Interactions between the P6 and P5-1 proteins of southern rice black-streaked dwarf fijivirus in yeast and plant cells

Southern rice black-streaked dwarf virus (SRBSDV) is a recently described member of the genus Fijivirus, family Reoviridae. The roles of the proteins encoded by the SRBSDV genome have rarely been studied. In a yeast two-hybrid (YTH) assay in which SRBSDV P6, a putatively multifunctional protein, was used as bait and an SRBSDV cDNA library was used as prey, there was a strong interaction between the P6 and P5-1 proteins. The interaction was confirmed by bimolecular fluorescence complement (BiFC) assay in plant cells. YTH analysis using truncated mutants showed that the N-terminal region (amino acids 9-231) of P5-1 is necessary for binding P5-1 to P6 and that the N-terminal fragment (amino acids 1-93) of P6 is necessary for its interaction with P5-1. SRBSDV P5-1 formed granules positioned at the cell periphery in Nicotiana benthamiana leaves; P6 was present in both the cytoplasm and the nucleus and formed punctate bodies associated with the cell periphery. Immunogold labeling showed that both P6 and P5-1 localized within viroplasms in infected cells of rice plants. These results suggest that the interaction between P5-1 and P6 of SRBSDV may be involved in the formation of viroplasms.