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 overlapping genes UL26 and UL26.5 form a scaffold around which the icosahedral capsid shell is assembled. In a series of cleavage events catalysed by the UL26-encoded protease, the full-length UL26 product is processed into capsid proteins VP24 and VP21 and the UL26.5 protein is converted 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 investigated using the baculovirus expression system, focusing on the function of the 25 residues cleaved from the UL26.5 protein. A key conclusion 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 proteins with the scaffold in the formation of intermediate capsids. When cells were multiply infected with baculoviruses expressing a truncated form of the UL26.5 product corresponding to VP22a and the essential components of the capsid shell, no capsids were detected, whereas large numbers of capsids were observed when the full-length UL26.5 product was used as a scaffold. The results are consistent with the proposal that cleavage of the UL26.5 product occurs after capsid assembly or when the UL26.5 protein is in a complex with one or more capsid shell proteins. Expression of VP22a in the absence or presence of capsid shell proteins resulted in the formation of large numbers of 60 nm scaffold-like particles. Since VP22a expressed from baculovirus was unable to participate in capsid assembly, these particles cannot be intermediates in the capsid assembly pathway but may be similar in structure to the protein cores present in HSV-1 immature (B) capsids.