Co-expression of the capsid proteins of Cowpea mosaic virus in insect cells leads to the formation of virus-like particles.

The regions of RNA-2 of Cowpea mosaic virus (CPMV) that encode the Large (L) and Small (S) coat proteins were expressed either individually or together in Spodoptera frugiperda (sf21) cells using baculovirus vectors. Co-expression of the two coat proteins from separate promoters in the same construct resulted in the formation of virus-like particles whose morphology closely resembled that of native CPMV virions. No such particles were formed when the individual L and S proteins were expressed. Sucrose gradient centrifugation of the virus-like particles showed that they had the sedimentation characteristics of empty (protein-only) shells. The results confirm that the 60 kDa L-S fusion is not an obligate intermediate in the virion assembly pathway and indicate that expression of the coat proteins in insect cells will provide a fruitful route for the study of CPMV morphogenesis.