Structural Evolution of Reoviridae Revealed by Oryzavirus in Acquiring the Second Capsid Shell

The conservation of the core structure and diversification of the external features among the turreted reoviruses appear to be relevant to structural evolution in facilitating the infection of diverse host species. The structure ofRice ragged stunt virus (RRSV), in the genusOryzavirus of the familyReoviridae , is determined to show a core composed of capsid shell, clamps, and long turrets. The RRSV core structure is equivalent to the core structure ofOrthoreovirus and the virion structure ofCytoplasmic polyhedrosis virus (CPV). In RRSV, five peripheral trimers surround each long turret and sit at the Q trimer position in the T=13l icosahedral symmetry, a structural feature unique to turreted reoviruses. That is, the core of RRSV is partially covered by 60 copies of the peripheral trimer. In contrast, the core ofOrthoreovirus is covered by 200 copies of the trimer that sit at the Q, R, S, and T trimer positions. Our results suggest that among the three viruses, RRSV has a structure intermediate between that ofOrthoreovirus and the CPV virion. This conclusion coincides with the results of the phylogenetic analysis of amino acid sequences of RNA-dependent RNA polymerases.