Crystallographic analysis reveals octamerization of viroplasm matrix protein P9-1 of Rice black streaked dwarf virus

The P9-1 protein of Rice black streaked dwarf virus accumulates in viroplasm inclusions, which are structures that appear to play an important role in viral morphogenesis and are commonly found in viruses in the family Reoviridae. Crystallographic analysis of P9-1 revealed structural features that allow the protein to form dimers via hydrophobic interactions. Each dimer has carboxy-terminal regions, resembling arms, that extend to neighboring dimers, thereby uniting sets of four dimers via lateral hydrophobic interactions, to yield cylindrical octamers. The importance of these regions for the formation of viroplasm-like inclusions was confirmed by the absence of such inclusions when P9-1 was expressed without its carboxy-terminal arm. The octamers are vertically elongated cylinders resembling the structures formed by NSP2 of rotavirus, even though there are no significant similarities between the respective primary and secondary structures of the two proteins. Our results suggest that an octameric structure with an internal pore might be important for the functioning of the respective proteins in the events that occur in the viroplasm, which might include viral morphogenesis.