Non-enveloped icosahedral double-stranded RNA (dsRNA) viruses possess multifunctional capsids required for their proliferation. Whereas protozoan/fungal dsRNA viruses have a relatively simple capsid structure, which suffices for the intracellular phase in their life cycle, metazoan dsRNA viruses have acquired additional structural features as an adaptation for extracellular cell-to-cell transmission in multicellular hosts. Here, we present the first atomic model of a metazoan dsRNA totivirus-like virus and the structure reveals three unique structural traits: a C-terminal interlocking arm, surface projecting loops, and an obstruction at the pore on the 5-fold symmetry axis. These traits are keys to understanding the capsid functions of metazoan dsRNA viruses, such as particle stability and formation, cell entry, and endogenous intraparticle transcription of mRNA. On the basis of molecular dynamics simulations of the obstructed pore, we propose a possible mechanism of intraparticle transcription in totivirus-like viruses, which dynamically switches between open and closed states of the pore(s).

Department of Medical Entomology National Institute of Infectious Diseases Tokyo Japan

ELI Beamlines Institute of Physics AS CR v v i Na Slovance 2 182 21 Prague 8 Czech Republic

Life Science Center for Survival Dynamics Tsukuba Advanced Research Alliance University of Tsukuba Tsukuba Japan

National Institute for Physiological Sciences Okazaki Japan

Program in Molecular Biophysics Department of Cell and Molecular Biology Uppsala University Uppsala Sweden

The Laboratory of Protein Synthesis and Expression Institute for Protein Research Osaka University Japan

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