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The Birnavirus Crystal Structure Reveals Structural Relationships among Icosahedral Viruses
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International audience. sified in two major categories according to their genome type: +sRNA and dsRNA. Among +sRNA eukary-Irina Gutsche, 1 Joan Pous, 1 Jorge Navaza, 1 otic viruses, the building block of the capsid-the "coat Stéphane Bressanelli, 1 Bernard Delmas, 2, * protein"-exhibits a special fold, the "jelly roll" β barrel and Félix A. Rey 1, * (Rossmann and Johnson, 1989). This protein forms a 1 Laboratoire de Virologie Moléculaire et Structurale tightly closed protein shell protecting the viral RNA, UMR 2472/1157 CNRS-INRA and IFR 115 with the jelly roll β barrel oriented such that the β 1 Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex strands run tangentially to the particle surface. Except 2 Unité de Virologie et Immunologie Moléculaires for the very small satellite +sRNA viruses-in which the INRA capsid contains only 60 copies of the coat protein-Domaine de Vilvert, 78350 Jouy-en-Josas most virus capsids contain 60 copies of a multimer France (Harrison, 2001b), organized in an icosahedral surface lattice following the rules of quasiequivalence (Caspar and Klug, 1962). Such an arrangement leads to al-Summary ternating 5-fold (I5 for "icosahedral 5-fold") and quasi-6-fold (Q6) contacts among coat proteins, distributed Double-stranded RNA virions are transcriptionally according to the triangulation of the surface lattice competent icosahedral particles that must translo-(Johnson and Speir, 1997). cate across a lipid bilayer to function within the cyto-In contrast to the +sRNA viruses, the crystal strucplasm of the target cell. Birnaviruses are unique tures of dsRNA virus particles revealed a characteristic among dsRNA viruses as they have a single T = 13 icosahedral shell composed of 120 subunits of a large icosahedral shell, lacking the characteristic inner coat protein, with a 3D fold completely unrelated to the capsid observed in the others. We determined the universal jelly roll described above (Harrison, 2001a). crystal structures of the T = 1 subviral particle (260 Å The corresponding surface lattice does not follow the in diameter) and of the T = 13 intact virus particle quasiequivalence principle, the building block being an (700 Å in diameter) of an avian birnavirus to 3 Å and asymmetric dimer. The simplest dsRNA virus of known 7 Å resolution, respectively. Our results show that structure, L-A virus, an intracellular yeast totivirus (Toti-VP2, the only component of the virus icosahedral viridae family), has a capsid organized in this way (Cascapsid, is homologous both to the capsid protein of ton et al., 1997; Naitow et al., 2002). positive-strand RNA viruses, like the T = 3 nodavi-Detailed structural data are available also for memruses, and to the T = 13 capsid protein of members bers of the Reoviridae family of dsRNA viruses (Grimes of the Reoviridae family of dsRNA viruses. Together, et al., 1998; Nakagawa et al., 2003; Reinisch et al., these results provide important insights into the 2000), which have multishelled architectures. The innermultiple functions of the birnavirus capsid and reveal most shell displays the same organization as that obunexpected structural relationships among icosaheserved in the totivirus capsid, with 60 asymmetric coat dral viruses. protein dimers. This structure is surrounded by a second icosahedral protein shell of triangulation T = 13,
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