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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-DUL37-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-1DUL37 . Aggregates of seemingly 'naked' nucleocapsids were present in the cytoplasm of African green monkey (Vero) or rabbit kidney (RK13) cells infected with HSV-1DUL37 or PrV-DUL37. Nuclear retention of nucleocapsids was not observed in either virus. However, in contrast to PrV-DUL37, HSV-1DUL37[86-1035] was unable to replicate productively in, and to form plaques on, either Vero or RK13 cells. Transcomplementation 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.

Homologs of the UL25 gene product of herpes simplex virus 1 (HSV-1) are highly conserved among the Herpesviridae. However, their exact function during viral replication is unknown. Current evidence suggests that in the alphaherpesvirus pseudorabies virus (PrV) the capsid-associated pUL25 plays a role in primary envelopment of DNA-containing mature capsids at the inner nuclear membrane. In the absence of pUL25, capsids were found in close association with the inner nuclear membrane, but nuclear egress was not observed Despite these apparently divergent functions, we wanted to assess whether the high sequence homology translates into functional homology. Therefore, we first analyzed a newly constructed HSV-1 UL25 deletion mutant in our assay system and observed a similar phenotype as in PrV. In the nuclei of infected cells, numerous electron-dense C capsids were detected, whereas primary envelopment of these capsids did not ensue. In agreement with results from PrV, vesicles were observed in the perinuclear space. Since these data indicated functional homology, we analyzed the ability of pUL25 of HSV-1 to complement a PrV UL25 deletion mutant and vice versa. Whereas a HSV-1 pUL25-expressing cell line partially complemented the pUL25 defect in PrV, reciprocal complementation of a HSV-1 UL25 deletion mutant by PrV pUL25 was not observed. Thus, our data demonstrate overlapping, although not identical functions of these two conserved herpesvirus proteins, and point to a conserved functional role in herpes virion formation.Capsid formation is morphologically similar in all herpesviruses and resembles that of tailed bacteriophages (10). After synthesis in the cytosol, capsid proteins are transported into the nucleus, where they assemble in an autocatalytic process. The newly replicated concatemeric DNA is cleaved to unit length during packaging into the preformed capsids with a concomitant loss of the scaffold protein. After encapsidation, capsids contact and bud at the inner nuclear membrane for primary envelopment, thus initiating nucleocapsid transit to the cytosol for final maturation (24,26,38). Viral proteins homologous to the products of the HSV-1 genes UL31 and UL34 are required for nuclear egress (8,9,15,35,39) and are sufficient for the formation of membranous vesicles resembling primary envelopes in PrV (17). pUL34 is a nuclear-membraneassociated phosphoprotein with a predicted type II integral topology that interacts with pUL31 (8,9,19,34,35,41). This complex formation is important for proper positioning of both proteins at the inner nuclear membrane (28,36,45). Moreover, coexpression of pUL31 and pUL34 alters lamin architecture (3,27,28,33,37), a prerequisite for intranuclear capsids to access the inner nuclear membrane. However, how intranuclear capsids are directed to the budding site is unknown.Current evidence suggests that the conserved pUL25 capsidassociated protein is also required for primary envelopment (16,44). Homologs of the UL25 gene product of HSV-1 have been identified in all three subfamilie...