Requirements for the nuclear-cytoplasmic translocation of infected-cell protein 0 of herpes simplex virus 1

Earlier studies have shown that wild-type infected-cell protein 0 (ICP0), a key herpes simplex virus regulatory protein, translocates from the nucleus to the cytoplasm of human embryonic lung (HEL) fibroblasts within several hours after infection (Y. Kawaguchi, R. Bruni, and B. Roizman, J. Virol. 71:1019-1024, 1997). Translocation of ICP0 was also observed in cells infected with the d120 mutant, in which both copies of the gene encoding ICP4, the major regulatory protein, had been deleted (V. Galvan, R. Brandimarti, J. Munger, and B. Roizman, J. Virol. 74:1931-1938, 2000). Furthermore, a mutant (R7914) carrying the D199A substitution in ICP0 does not bind or stabilize cyclin D3 and is retained in the nucleus (C. Van Sant, P. Lopez, S. J. Advani, and B. Roizman, J. Virol. 75:1888-1898, 2001). Studies designed to elucidate the requirements for the translocation of ICP0 between cellular compartments revealed the following. (i) Translocation of ICP0 to the cytoplasm in productive infection maps to the D199 amino acid, inasmuch as wild-type ICP0 delivered in trans to cells infected with an ICP0 null mutant was translocated to the cytoplasm whereas the D199A-substituted mutant ICP0 was not. (ii) Translocation of wild-type ICP0 requires a function expressed late in infection, inasmuch as phosphonoacetate blocked the translocation of ICP0 in wild-type virus-infected cells but not in d120 mutant-infected cells. Moreover, whereas in d120 mutant-infected cells ICP0 was translocated rapidly from the cytoplasm to the nucleus at approximately 5 h after infection, the translocation of ICP0 in wild-type virus-infected cells extended from 5 to at least 9 h after infection. (iii) In wild-type virus-infected cells, the MG132 proteasomal inhibitor blocked the translocation of ICP0 to the cytoplasm early in infection, but when added late in infection, it caused ICP0 to be relocated back to the nucleus from the cytoplasm. (iv) MG132 blocked the translocation of ICP0 in d120 mutant-infected cells early in infection but had no effect on the ICP0 aggregated in vesicle-like structures late in infection. However, in d120 mutant-infected cells treated with MG132 at late times, proteasomes formed a shell-like structure around the aggregated ICP0. These structures were not seen in wild-type virus or R7914 mutant-infected cells. The results indicate the following. (i) In the absence of beta or gamma protein synthesis, ICP0 dynamically associates with proteasomes and is translocated to the cytoplasm. (ii) In cells productively infected beyond alpha gene expression, ICP0 is retained in the nucleus until after the onset of viral DNA synthesis and the synthesis of gamma2 proteins. (iii) Late in infection, ICP0 is actively sequestered in the cytoplasm by a process mediated by proteasomes, inasmuch as interference with proteasomal function causes rapid relocation of ICP0 to the nucleus.     

Herpes simplex virus type 1 gene UL14: phenotype of a null mutant and identification of the encoded protein

Herpes simplex virus type 1 (HSV-1) gene UL14 is located between divergently transcribed genes UL13 and UL15 and overlaps the promoters for both of these genes. UL14 also exhibits a substantial overlap of its coding region with that of UL13. It is one of the few HSV-1 genes for which a phenotype and protein product have not been described. Using mass spectrometric and immunological approaches, we demonstrated that the UL14 protein is a minor component of the virion tegument of 32 kDa which is expressed late in infection. In infected cells, the UL14 protein was detected in the nucleus at discrete sites within electron-dense nuclear bodies and in the cytoplasm initially in a diffuse distribution and then at discrete sites. Some of the UL14 protein was phosphorylated. A mutant with a 4-bp deletion in the central region of UL14 failed to produce the UL14 protein and generated small plaques. The mutant exhibited an extended growth cycle at low multiplicity of infection and appeared to be compromised in efficient transit of virus particles from the infected cell. In mice injected intracranially, the 50% lethal dose of the mutant was reduced more than 30,000-fold. Recovery of the mutant from the latently infected sacral ganglia of mice injected peripherally was significantly less than that of wild-type virus, suggesting a marked defect in the establishment of, or reactivation from, latent infection.     

Isolation of herpes simplex virus procapsids from cells infected with a protease-deficient mutant virus

Herpes simplex virus type 1 (HSV-1) capsid proteins assemble in vitro into spherical procapsids that differ markedly in structure and stability from mature polyhedral capsids but can be converted to the mature form. Circumstantial evidence suggests that assembly in vivo follows a similar pathway of procapsid assembly and maturation, a pathway that resembles those of double-stranded DNA bacteriophages. We have confirmed the above pathway by isolating procapsids from HSV-1-infected cells and characterizing their morphology, thermal sensitivity, and protein composition. Experiments were carried out with an HSV-1 mutant (m100) deficient in the maturational protease for which it was expected that procapsids-normally, short-lived intermediates-would accumulate in infected cells. Particles isolated from m100-infected cells were found to share the defining properties of procapsids assembled in vitro. For example, by electron microscopy, they were found to be spherical rather than polyhedral in shape, and they disassembled at 0 degrees C, unlike mature capsids, which are stable at this temperature. A three-dimensional reconstruction computed at 18-A resolution from cryoelectron micrographs showed m100 procapsids to be structurally indistinguishable from procapsids assembled in vitro. In both cases, their predominant components are the four essential capsid proteins: the major capsid protein (VP5), the scaffolding protein (pre-VP22a), and the triplex proteins (VP19C and VP23). VP26, a small, abundant but dispensable capsid protein, was not found associated with m100 procapsids, suggesting that it binds to capsids only after they have matured into the polyhedral form. Procapsids were also isolated from cells infected at the nonpermissive temperature with the HSV-1 mutant tsProt.A (a mutant with a thermoreversible lesion in the protease), and their identity as procapsids was confirmed by cryoelectron microscopy. This analysis revealed density on the inner surface of the procapsid scaffolding core that may correspond to the location of the maturational protease. Upon incubation at the permissive temperature, tsProt.A procapsids transformed into polyhedral, mature capsids, providing further confirmation of their status as precursors.     

Herpes simplex virus 1 protein kinase Us3 and major tegument protein UL47 reciprocally regulate their subcellular localization in infected cells

Us3 is a serine-threonine protein kinase encoded by herpes simplex virus 1 (HSV-1). We have identified UL47, a major virion protein, as a novel physiological substrate of Us3. In vitro kinase assays and systematic analysis of mutations at putative Us3 phosphorylation sites near the nuclear localization signal of UL47 showed that serine at residue 77 (Ser-77) was required for Us3 phosphorylation of UL47. Replacement of UL47 Ser-77 by alanine produced aberrant accumulation of UL47 at the nuclear rim and impaired the nuclear localization of UL47 in a significant fraction of infected cells. The same defect in UL47 localization was produced by an amino acid substitution in Us3 that inactivated its protein kinase activity. In contrast, a phosphomimetic mutation at UL47 Ser-77 restored wild-type nuclear localization. The UL47 S77A mutation also reduced viral replication in the mouse cornea and the development of herpes stromal keratitis in mice. In addition, UL47 formed a stable complex with Us3 in infected cells, and nuclear localization of Us3 was significantly impaired in the absence of UL47. These results suggested that Us3 phosphorylation of UL47 Ser-77 promoted the nuclear localization of UL47 in cell cultures and played a critical role in viral replication and pathogenesis in vivo. Furthermore, UL47 appeared to be required for efficient nuclear localization of Us3 in infected cells. Therefore, Us3 protein kinase and its substrate UL47 demonstrated a unique regulatory feature in that they reciprocally regulated their subcellular localization in infected cells.     

Herpes simplex virus type 1 UL51 protein is involved in maturation and egress of virus particles

The UL51 gene of herpes simplex virus type 1 (HSV-1) encodes a phosphoprotein whose homologs are conserved throughout the herpes virus family. Recently, we reported that UL51 protein colocalizes with Golgi marker proteins in transfected cells and that targeting of UL51 protein to the Golgi apparatus depends on palmitoylation of its N-terminal cysteine at position 9 (N. Nozawa, T. Daikoku, T. Koshizuka, Y. Yamauchi, T. Yoshikawa, and Y. Nishiyama, J. Virol. 77:3204-3216, 2003). However, its role in the HSV replication cycle was unknown. Here, we generated UL51-null mutants (FDL51) in HSV-1 to uncover the function of UL51 protein. We show that the mutant plaques were much smaller in size and that maximal titers were reduced nearly 100-fold compared to wild-type virus. Electron microscopy indicated that the formation of nucleocapsids was not affected by the deletion of UL51 but that viral egress from the perinuclear space was severely compromised. In FDL51-infected cells, a large number of enveloped nucleocapsids were observed in the perinuclear space, but enveloped mature virions in the cytoplasm, as well as extracellular mature virions, were rarely detected. These defects were fully rescued by reinsertion of the UL51 gene. These results indicate that UL51 protein is involved in the maturation and egress of HSV-1 virus particles downstream of the initial envelopment step.     

Herpes simplex virus 1 mutant deleted in the alpha 22 gene: growth and gene expression in permissive and restrictive cells and establishment of latency in mice.

R325-beta TK+, a herpes simplex virus 1 mutant carrying a 500-base-pair deletion in the alpha 22 gene and the wild-type (beta) thymidine kinase (TK) gene, was previously shown to grow efficiently in HEp-2 and Vero cell lines. We report that in rodent cell lines exemplified by the Rat-1 line, plating efficiency was reduced and growth was multiplicity dependent. A similar multiplicity dependence for growth and lack of virus spread at low multiplicity was seen in resting, confluent human embryonic lung (HEL) cells. The shutoff of synthesis of beta proteins was delayed and the duration of synthesis of gamma proteins was extended in R325-beta TK+-infected HEL cells relative to cells infected with the wild-type parent, but no significant differences were seen in the total accumulation of viral DNA. To quantify the effect on late (gamma 2) gene expression, a recombinant carrying the deletion in the alpha 22 gene and a gamma 2-TK gene (R325-gamma 2 TK) was constructed and compared with a wild-type virus (R3112) carrying a chimeric gamma 2-TK gene. In Vero cells, the gamma 2-TK gene of R325-gamma 2TK was expressed earlier than and at the same level as the gamma 2-TK gene of R3112. In the confluent resting HEL cells, the expression of the gamma 2-TK gene of the alpha 22- virus was grossly reduced relative to that of the alpha 22+ virus. Electron microscopic studies indicated that the number of intranuclear capsids of R325-beta TK+ virus was reduced relative to that of the parent virus in resting confluent HEL cells, but the number of DNA-containing capsids was higher. Notwithstanding the grossly reduced neurovirulence on intracerebral inoculation in mice, R325-beta TK+ virus was able to establish latency in mice. We conclude that (i) the alpha 22 gene affects late (gamma 2) gene expression, and (ii) a host cell factor complements that function of the alpha 22 gene to a greater extent in HEp-2 and Vero cells than in confluent, resting HEL cells.     

Herpes simplex virus tegument protein VP16 is a component of primary enveloped virions

Immunogold electron microscopy was used to determine whether the tegument proteins VP13/14, VP22, and VP16 of herpes simplex virus type 1 (HSV1) are components of primary enveloped virions. Whereas VP13/14 and VP22 were not detected in virus particles in the perinuclear space and were present in only mature extracellular virions, VP16 was acquired prior to primary envelopment of the virus at the inner nuclear membrane. This finding highlights potential similarities and differences between HSV1 and the related alphaherpesvirus, pseudorabies virus, in which the homologues of all three of these tegument proteins are not incorporated into the virion until secondary envelopment.     

Differential feulgen-deoxyribonucleic acid hydrolysis patterns of Herpes simplex virus type 1 and type 2 infected cells.

Infection of human embryonic lung cells with herpes simplex virus type 1 (HSV-1) and herpes simplex type 1 (HSV-2) resulted in: (a) qualitative (nuclear cytopathologic) alterations and quantitative (nuclear area) differences in infected compared to control nuclei; (b) increased Feulgen-deoxyribonucleic acid (F-DNA) amounts in infected cells, probably due to viral DNA; (c) higher F-DNA levels in HSV-2 infected cells; and (d) increased rates of F-DNA hydrolysis in viral-infected as compared to uninfected nuclei.     

Herpes simplex virus type-1 immediate-early gene expression and shut off of host protein synthesis are inhibited in neomycin-treated human epidermoid carcinoma 2 cells

Infection of human epidermoid carcinoma-2 (HEp-2) cells by Herpes simplex virus type 1 (HSV-1) leads to significant activation of inositol phospholipid turnover after 15 min. The effect of neomycin, an inhibitor of inositol phospholipid turnover, has been investigated for its effect on HSV-1 multiplication in HEp-2 cells. HSV-1 multiplication is inhibited by neomycin. This inhibition is not due to a block of virus adsorption or penetration. Neomycin inhibits the expression of virus immediate-early genes, as well as expression of early genes and viral DNA synthesis. In neomycin-treated cells, the usual virion-associated shut off of host protein synthesis does not occur. These results indicate that the inositol phospholipid pathway is involved in immediate-early gene expression and shut off of host protein synthesis in HEp-2 cells.     

Phenotype of a herpes simplex virus type 1 mutant that fails to express immediate-early regulatory protein ICP0

Herpes simplex virus type 1 (HSV-1) immediate-early (IE) regulatory protein ICP0 is required for efficient progression of infected cells into productive lytic infection, especially in low-multiplicity infections of limited-passage human fibroblasts. We have used single-cell-based assays that allow detailed analysis of the ICP0-null phenotype in low-multiplicity infections of restrictive cell types. The major conclusions are as follows: (i) there is a threshold input multiplicity above which the mutant virus replicates normally; (ii) individual cells infected below the threshold multiplicity have a high probability of establishing a nonproductive infection; (iii) such nonproductively infected cells have a high probability of expressing IE products at 6 h postinfection; (iv) even at 24 h postinfection, IE protein-positive nonproductively infected human fibroblast cells exceed the number of cells that lead to plaque formation by up to 2 orders of magnitude; (v) expression of individual IE proteins in a proportion of the nonproductively infected cells is incompletely coordinated; (vi) the nonproductive cells can also express early gene products at low frequencies and in a stochastic manner; and (vii) significant numbers of human fibroblast cells infected at low multiplicity by an ICP0-deficient virus are lost through cell death. We propose that in the absence of ICP0 expression, HSV-1 infected human fibroblasts can undergo a great variety of fates, including quiescence, stalled infection at a variety of different stages, cell death, and, for a minor population, initiation of formation of a plaque.     

Release of a virus coded glycoprotein from herpes simplex virus type 1 infected cells

HEp-2 cells, which were infected with HSV-1, excrete besides other proteins a soluble glycoprotein (Mr 125000–130000) related to the virus protein gC. The excretion of the glycoprotein and the production of extracellular virus particles is reduced to a similar extent when the cells were treated with monensin. Possible consequences of the excretion of soluble viral proteins to a modulation of the immune response are discussed.Abbreviations HSV-1 Herpes simplex virus type 1 - PAGE Polyacrylamide gel electrophoresis - SDS Sodium dodecylsulfate     

Herpes simplex virus type 2 glycoprotein G is targeted by the sulfated oligo- and polysaccharide inhibitors of virus attachment to cells

Variants of herpes simplex virus type 2 (HSV-2) generated by virus passage in GMK-AH1 cells in the presence of the sulfated oligosaccharide PI-88 were analyzed. Many of these variants were substantially resistant to PI-88 in their initial infection of cells and/or their cell-to-cell spread. The major alteration detected in all variants resistant to PI-88 in the initial infection of cells was a frameshift mutation(s) in the glycoprotein G (gG) gene that resulted in the lack of protein expression. Molecular transfer of the altered gG gene into the wild-type background confirmed that the gG-deficient recombinants were resistant to PI-88. In addition to PI-88, all gG-deficient variants of HSV-2 were resistant to the sulfated polysaccharide heparin. The gG-deficient virions were capable of attaching to cells, and this activity was relatively resistant to PI-88. In addition to having a drug-resistant phenotype, the gG-deficient variants were inefficiently released from infected cells. Purified gG bound to heparin and showed the cell-binding activity which was inhibited by PI-88. Many PI-88 variants produced syncytia in cultured cells and contained alterations in gB, including the syncytium-inducing L792P amino acid substitution. Although this phenotype can enhance the lateral spread of HSV in cells, it conferred no virus resistance to PI-88. Some PI-88 variants also contained occasional alterations in gC, gD, gE, gK, and UL24. In conclusion, we found that glycoprotein gG, a mucin-like component of the HSV-2 envelope, was targeted by sulfated oligo- and polysaccharides. This is a novel finding that suggests the involvement of HSV-2 gG in interactions with sulfated polysaccharides, including cell surface glycosaminoglycans.