© 2006 Nature Publishing Group
Architecture of ribonucleoprotein complexes in
influenza A virus particles
Takeshi Noda1,2,3, Hiroshi Sagara4, Albert Yen5, Ayato Takada3,6†, Hiroshi Kida2, R. Holland Cheng5,7
& Yoshihiro Kawaoka1,3,6,8
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 120nm 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
PB1 and PB2; the resultant ribonucleoprotein complexes (RNPs)
resemble a twisted rod (10–15nm in width and 30–120nm in
length) that is folded back and coiled on itself2–4. 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
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 thatenable the RNPstobe recruited
and packaged as a complete set6–12. A selective mechanism of RNP
incorporation into virions and the unique organization of the
eight RNPsegments may be crucial tomaintaining the integrityof
the viral genome during repeated cycles of replication.
To elucidate the architecture of the virion interior, we longitudi-
nally and transversely sectioned A/WSN/33 (H1N1) virions budding
from Madin–Darby canine kidney (MDCK) cells at 10h after
are spherical in shape13, the budding virions in longitudinal sections
were elongated and contained rod-like structures that were associ-
ated with host-derived lipid bilayer envelopes and oriented perpen-
dicular to the budding tip of the virion (Fig. 1a and Supplementary
Fig. 1). They were ,12nm in width and up to 130nm in length,
consistent with the sizes of purified RNPs2–4. In transversely sec-
tioned budding virions, electron-dense dots, representing the rod-
like structures seen in longitudinal sections, were apparent inside
each virion (Fig. 1b). Notably, many of the virions contained eight
dots, arranged as seven in a circle surrounding one at the centre
(Fig. 1c); not more than eight dots were observed in a virion. To
determine the lengths of the rod-like structures, we compared
serial transverse sections of whole budding virions (Fig. 1d and
Supplementary Fig. 2). In all virions examined, the number of dots
decreased progressively with increasing distance from the budding
tip of the virion. Together, these results indicate that influenza A
virions contain a highly organized set of eight rod-like structures of
different lengths (Fig. 2).
Figure 1 | Budding virions show a specific arrangement of eight rod-like
structures of different lengths. a, Rod-like structures, 12nm in width, are
associated with the viral envelope at the distal end of the budding virion.
b, c, Electron-dense dots (N # 8), representing transversely sectioned rods,
were observed in each virion in a characteristic configuration (several
peripheral dots surrounding a core dot). d, Serial sections of a virion cut
from the distal end. As the distance from the end increased the number of
dots decreased, suggesting that the eight rod-like structures differed in
length. e, Lower magnification views of the serial ultrathin sections in d,
demonstrating that the serial section shown in d represents the same virion
(a, c, d); 200nm (b, e).
1Internal Research Center for Infectious Diseases, Institute of Medical Science, University of Tokyo, Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.2Laboratory of
Microbiology, Department of Disease Control, Graduate School of Veterinary Medicine, Hokkaido University, Kita-ku, Sapporo 060-0818, Japan.3Core Research for Evolutional
Science and Technology, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan.4Fine Morphology Laboratory, Department of Basic Medical Science, and
Division of Virology, Institute of Medical Science, University of Tokyo, Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.5Department of Biosciences, Karolinska Institute,
141 57 Huddinge, Sweden.6Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.
7Molecular and Cellular Biology, University of California, Davis, California 95616, USA.8Department of Pathological Science, School of Veterinary Medicine, University of
Wisconsin-Madison, Madison, Wisconsin 53706, USA. †Present address: Research Center for Zoonosis Control, Hokkaido University, Kita-ku, Sapporo 060-0818, Japan.
Vol 439|26 January 2006|doi:10.1038/nature04378
© 2006 Nature Publishing Group
To determine whether or not the arrangement of the virion
interior in spherical A/WSN/33 virus is unique to that strain, we
examined MDCK cells infected with other strains of spherical
influenza A viruses isolated from humans (A/Aichi/2/68 (H3N2)
and A/Puerto Rico/8/34 (H1N1)), ducks (A/duck/Hong Kong/836/
80 (H3N1) and A/mallard/New York/6750/78 (H2N2)), and swine
(A/swine/Chiba/1/91 (H1N2); see Supplementary Fig. 3). In all
strains tested, the virions showed the same organization of rod-like
structures, in which a central dot was surrounded by seven dots,
duplicating the pattern seen in A/WSN/33 virions.
are filamentous1, we next examined the architecture of the filamen-
sections of filamentous virions lacked electron-dense material (Sup-
plementary Fig. 4a), some showed the typical configuration of seven
sectioned filamentous virions, the rod-like structures were confined
the remainder of the virion was empty, consistent with the apparent
lack of internal structures in transverse sections of filamentous
virions. Thus, both spherical and filamentous influenza virions
budding from cells possess an organized set of eight rod-like
structures that are associated with the envelope at the budding end
of the virion.
Our serendipitous observation that some influenza A virions are
partially disrupted when freeze-dried enabled us to study the nature
of the rod-like structures inside virions. The widths of the structures
released by freeze-dried, negatively stained A/Puerto Rico/8/34
virions were ,12nm on average (Fig. 3a), similar to the widths of
structures inside sectioned virions (Fig. 1). These released structures
were morphologically indistinguishable from the RNPs purified from
virions in previous studies2–4and reacted with anti-nucleoprotein
monoclonal antibodies conjugated to colloidal gold (Fig. 3b). To
determine further the nature of the rod-like structures observed
in virions, we performed ultrathin-section immunoelectron
microscopy using mouse monoclonal antibody against nucleo-
protein. The electron-dense dots in transversely sectioned virions
reacted with anti-nucleoprotein monoclonal antibodies conjugated
to gold particles (Fig. 3c), indicating that they were indeed viral
To analyse the interior architecture of virions in greater detail, we
used electron tomography, which constructs a three-dimensional
density map of molecules, the ‘tomogram’, at a near-molecular
resolution14. Taking computational thin slices from the map, Fig. 4
shows the results of electron tomography applied to virions budding
from MDCK cells. Within a virion, outlined in Fig. 4 by a bilayered
membrane with protruding viral glycoproteins, were eight RNPs
with a distorted round or square shape. Although some of the RNPs
were clearly isolated from each other, there were instances in which
peripheral RNPs, as well as central and peripheral RNPs, appeared to
be in close contact (Fig. 4, arrows). Whether such contact represents
nonspecific association of the RNPs inside the virion or specific
interactions with functional significance remains to be determined.
Our findings address a long-standing controversy in influenza
virus research: are viral RNA segments incorporated randomly5or
selectively6,7into virions? Together with studies showing that all viral
RNPs possess segment-specific packaging signals (refs 8–12; and
Y. Muramoto and Y.K., and M. Ozawa and Y.K., unpublished data),
our morphological findings clearly favour a model of selective
incorporation. Still unclear is how the eight RNPs are organized
into a characteristic arrangement within virions. One possibility is
that coding regions of the viral RNAs possess signal sequences that
may then control intersegmental association. Our study contributes
Figure 2 | Rod-like structures in a developing virion. a–c, Rod-like
budding virion contains a set of eight rod-like structures. Virions in the
upper row are viewed from the top; those in the lower row are shown in side
Figure 3 | Identification of rod-like structures in virions as viral RNPs.
a, Virions ruptured by freeze-drying release twisted rods, 12nm in width,
conjugated to 5-nm gold particles. c, Immunogold labelling of electron-
dense dots within transversely sectioned virions with anti-NP monoclonal
antibodies conjugated to 10-nm gold particles. Scale bars, 50nm.
NATURE|Vol 439|26 January 2006
© 2006 Nature Publishing Group
nisms of genome incorporation into virions. Defining the interior
architecture of influenza virions may speed up the development of
both antiviral compounds and more efficient methods of gene
delivery and expression.
Cells and viruses. A stock of A/WSN/33 (H1N1) was prepared by growing the
virus in MDCK cells. Stocks of A/Aichi/2/68 (H3N2), A/Puerto Rico/8/34
(H1N1), A/duck/Hong Kong/836/80 (H3N1), A/mallard/New York/6750/78
(H2N2), A/swine/Chiba/1/91 (H1N2) and A/Udorn/307/72 (H3N2) were
prepared in embryonated chicken eggs aged 10–11d.
Ultrathin section electron microscopy. Electron microscopy was done as
described15. In brief, MDCK cells were infected with virus at a multiplicity of
infection of more than 10 and were then prefixed with 2.5% glutaraldehyde in
buffer, the cells were post-fixed with 2% osmium tetraoxide on ice for 1h,
stainedwith uranyl aqueous solutionen bloc, dehydrated with a series of ethanol
gradients followed by propylene oxide treatment, and embedded in Epon 812
Resin mixture. Ultrathin sections (,60-nm thick) were stained with 2% uranyl
acetate in 70% ethanol for 3min at room temperature and in Raynold’s lead for
3min at room temperature, and then examined with a JEM-1200EX electron
microscope (Jeol) operated at 80kV. For ultrathin section-immunoelectron
microscopy, the sections were prepared on nickel grids as described above and
incubated with saturated sodium periodate solution16,17, followed by 0.2M
glycine in PBS buffer. After being washed with PBS, the sections were incubated
with 1% bovine serum albumin (BSA) in PBS, and then with anti-influenza A
virus nucleoprotein mouse monoclonal antibodies. They were then washed
with PBS and incubated with a goat anti-mouse immunoglobulin conjugated to
10-nm gold particles (1:100 dilution, BBInternational).
Preparation of serial ultrathin sections. Samples for serial ultrathin sections
were prepared as described above. Sections (,20-nm thick) were continuously
cut with an Ultracut S ultramicrotome (Leica). The resultant series of ultrathin
sections were placed on a single-slot copper grid and examined with an H-7500
electron microscope (Hitachi) operated at 60kV.
Negative staining of freeze-dried samples. Purified virions were adsorbed to
Formvar-carbon-coated copper grids and negatively stained with 2% phospho-
tungstic acid solution (PTA). Grids were immediately placed in liquid nitrogen
and transferred to a JEE-4X vacuum evaporator (Jeol) for freeze-drying.
Immunoelectron microscopy was done with freeze-dried samples on nickel
grids treated as described above, except that the 2% PTA staining step was
omitted. The grids were then treated with 1% BSA in PBS and mouse
monoclonal antibodies against influenza A virus nucleoprotein, washed with
PBS, and incubated with a goat anti-mouse immunoglobulin conjugated to
5-nm gold particles (1:50 dilution, BBInternational). After being washed, the
samples were fixed with 2% glutaraldehydeandnegatively stainedwith 2% PTA.
Tomographic reconstruction. Sections (,50-nm thick) were prepared as
described above and affixed to 10-nm colloidal gold particles on the upper
surface, which served as fiducial markers for subsequent image alignment. They
were then placed in a CompStage specimen holder (FEI) and imaged in a Tecnai
G2 Shera (FEI) operating at 200kV. Images were taken at a magnification of
58,000 every 28 (from 2658 to þ708) and recorded by a Gatan CCD camera
(1,024 £ 1,024pixelswith0.43nmperpixel)withanaccumulativeelectrondose
of 400e2A22. The alignment of the projections was calculated by using IMOD
software18and the 10-nm gold beads, and the three-dimensional reconstruction
was based on an R-weighted back projection. Galleries of slice tomograms were
displayed with Amira software (Template Graphics) at intervals of 2.15-nm
Received 26 August; accepted 26 October 2005.
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0end of the PB2
Supplementary Information is linked to the online version of the paper at
Acknowledgements 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.
Author Information Reprints and permissions information is available at
npg.nature.com/reprintsandpermissions. The authors declare no competing
financial interests. Correspondence and requests for materials should be
addressed to Y.K. (Kawaoka@ims.u-tokyo.ac.jp).
Figure 4 | Electrontomography of RNP complexesin a budding virion. The
virion was reconstructed in three dimensions by electron tomography, and
three tomograms of its upper portion (at 2.15-nm intervals) containing
eight RNPs are shown. Note the close proximities (possible associations) of
the RNPs (arrows). Scale bar, 50nm.
NATURE|Vol 439|26 January 2006