Large conformational changes in the maturation of a simple RNA virus, Nudaurelia capensis omega virus (N omega V)

An assembly intermediate of a small, non-enveloped RNA virus has been disco vered that exhibits striking differences from the mature virion. Virus-like particles (VLPs) of Nudaurelia capensis omega virus (N omega V), a T = 4 i cosahedral virus infecting Lepidoptera insects, were produced in insect cel ls using a baculovirus vector expressing the coat protein. A procapsid form was discovered when N omega V 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 struct urally indistinguishable from the authentic virion and the subunits autopro teolyzed. The N omega V protein subunit coordinates, which were previously determined crystallographically, were modelled into the 28 Angstrom resolut ion cryoEM map of the procapsid. The resulting pseudo-atomic model of the N omega V 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 re arrangements of interior helical regions are responsible for the large conf ormational change. These results are surprising because large structural re arrangements have not been found in the maturation of any other small RNA v iruses. However, similarities of this conformational change to the maturati onal processes of more complex DNA viruses (e.g. bacteriophages and herpesv irus) and to the swelling of simple plant viruses suggest that structural c hanges in icosahedral viruses, which are integral to their function, have s imilar strategies and perhaps mechanisms. (C) 2000 Academic Press.