Structure of an insect parvovirus (Junonia coenia Densovirus) determined by cryo-electron microscopy.

Junonia coenia densovirus (JcDNV) belongs to the densovirus genus of the Parvoviridae family and infects the larvae of the Common Buckeye butterfly. Its capsid is icosahedral and consists of viral proteins VP1 (88 kDa), VP2 (58 kDa), VP3 (52 kDa) and VP4 (47 kDa). Each viral protein has the same C terminus but differs in the length of its N-terminal extension. Virus-like-particles (VLPs) assemble spontaneously when the individual viral proteins are expressed by a recombinant baculovirus. We present here the structure of native JcDNV at 8.7A resolution and of the two VLPs formed essentially from VP2 and VP4 at 17 A resolution, as determined by cryo-electron microscopy. The capsid displays a remarkably smooth surface, with only two very small spikes that define a pentagonal plateau on the 5-fold axes. JcDNV is very closely related to Galleria mellonella densovirus (GmDNV), whose structure is known (94% sequence identity with VP4 and 96% similarity). We compare these structures in order to locate the structural changes and mutations that may be involved in the species shift of these densoviruses. A single mutation at the tip of one of the two small spikes is a strong candidate as a species shift determinant. Difference imaging reveals that the 21 disordered amino acid residues at the N terminus of the capsid protein VP4 are located inside the capsid at the 5-fold axis, but the additional 94 amino acid residue extension of VP2 is not visible, suggesting that it is highly disordered. There is strong evidence of DNA ordering associated with the 3-fold axes of the capsid.