EFFICIENT HERPES-SIMPLEX VIRUS TYPE-1 (HSV-1) CAPSID FORMATION DIRECTED BY THE VARICELLA-ZOSTER VIRUS SCAFFOLDING PROTEIN REQUIRES THE CARBOXY-TERMINAL SEQUENCES FROM THE HSV-1 HOMOLOG

The scaffolding protein and associated protease of the human herpesvir us varicella-zoster virus (VZV), encoded by genes 33.5 and 33 respecti vely, were synthesized in insect cells using a baculovirus expression system. The expressed 33.5 product formed numerous long, flexible, hol low rods, and in this respect differed from the herpes simplex virus t ype 1 (HSV-1) homologue which forms large aggregates consisting mainly of fibrous material interspersed with scaffold-like particles. Remova l of 27 amino acids from the carboxy terminus of the VZV scaffolding p rotein by the gene 33 protease or expression of the cleaved product di d not result in any discernible change in the morphology of the scaffo lding protein. Again, this was in marked contrast to the situation in HSV-1 where removal of the 25 carboxy-terminal amino acids from the sc affolding protein by the associated protease or expression of VP22a re sults in the formation of large numbers of scaffold-like particles. De spite these differences, when cells were multiply infected with baculo viruses expressing the HSV-1 capsid shell proteins and the VZV scaffol ding protein complete capsids were observed, suggesting that the VZV p rotein could act as a scaffold for the assembly of the HSV-I capsid sh ell. The efficiency of capsid assembly was increased substantially by exchanging the 23 carboxy-terminal amino acids of the VZV scaffolding protein for the corresponding 22 carboxy-terminal amino acids of the H SV-1 homologue, supporting previous work which showed that this region was critical for the formation of intact capsids.