The expressed VP4 protein of bluetongue virus binds GTP and is the candidate guanylyl transferase of the virus.

A minor core protein, VP4, of bluetongue virus serotype 10 (BTV-10) has been synthesized in insect cells infected with a genetically manipulated recombinant baculovirus. When insect cells were coinfected by this recombinant virus and a recombinant baculovirus expressing the two major core proteins (VP3 and VP7) of the virus, core-like particles (CLPs) consisting of all three proteins were formed. Purified CLPs reacted with [32P]GTP which was covalently bound to VP4 only. Similarly reconstituted CLPs with VP1 or VP6 did not form covalent complexes with [32P]GTP. The virion-derived VP4 was also shown to have GTP-binding activity. The covalent binding of GTP indicates that expressed VP4 not only is biologically active but also is the candidate guanylyl transferase of the virus. The optimum reaction conditions for GTP binding by VP4 have been investigated.