THE 25 AMINO-ACID-RESIDUES AT THE CARBOXY-TERMINUS OF THE HERPES-SIMPLEX VIRUS TYPE-1 UL26.5 PROTEIN ARE REQUIRED FOR THE FORMATION OF THE CAPSID SHELL AROUND THE SCAFFOLD

Herpes simplex virus type 1 (HSV-1) polypeptides specified by overlapp ing genes UL26 and UL26.5 form a scaffold around which the icosahedral capsid shell is assembled. In a series of cleavage events catalysed b y the UL26-encoded protease, the full-length UL26 product is processed into capsid proteins VP24 and VP21 and the UL26.5 protein is converte d into the capsid protein VP22a by the loss of 25 amino acids from its carboxy terminus. The roles of the UL26 and UL26.5 products were inve stigated using the baculovirus expression system, focusing on the func tion of the 25 residues cleaved from the UL26.5 protein. A key conclus ion from electron microscopic analysis and protein expression studies is that the 25 amino acids at the carboxy terminus of the full-length UL26.5 protein are required for the interaction of the capsid shell pr oteins with the scaffold in the formation of intermediate capsids. Whe n cells were multiply infected with baculoviruses expressing a truncat ed form of the UL26.5 product corresponding to VP22a and the essential components of the capsid shell, no capsids were detected, whereas lar ge numbers of capsids were observed when the full-length UL26.5 produc t was used as a scaffold. The results are consistent with the proposal that cleavage of the UL26.5 product occurs after capsid assembly or w hen the UL26.5 protein is in a complex with one or more capsid shell p roteins. Expression of VP22a in the absence or presence of capsid shel l proteins resulted in the formation of large numbers of 60 nm scaffol d-like particles. Since VP22a expressed from baculovirus was unable to participate in capsid assembly, these particles cannot be intermediat es in the capsid assembly pathway but may be similar in structure to t he protein cores present in HSV-1 immature (B) capsids.