THE C-TERMINAL-25 AMINO-ACIDS OF THE PROTEASE AND ITS SUBSTRATE ICP35OF 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 itse lf at the C-terminal site (maturation site) and also cleaves ICP35 at an identical site, releasing a 25-amino-acid (aa) peptide from each pr otein. To determine whether these 25 aa play a role in capsid assembly , we constructed a mutant virus expressing only Prb, the protease with out 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 gr owth in the presence of ICP35; (ii) in contrast to the ICP35 null muta nt Delta ICP35 virus, the Prb virus fails to grow in the absence of IC P35; and (iii) trans-complementation experiments indicated that full-l ength ICP35 (ICP35 c,d), but not the cleaved form (ICP35 e,f), complem ents the growth of the Prb virus. The most striking phenotype of the P rb virus is that only unsealed aberrant capsid structures are observed by electron microscopy in mutant-infected Vero cells. Our results dem onstrate that the growth of herpes simplex virus type 1 requires the C -terminal 25 aa of either the protease or its substrate, ICP35, and th at the C-terminal 25 aa are involved in the formation of sealed capsid s.