Structure and components of the globular and filamentous viroplasms induced by Rice black-streaked dwarf virus

Viroplasms of members of the family Reoviridae are considered to be viral factories for genome replication and virion assembly. Globular and filamentous phenotypes have different components and probably have different functions. We used transmission electron microscopy and electron tomography to examine the structure and components of the two viroplasm phenotypes induced by Rice black-streaked dwarf virus (RBSDV). Immuno-gold labeling was used to localize each of the 13 RBSDV encoded proteins as well as double-stranded RNA, host cytoskeleton actin-11 and α-tubulin. Ten of the RBSDV proteins were localized in one or both types of viroplasm. P5-1, P6 and P9-1 were localized on both viroplasm phenotypes but P5-1 was preferentially associated with filaments and P9-1 with the matrix. Structural analysis by electron tomography showed that osmiophilic granules 6-8 nm in diameter served as the fundamental unit for constructing both of the viroplasm phenotypes but were more densely packed in the filamentous phenotype.