Efficient generation of cowpea mosaicvirus empty virus-like particles by the proteolytic processing of precursors in insect cells and plants

To elucidate the mechanism of formation of cowpea mosaic virus (CPMV) particles, RNA-2-encoded precursor proteins were expressed in Spodoptera frugiperda cells. Processing of the 105K and 95K polyproteins in trans to give the mature Large (L) and Small (S) coat proteins required both the 32K proteinase cofactor and the 24K proteinase itself, while processing of VP60, consisting of the fused L–S protein, required only the 24K proteinase. Release of the L and S proteins resulted in the formation of virus-like particles (VLPs), showing that VP60 can act as a precursor of virus capsids. Processing of VP60 expressed in plants also led to efficient production of VLPs. Analysis of the VLPs produced by the action of the 24K proteinase on precursors showed that they were empty (RNA-free). This has important implications for the use of CPMV VLPs in biotechnology and nanotechnology as it will permit the use of noninfectious particles.