Large conformational changes in the maturation of a simple RNA virus, Nudaurelia capensis ω virus (NωV)

An assembly intermediate of a small, non-enveloped RNA virus has been discovered that exhibits striking differences from the mature virion. Virus-like particles (VLPs) of Nudaurelia capensis ω virus (NωV), a T = 4 icosahedral virus infecting Lepidoptera insects, were produced in insect cells using a baculovirus vector expressing the coat protein. A procapsid form was discovered when NωV VLPs were purified at neutral pH conditions. These VLPs were fragile and did not undergo the autoproteolytic maturation that occurs in the infectious virus. Electron cryo-microscopy (cryoEM) and image analysis showed that, compared with the native virion, the VLPs were 16 % larger in diameter, more rounded, porous, and contained an additional internal domain. Upon lowering the pH to 5.0, the VLP capsids became structurally indistinguishable from the authentic virion and the subunits autoproteolyzed. The NωV protein subunit coordinates, which were previously determined crystallographically, were modelled into the 28 Å resolution cryoEM map of the procapsid. The resulting pseudo-atomic model of the NωV procapsid demonstrated the large rearrangements in quaternary and tertiary structure needed for the maturation of the VLPs and presumably of the virus. Based on this model, we propose that electrostatically driven rearrangements of interior helical regions are responsible for the large conformational change. These results are surprising because large structural rearrangements have not been found in the maturation of any other small RNA viruses. However, similarities of this conformational change to the maturational processes of more complex DNA viruses (e.g. bacteriophages and herpesvirus) and to the swelling of simple plant viruses suggest that structural changes in icosahedral viruses, which are integral to their function, have similar strategies and perhaps mechanisms.