The C-terminal 25 amino acids of the protease and its substrate ICP35 of herpes simplex virus type 1 are involved in the formation of sealed capsids.

The herpes simplex virus type 1 protease and its substrate, ICP35, are involved in the assembly of viral capsids. Both proteins are encoded by a single open reading frame from overlapping mRNAs. The protease is autoproteolytically processed at two sites. The protease cleaves itself at the C-terminal site (maturation site) and also cleaves ICP35 at an identical site, releasing a 25-amino-acid (aa) peptide from each protein. To determine whether these 25 aa play a role in capsid assembly, we constructed a mutant virus expressing only Prb, the protease without the C-terminal 25 aa. Phenotypic analysis of the Prb virus in the presence and absence of ICP35 shows the following: (i) Prb retains the functional activity of the wild-type protease which supports virus growth in the presence of ICP35; (ii) in contrast to the ICP35 null mutant delta ICP35 virus, the Prb virus fails to grow in the absence of ICP35; and (iii) trans-complementation experiments indicated that full-length ICP35 (ICP35 c,d), but not the cleaved form (ICP35 e,f), complements the growth of the Prb virus. The most striking phenotype of the Prb virus is that only unsealed aberrant capsid structures are observed by electron microscopy in mutant-infected Vero cells. Our results demonstrate that the growth of herpes simplex virus type 1 requires the C-terminal 25 aa of either the protease or its substrate, ICP35, and that the C-terminal 25 aa are involved in the formation of sealed capsids.