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The 2.8 Å Structure of aT=4 Animal Virus and its Implications for Membrane Translocation of RNA

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Abstract

Simple RNA animal viruses generally enter cells through receptor-mediated endocytosis followed by acid pH dependent release and translocation of RNA across the endosomal membrane. TheT=3 nodaviruses contain prefabricated pentameric helical bundles that are cleaved from the remainder of the subunits by an assembly-dependent auto-proteolysis and they are positioned for release through 5-fold axes of the particle. We previously proposed that these bundles may serve as conduits for RNA membrane translocation. Additional support for this hypothesis is now provided by the first atomic resolution structure of aT=4 RNA virus, where we find cleavage sites and helical bundles nearly identical with those observed inT=3 nodaviruses. The helices are of sufficient length to span a membrane bilayer and the internal diameter of the coiled bundle could accommodate ssRNA. TheT=4 particle has a mean outer diameter of 410 Å and is formed by 240 copies of a single subunit type. The sub- unit is composed of a helical inner domain (where the cleavage occurs) containing residues preceding and following a canonical, viral, eight-stranded β-sandwich that forms the contiguous shell. Inserted between two strands of the shell domain are 133 residues with an immunoglobulin c-type fold. The initial gene product consists of 644 amino acid residues and is cleaved between residues Asn570 and Phe571 in the mature particle determined in this analysis.

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Present addresses: S. Munshi, WP44-B122, Merck Research Laboratory, Lansdale, PA 19486, USA; L. Liljas, University of Uppsala, Biomedical Center, Department of Molecular Biology, Box 590, S-751 24 Uppsala, Sweden; J. Cavarelli, UPR 9004 de Biologie Structurale du CNRS, IGBMC, Parc d-Innovation, BP 163, 67404 Illkirch Cedex, France; W. Bomu, Institute of Biophysics, Academia Sinica, Beijing 100080, China; V. Reddy and J. E. Johnson, Department of Molecular Biology, The Scripps Research Institute, 10666 N. Torrey Pines Road, LaJolla, CA 92037, USA.

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