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Characterization of the herpes simplex virus (HSV)-1 tegument protein VP1-2 during infection with the HSV temperature-sensitive mutant tsB7.

VP1-2, encoded by the UL36 gene of herpes simplex virus (HSV), is a large structural protein, conserved across the family Herpesviridae, that is assembled into the tegument and is essential for virus replication. Current evidence indicates that VP1-2 is a central component in the tegumentation and envelopment processes and that it also possesses important roles in capsid transport and entry. However, any detailed mechanistic understanding of VP1-2 function(s) remains limited. This study characterized the replication of HSV-1 tsB7, a temperature-sensitive mutant restricted at the non-permissive temperature due to a defect in VP1-2 function. A tsB7 virus expressing green fluorescent protein-fused VP16 protein was used to track the accumulation and location of a major tegument protein. After infection at the permissive temperature and shift to the non-permissive temperature, the production of infectious virus ceased. VP1-2 accumulated in altered cytosolic clusters, together with VP16 and other virion proteins. Furthermore, correlating with the results of immunofluorescence, electron microscopy demonstrated abnormal cytosolic capsid clustering and a block in envelopment. As VP1-2 encompasses a ubiquitin-specific protease domain, the occurrence of ubiquitin-conjugated proteins during tsB7 infection was also examined at the non-permissive temperature. A striking overaccumulation was observed of ubiquitin-specific conjugates in cytoplasmic clusters, overlapping and adjacent to the VP1-2 clusters. These results are discussed in relation to the possible functions of VP1-2 in the assembly pathway and the nature of the defect in tsB7.