Acquisition of functions on the outer capsid surface during evolution of double-stranded RNA fungal viruses

Unlike their counterparts in bacterial and higher eukaryotic hosts,most fungal viruses are transmitted intracellularly and lack an extracellular phase. here we determined the cryo-em structure at 3.7 å resolution of rosellinia necatrix quadrivirus 1 (rnqv1),a fungal double-stranded (ds)rna virus. rnqv1,the type species of the family quadriviridae,has a multipartite genome consisting of four monocistronic segments. whereas most dsrna virus capsids are based on dimers of a single protein,the ~450-å-diameter,t = 1 rnqv1 capsid is built of p2 and p4 protein heterodimers,each with more than 1000 residues. despite a lack of sequence similarity between the two proteins,they have a similar α-helical domain,the structural signature shared with the lineage of the dsrna bluetongue virus-like viruses. domain insertions in p2 and p4 preferential sites provide additional functions at the capsid outer surface,probably related to enzyme activity. the p2 insertion has a fold similar to that of gelsolin and profilin,two actin-binding proteins with a function in cytoskeleton metabolism,whereas the p4 insertion suggests protease activity involved in cleavage of the p2 383-residue c-terminal region,absent in the mature viral particle. our results indicate that the intimate virus-fungus partnership has altered the capsid genome-protective and/or receptor-binding functions. fungal virus evolution has tended to allocate enzyme activities to the virus capsid outer surface. © 2017 rossi et al.