Infectious Bursal Disease Virus: Ribonucleoprotein Complexes of a Double-Stranded RNA Virus

Genome-binding proteins with scaffolding and/or regulatory functions are common in living organisms and include histones in eukaryotic cells, histone-like proteins in some double-stranded DNA (dsDNA) viruses, and the nucleocapsid proteins of single-stranded RNA viruses. dsRNA viruses nevertheless lack these ribonucleoprotein (RNP) complexes and are characterized by sharing an icosahedral T = 2 core involved in the metabolism and insulation of the dsRNA genome. The birnaviruses, with a bipartite dsRNA genome, constitute a well-established exception and have a single-shelled T = 13 capsid only. Moreover, as in many negative single-stranded RNA viruses, the genomic dsRNA is bound to a nucleocapsid protein (VP3) and the RNA-dependent RNA polymerase (VPg). We used electron microscopy and functional analysis to characterize these RNP complexes of infectious bursal disease virus, the best characterized member of the Birnaviridae family. Mild disruption of viral particles revealed that VP3, the most abundant core protein, present at ∼ 450 copies per virion, is found in filamentous material tightly associated with the dsRNA. We developed a method to purify RNP and VPg–dsRNA complexes. Analysis of these complexes showed that they are linear molecules containing a constant amount of protein. Sensitivity assays to nucleases indicated that VP3 renders the genomic dsRNA less accessible for RNase III without introducing genome compaction. Additionally, we found that these RNP complexes are functionally competent for RNA synthesis in a capsid-independent manner, in contrast to most dsRNA viruses.