Evolutionary and taxonomic implications of conserved structural motifs between picornaviruses and insect picorna-like viruses

A comparison of the recently determined structure of an insect picorna-like virus, Cricket paralysis virus (CrPV), with that of the mammalian picornaviruses shows that several structural features are highly conserved between these viruses. These conserved features include the topology of the coat proteins, the conformation of most loops, and the general arrangement of the internally located N-terminal arms of the coat proteins. The conformational conservation of the N-termini of the three major coat proteins between CrPV and the picornaviruses suggests a putative ancestral T = 3 virus. Comparisons of the genome structure and amino-acid sequence of the coat proteins of CrPV with a number of other insect picorna-like viruses show that most of them belong to a novel group, recently given the interim name Cricket paralysis-like viruses. Two other insect picorna-like viruses, Infectious flacherie virus (IFV) and Sacbrood virus (SBV), for which the genome sequences have recently been determined, have very different coat protein sequences and a genome organization more like the picornaviruses. However, the position of the small VP4 protein in the structural protein polyprotein as well as the mechanism for its cleavage from VP3 upon assembly strongly suggests an evolutionary link to the "Cricket paralysis-like viruses". We propose that the picornaviruses, Cricket paralysis-like viruses and IFV/SBV group are a natural assemblage. The ancestor for this assemblage had a structure based upon the CrPV/picornavirus paradigm and a genome encoding a single major coat protein; gene duplication and rearrangements have subsequently produced the viruses that we observe today. We also discuss the possible relatives of the proposed assemblage and the likely implications of future structural studies that may be carried out on the putative relatives.