The Interaction of Bluetongue Virus VP6 and Genomic RNA Is Essential for Genome Packaging

The genomes of the Reoviridae, including the animal pathogen bluetongue virus (BTV), are multisegmented double-stranded RNA (dsRNA). During replication, single-stranded (ss) positive-sense RNA segments are packaged into the assembling virus capsid, triggering genomic dsRNA synthesis. However, exactly how this packaging event occurs is not clear. A minor capsid protein, VP6, unique for the orbiviruses, has been proposed to be involved in the RNA-packaging process. In this study, we sought to characterize the RNA binding activity of VP6 and its functional relevance. A novel proteomic approach was utilized to map the ssRNA/dsRNA binding sites of a purified recombinant protein and the genomic dsRNA binding sites of the capsid-associated VP6. The data revealed that each VP6 protein has multiple distinct RNA-binding regions and that only one region is shared between recombinant and capsid-associated VP6. A combination of targeted mutagenesis and reverse genetics identified the RNA-binding region that is essential for virus replication. Using an in vitro RNA-binding competition assay, a unique cell-free assembly assay, and an in vivo single-cycle replication assay, it was possible to identify a motif within the shared binding region that binds BTV ssRNA preferentially in a manner consistent with specific RNA recruitment during capsid assembly. These data highlight the critical roles that this unique protein plays in orbivirus genome packaging and replication. IMPORTANCE Genome packaging is a critical stage during virus replication. For viruses with segmented genomes, the genome segments need to be correctly packaged into a newly formed capsid. However, the detailed mechanism of this packaging is unclear. Here we focus on VP6, a minor viral protein of bluetongue virus, which is critical for genome packaging. We used multiple approaches, including a robust RNA-protein fingerprinting assay, to map the ssRNA binding sites of recombinant VP6 and the genomic dsRNA binding sites of capsid-associated VP6. By these means, together with virological and biochemical methods, we identify the viral RNA-packaging motif of a segmented dsRNA virus for the first time.