Structures of two main components of the virophage and Marseilleviridae virions extend the range of unrelated viruses using fiber head as common receptor binding fold

The detailed proteomic analysis of Marseilleviridae icosahedral capsids revealed that the two most abundant protein components of the virions were the Major Capsid Protein (MCP) and the product of an ORFan gene conserved in all Marseilleviridae. The noumeavirus NMV_189 3D structure revealed a common fold with fiber head proteins used by a variety of viruses to recognize their cellular receptor. However, the trimeric structure of NMV_189 uniquely lacking a tail domain, presented a deep concave site suggesting it could be directly anchored to the pseudo-hexagonal capsomers of the virion. This was confirmed by the unambiguous fit of the structure in the melbournevirus 4.4 [A] cryo-EM map. In parallel, our structural genomic study of zamilon vitis virophage capsid proteins revealed that Zav_19 shared the same trimeric fiber head fold, but presented an N-terminal tail with a unique {beta}-prism fold. The fiber head fold thus appears to be conserved in all types of non-enveloped icosahedral virions independently of their genomic contents (dsDNA, ssRNA, dsRNA). This could be a testimony of a common origin or the result of convergent evolution for receptor binding function. IMPORTANCEGiant viruses and their associated virophages exhibit a large proportion ([≥]60%) of orphan genes, i.e. genes without homologs in databases, and thus a vast majority of their proteins are of unknown function. The structural characterization of two ORFans, NMV_189 and Zav_19, both major components of noumeavirus and zamilon virophage capsids, respectively, revealed that despite a total lack of sequence homology, the two proteins share a common trimeric fold typical of viral receptor binding proteins and could be responsible for host receptor recognition. These two structures extend the range of unrelated viruses using fiber head structures as common receptor binding fold.