Herpes simplex virus type 1 VP26 is not essential for replication in cell culture but influences production of infectious virus in the nervous system of infected mice.

VP26 is the smallest capsid protein of herpes simplex virus type 1 and is encoded by the UL35 open reading frame. It resides on the outer capsid surface, interacting with VP5 in a one to one stoichiometry in the hexons that comprise capsids. A null mutation in the gene encoding VP26 was generated and transferred into the KOS genome. Recombinant viruses were isolated on Vero cells, which indicated that the absence of VP26 was not required for growth of the virus in cell culture. This was confirmed by the characterization of the VP26 null mutant, designated K delta 26Z. The yield of virus from K delta 26Z-infected Vero cells was decreased only twofold relative to wild-type-infected cells, as judged by the burst size. All three types of capsids (A, B, and C) were observed after sedimentation analysis of K delta 26Z-infected cell extracts. These capsids were similar in composition to wild-type capsids except for the absence of VP26. The mouse ocular model was used to determine if VP26 played a major role in vivo. The yield of the mutant virus relative to wild-type virus was decreased twofold in the eye; however, the mutant virus yields were decreased 30- to 100-fold in the trigeminal ganglia. Reactivation of the mutant virus as determined by cocultivation assays was also reduced. To determine the effect of VP26 on capsid translocation, the VP26 null mutation was transferred into a virus specifiying a thymidine kinase mutation that by itself is transported to the trigeminal ganglia but whose DNA is not replicated in the ganglia. Using quantitative PCR assays the number of viral genomes detected in the ganglia was similar in the presence or the absence of VP26. Therefore, VP26 does not appear to aid in the translocation of the virus capsid from the mouse eye to the trigeminal ganglia but is important for infectious virus production in the ganglia.