Phenotypic similarities and differences between UL37-deleted pseudorabies virus and herpes simplex virus type 1

In the absence of the tegument protein pUL37, virion formation of pseudorabies virus (PrV) and herpes simplex virus type 1 (HSV-1) is severely impaired. Non-enveloped nucleocapsids accumulate in clusters in the cytoplasm, whereas only a few enveloped particles can be detected. Although a contribution of pUL37 to nuclear egress of HSV-1 has been suggested, the nuclear stages of morphogenesis are not impaired in PrV-DeltaUL37-infected cells. Moreover, HSV-1 pUL37 has been described as essential for replication, whereas PrV is able to replicate productively without pUL37, although to lower titres than wild-type virus. Thus, there may be functional differences between the respective pUL37 proteins. This study compared the phenotypes of UL37-deleted PrV and HSV-1 in parallel assays, using a novel pUL37 deletion mutant of HSV-1 strain KOS, HSV-1DeltaUL37[86-1035]. Aggregates of seemingly 'naked' nucleocapsids were present in the cytoplasm of African green monkey (Vero) or rabbit kidney (RK13) cells infected with HSV-1DeltaUL37[86-1035] or PrV-DeltaUL37. Nuclear retention of nucleocapsids was not observed in either virus. However, in contrast to PrV-DeltaUL37, HSV-1DeltaUL37[86-1035] was unable to replicate productively in, and to form plaques on, either Vero or RK13 cells. Trans-complementation of respective deletion mutants with the heterologous pUL37 did not ensue. These data demonstrate that the conserved pUL37 in HSV-1 and PrV have similar but distinct functions.