Interactions between bluetongue virus core and capsid proteins translated in vitro

To determine whether the two major core proteins (VP3 and VP7) of bluetongue virus can interact in vitro to form morphological structures, linearized VP3 and VP7 cDNA clones were transcribed using SP6 polymerase and the resultant transcripts were co-translated using rabbit reticulocyte lysates. The structures derived were isolated by sedimentation through a sucrose gradient and found to resemble VP3-VP7 core-like particles (CLPs) expressed in vivo. Reacting CLPs synthesized in vivo with outer capsid proteins translated in vitro (VP2 or VP5) indicated that each outer capsid protein has the capacity to bind to a preformed CLP. This was confirmed by in vivo expression of the appropriate genes using baculovirus vectors. The interaction of VP2 or VP5 with the CLP was analysed by electron microscopy and by using immunogold-labelled monoclonal antibody.