architecture of ribonucleoprotein complexes in influenza A virus particles


In viruses, as in eukaryotes, elaborate mechanisms have evolved to protect the genome and to ensure its timely replication and reliable transmission to progeny. Influenza A viruses are enveloped, spherical or filamentous structures, ranging from 80 to 120 nm in diameter1. Inside each envelope is a viral genome consisting of eight single-stranded negative-sense RNA segments of 890 to 2,341 nucleotides each1. These segments are associated with nucleoprotein and three polymerase subunits, designated PA, PB1 and PB2; the resultant ribonucleoprotein complexes (RNPs) resemble a twisted rod (10–15 nm in width and 30–120 nm in length) that is folded back and coiled on itself2,3,4. Late in viral infection, newly synthesized RNPs are transported from the nucleus to the plasma membrane, where they are incorporated into progeny virions capable of infecting other cells. Here we show, by transmission electron microscopy of serially sectioned virions, that the RNPs of influenza A virus are organized in a distinct pattern (seven segments of different lengths surrounding a central segment). The individual RNPs are suspended from the interior of the viral envelope at the distal end of the budding virion and are oriented perpendicular to the budding tip. This finding argues against random incorporation of RNPs into virions5, supporting instead a model in which each segment contains specific incorporation signals that enable the RNPs to be recruited and packaged as a complete set6,7,8,9,10,11,12. A selective mechanism of RNP incorporation into virions and the unique organization of the eight RNP segments may be crucial to maintaining the integrity of the viral genome during repeated cycles of replication.

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Figure 1: Budding virions show a specific arrangement of eight rod-like structures of different lengths.
Figure 2: Rod-like structures in a developing virion.
Figure 3: Identification of rod-like structures in virions as viral RNPs.
Figure 4: Electron tomography of RNP complexes in a budding virion.


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We thank J. Gilbert for editing the manuscript; M. Imai, Y. Muramoto and K. Fujii for discussion; Y. Hirata, K. Aoyama and K. Inoke for technical assistance with electron microscopic tomography; and Y. Kawaoka for illustrations. This work was supported by CREST (Japan Science and Technology Agency), by Grants-in-Aid by the Ministry of Education, Culture, Sports, Science and Technology, by the Ministry of Health, Labor and Welfare, Japan, and by a National Institute of Allergy and Infectious Disease Public Health Service research grant (to Y.K.); and by Swedish Research Council grants and the STINT Foundation (to R.H.C.). T.N. was the recipient of a fellowship from the incorporated foundation SYOUSHISYA and a research fellowship from the Japan Society for the Promotion of Science for Young Scientists.

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Correspondence to Yoshihiro Kawaoka.

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Noda, T., Sagara, H., Yen, A. et al. Architecture of ribonucleoprotein complexes in influenza A virus particles. Nature 439, 490–492 (2006).

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