Role of cyclin D3 in the biology of herpes simplex virus 1 ICPO

A comprehensive analysis of avian lymphoid leukosis-like lymphoma transcriptomes including identification of LncRNAs and the expression profiles

Avian lymphoid leukosis-like (LL-like) lymphoma has been observed in some experimental and commercial lines of chickens that are free of exogenous avian leukosis virus. Reported cases of avian lymphoid leukosis-like lymphoma incidences in the susceptible chickens are relatively low, but the apathogenic subgroup E avian leukosis virus (ALV-E) and the Marek’s disease vaccine, SB-1, significantly escalate the disease incidence in the susceptible chickens. However, the underlying mechanism of tumorigenesis is poorly understood. In this study, we bioinformatically analyzed the deep RNA sequences of 6 lymphoid leukosis-like lymphoma samples, collected from susceptible chickens post both ALV-E and SB-1 inoculation, and identified a total of 1,692 novel long non-coding RNAs (lncRNAs). Thirty-nine of those novel lncRNAs were detected with altered expression in the LL-like tumors. In addition, 13 lncRNAs whose neighboring genes also showed differentially expression and 2 conserved novel lncRNAs, XLOC_001407 and XLOC_022595, may have previously un-appreciated roles in tumor development in human. Furthermore, 14 lncRNAs, especially XLOC_004542, exhibited strong potential as competing endogenous RNAs via sponging miRNAs. The analysis also showed that ALV subgroup E viral gene Gag/Gag-pol and the MD vaccine SB-1 viral gene R-LORF1 and ORF413 were particularly detectable in the LL-like tumor samples. In addition, we discovered 982 novel lncRNAs that were absent in the current annotation of chicken genome and 39 of them were aberrantly expressed in the tumors. This is the first time that lncRNA signature is identified in avian lymphoid leukosis-like lymphoma and suggests the epigenetic factor, lncRNA, is involved with the avian lymphoid leukosis-like lymphoma formation and development in susceptible chickens. Further studies to elucidate the genetic and epigenetic mechanisms underlying the avian lymphoid leukosis-like lymphoma is indeed warranted.

Timing Is Everything

N. Drayman et al. in their recent article (mBio 8:e01612-17, 2017, https://doi.org/10.1128/mBio.01612-17) have used dynamic proteomics and machine learning to show that the cell cycle state of any individual cell affects the outcome of a productive herpes simplex virus 1 (HSV-1) infection. Cells infected from early G₁ through S were most permissive for expression of genes from the HSV-1 genome, whereas cells infected in late G₂ to mitosis were much less so. Most of the infected cells that underwent mitosis became permanently nonpermissive for HSV-1 gene expression afterward. The cell cycle stage accounted for 60% of the success of infection, and cell density and motility accounted for most of the rest. To successfully reactivate, HSV-1 must express its genes in neurons and cells of the spinosum and granulosum epidermis strata. These cells are permanently in the cell cycle stages most permissive for HSV-1 gene expression, and none reenters mitosis, thus maximizing the efficiency of a successful HSV-1 reactivation before the adaptive immunity can control it.

Infected cell protein 0 functional domains and their coordination in herpes simplex virus replication

Herpes simplex virus 1 (HSV-1) is a ubiquitous human pathogen that establishes latent infection in ganglia neurons. Its unique life cycle requires a balanced “conquer and compromise” strategy to deal with the host anti-viral defenses. One of HSV-1 α (immediate early) gene products, infected cell protein 0 (ICP0), is a multifunctional protein that interacts with and modulates a wide range of cellular defensive pathways. These pathways may locate in different cell compartments, which then migrate or exchange factors upon stimulation, for the purpose of a concerted and effective defense. ICP0 is able to simultaneously attack multiple host pathways by either degrading key restrictive factors or modifying repressive complexes. This is a viral protein that contains an E3 ubiquitin ligase, translocates among different cell compartments and interacts with major defensive complexes. The multiple functional domains of ICP0 can work independently and at the same time coordinate with each other. Dissecting the functional domains of ICP0 and delineating the coordination of these domains will help us understand HSV-1 pathogenicity as well as host defense mechanisms. This article focuses on describing individual ICP0 domains, their biochemical properties and their implication in HSV-1 infection. By putting individual domain functions back into the picture of host anti-viral defense network, this review seeks to elaborate the complex interactions between HSV-1 and its host.

Novel roles of cytoplasmic ICP0: proteasome-independent functions of the RING finger are required to block interferon-stimulated gene production but not to promote viral replication

The immediate-early protein ICP0 from herpes simplex virus 1 (HSV-1) plays pleiotropic roles in promoting viral lytic replication and reactivation from latency. Most of the known actions of ICP0 occur in the nucleus and are thought to involve the E3 ubiquitin ligase activity of its RING finger domain, which targets proteins for degradation via the proteasome. Although ICP0 translocates to the cytoplasm as the infection progresses, little is known about its activities in this location. Here, we show that cytoplasmic ICP0 has two distinct functions. In primary cell cultures and in an intravaginal mouse model, cytoplasmic ICP0 promotes viral replication in the absence of an intact RING finger domain. Additionally, ICP0 blocks the activation of interferon regulatory factor 3 (IRF3), a key transcription factor of the innate antiviral response, in a mechanism that requires the RING finger domain but not the proteasome. To our knowledge, this is the first observation of a proteasome-independent function of the RING finger domain of ICP0. Collectively, these results underscore the importance of cytoplasm-localized ICP0 and the diverse nature of its activities. Importance: Despite ICP0 being a well-studied viral protein, the significance of its cytoplasmic localization has been largely overlooked. This is, in part, because common experimental manipulations result in the restriction of ICP0 to the nucleus. By overcoming this constraint, we both further characterize the ability of cytoplasmic ICP0 to inhibit antiviral signaling and show that ICP0 at this site has unexpected activities in promoting viral replication. This demonstrates the importance of considering location when analyzing protein function and adds a new perspective to our understanding of this multifaceted protein.

The stability of herpes simplex virus 1 ICP0 early after infection is defined by the RING finger and the UL13 protein kinase

Herpes simplex virus 1 (HSV-1)-infected cell protein 0 (ICP0) is a multifunctional protein that plays a key role in overcoming numerous facets of host innate immunity. A key function of ICP0 that requires an intact RING finger domain is that of an ubiquitin E3 ligase: ICP0 interacts with at least three ubiquitin-conjugating enzymes of which one, UbcH5a, is required for degradation of PML and SP100. A preceding report showed that ICP0 is highly unstable at very early times after infection but becomes stable at later times. We report here that (i) the degradation of ICP0 is not infected cell specific, (ii) the degradation does not require the interaction of ICP0 with either UbcH5a, UbcH6, or UbcH9, (iii) ICP0 is degraded both early and late in cells infected with a mutant lacking the UL13 protein kinase, (iv) ICP0 encoded by wild-type virus or the ΔUL13 mutant is stable in cells transfected with a plasmid encoding UL13 before infection, (v) ICP0 carrying mutations in the RING finger domain is stable both early and late in infection, and, finally, (vi) in cells infected with both wild type and RING finger mutant only the wild-type ICP0 is rapidly degraded at early times. The results suggest that the stability of ICP0 is mediated by the UL13 protein kinase and that the target of proteolysis is a site at or near the RING domain of ICP0.

IMPORTANCE

ICP0, a major regulatory protein of HSV-1, turns over rapidly early in infection but becomes stable at late times. We report that stabilization requires the presence of UL13 protein kinase and that an ICP0 with mutations in RING finger is stable. In mixed infections mutant ICP0 is stable, whereas the wild-type ICP0 is degraded. Our findings suggest that the lifestyle of HSV-1 requires an ICP0 that turns over rapidly if late proteins are absent.

Use of biotinylated plasmid DNA as a surrogate for HSV DNA to identify proteins that repress or activate viral gene expression

ICP0, a key herpes simplex virus regulatory protein, functions first in the nucleus and then in the cytoplasm. The duration of its nuclear sojourn in cells transfected with DNA and then infected is related to the quantity of transfected DNA. Furthermore, ICP0 transactivates both viral genes and genes encoded by the transfected DNA. The data support the hypothesis that ICP0 is retained in the nucleus until it completes the replacement of repressive chromatin with effector proteins that enable transcription of both DNA templates.To identify the effector proteins, we transfected cells with biotinylated DNA encoding a nonviral gene and then infected the cells with wild-type virus. Proteins bound to transfected biotinylated plasmid recovered from mock-treated and infected cells were identified using mass spectrometry followed by appropriate database search. The transfected DNA from mock-infected cells yielded proteins associated with repression, whereas DNA recovered from infected cells included proteins known to enable transcription and proteins that have not been previously associated with that role. To test the hypothesis that the proteins hitherto not known to associate with viral gene expression are nevertheless essential, we tested the role of the DEAD-box helicase Ddx17. We report that Ddx17 plays a critical role in the expression of early and late viral genes. Thus, biotinylated DNA recovered from transfected infected cells can function as a surrogate for viral DNA and is a rich source of proteins that play a role in viral gene expression but which have not been previously identified in that role.

Activities of ICP0 involved in the reversal of silencing of quiescent herpes simplex virus 1

ICP0 is a transcriptional activating protein required for the efficient replication and reactivation of latent herpes simplex virus 1 (HSV-1). Multiple regions of ICP0 contribute its activity, the most prominent of which appears to be the RING finger, which confers E3 ubiquitin ligase activity. A region in the C terminus of ICP0 has also been implicated in several activities, including the disruption of a cellular repressor complex, REST/CoREST/HDAC1/2/LSD1. We used quiescent infection of MRC-5 cells with a virus that does not express immediate-early proteins, followed by superinfection with various viral mutants to quantify the ability of ICP0 variants to reactivate gene expression and alter chromatin structure. Superinfection with wild-type virus resulted in a 400-fold increase in expression from the previously quiescent d109 genome, the removal of heterochromatin and histones from the viral genome, and an increase in histone marks associated with activated transcription. RING finger mutants were unable to reactivate transcription or remove heterochromatin from d109, while mutants that are unable to bind CoREST activate gene expression from quiescent d109, albeit to a lesser degree than the wild-type virus. One such mutant, R8507, resulted in the partial removal of heterochromatin. Infection with R8507 did not result in the hyperacetylation of H3 and H4. The results demonstrate that (i) consistent with previous findings, the RING finger domain of ICP0 is required for the activation of quiescent genomes, (ii) the RF domain is also crucial for the ultimate removal of repressive chromatin, (iii) activities or interactions specified by the carboxy-terminal region of ICP0 significantly contribute to activation, and (iv) while the effects of the R8507 on chromatin are consistent with a role for REST/CoREST/HDAC1/2/LSD1 in the repression of quiescent genomes, the mutation may also affect other activities involved in derepression.

The herpes simplex virus immediate-early ubiquitin ligase ICP0 induces degradation of the ICP0 repressor protein E2FBP1

E2FBP1/hDRIL1, a DNA-binding A/T-rich interaction domain (ARID) family transcription factor, is expressed ubiquitously in human tissues and plays an essential role in maintaining the proliferation potential of passage-limited human fibroblasts by dissociating promyelocytic leukemia nuclear bodies (PML-NBs). This effect on PML-NBs is similar to that of viral immediate-early gene products, such as infected cellular protein 0 (ICP0) from human herpes simplex virus 1 (HSV-1), which also disrupts PML-NBs to override the intrinsic cellular defense. Here we report that E2FBP1 inhibits accumulation of ICP0 RNA and, at the same time, is degraded via ICP0's herpes ubiquitin ligase 2 (HUL-2) activity upon HSV-1 infection. These reciprocal regulatory roles of ICP0 and E2FBP1 are linked in an ARID-dependent fashion. Our results suggest that E2FBP1 functions as an intrinsic cellular defense factor in spite of its PML-NB dissociation function.

Herpes simplex virus immediate-early ICP0 protein inhibits Toll-like receptor 2-dependent inflammatory responses and NF-kappaB signaling

The discovery of the Toll-like receptors (TLRs) and their importance in the regulation of host responses to infection raised attention to the complex interplay between viral gene products and the host innate immune responses in determining the outcome of virus infection. Robust inflammatory cytokine responses are observed in herpes simplex virus (HSV)-infected animals and cells. Our studies have demonstrated that Toll-like receptor 2 (TLR2) activation by HSV results in NF-κB activation with concomitant inflammatory cytokine production and that TLR2 activation plays a critical role in HSV-induced pathology and mortality. Here we demonstrate that the HSV-1 immediate-early ICP0 protein reduces the TLR2-mediated inflammatory response to HSV 1 (HSV-1) infection. Expression of ICP0 alone is sufficient to block TLR2-driven responses to both viral and nonviral ligands at or downstream of the MyD88 adaptor and upstream of p65. ICP0 alone can also reduce the levels of MyD88 and Mal (TIRAP). In HSV-infected cells, the E3 ligase function of ICP0 and cellular proteasomal activity are required for the inhibitory activity. Our results argue for a model in which ICP0 promotes the degradation of TLR adaptor molecules and inhibition of the inflammatory response, much as it inhibits the interferon response by sequestration and degradation of interferon regulatory factor 3 (IRF-3).

Interwoven roles of cyclin D3 and cdk4 recruited by ICP0 and ICP4 in the expression of herpes simplex virus genes

Elsewhere this laboratory reported that (i) ICP0 interacts with cyclin D3 but not D1 or D2. The 3 cyclins independently partially rescue DeltaICP0 mutants. (ii) Interaction with cyclin D3 is required for the switch from nuclear to cytoplasmic accumulation of ICP0. (iii) In infected cells cdk4 is activated whereas cdk2 is not. Inhibition of cdk4 results in nuclear retention of ICP0. Overexpression of cyclin D3 reverses the effect of the inhibitor. Here we report the following. (i) cdk4 interacts with ICP0, ICP4, and possibly with ICP8. This interaction is required to recruit cdk4 initially to ND10 and later to the viral replication compartments. (ii) cdk4 inhibitor I reduced or delayed the transcription and ultimately translation of mRNAs of ICP4, ICP27, or ICP8 and to a lesser extent that of the ICP0 gene in wild-type virus-infected cells. (iii) Overexpression of cyclin D3 resulted in a more rapid transcription of these genes. In the presence of inhibitor, the rates of accumulation of the products of these genes resemble those of wild-type virus in the absence of inhibitor. (iv) Overexpression of cyclin D3 also results in mobilization of cdk6 in nuclei of infected cells. We conclude that ICP0 encodes a function that enhances the recruitment of cyclin D3 to ND10 structures to activate cdk4 and that ICP0 along with other viral proteins recruits cdk4 to ND10 structures and ultimately to replication compartments for enhanced expression of viral genes and viral DNA synthesis.

Recruitment of herpes simplex virus type 1 immediate-early protein ICP0 to the virus particle

Although the herpes simplex virus type 1 (HSV-1) tegument is comprised of a large number of viral and cellular proteins, how and where in the cell these proteins are recruited into the virus structure is poorly understood. We have shown previously that the immediate-early gene product ICP0 is packaged by a mechanism dependent on the major tegument protein VP22, while others have shown a requirement for ICP27. We now extend our studies to show that ICP0 packaging correlates directly with the ability of ICP0 to complex with VP22 in infected cells. ICP27 is not, however, present in this VP22-ICP0 complex but is packaged into the virion in a VP22- and ICP0-independent manner. Biochemical fractionation of virions indicated that ICP0 associates tightly with the virus capsid, but intranuclear capsids contained no detectable ICP0. The RING finger domain of ICP0 and the N terminus of VP22 were both shown to be essential but not sufficient for ICP0 packaging and complex formation. Strikingly, however, the N-terminal region of VP22, while unable to form a complex with ICP0, inhibited its translocation from the nucleus to the cytoplasm. PML degradation by ICP0 was efficient in cells infected with this VP22 mutant virus, confirming that ICP0 retains activity. Hence, we would suggest that VP22 is an important molecular partner of ICP0 that controls at least one of its activities: its assembly into the virion. Moreover, we propose that the pathway by which VP22 recruits ICP0 to the virion may begin in the nucleus prior to ICP0 translocation to its final site of assembly in the cytoplasm.

During its nuclear phase the multifunctional regulatory protein ICP0 undergoes proteolytic cleavage characteristic of polyproteins

ICP0 is a multifunctional herpes simplex virus protein known primarily as a promiscuous transactivator. In the course of productive infection, it is localized during the first 5-7 h in the nucleus and later in the cytoplasm. In the nucleus, its primary activities are to suppress the silencing of viral DNA by host proteins, activate cdk4 through recruitment of cyclin D3 to the sites of formation of replication compartments, and degrade several cellular proteins including PML and Sp100, key components of the ND10 nuclear bodies. ICP0 is not translocated to the cytoplasm in cells infected with mutants incapable of performing these tasks. We report the unexpected finding that ICP0 is cleaved into several discrete polypeptides by a proteasome-independent process. The products of this cleavage accumulate in cells infected with ICP0 mutants incapable of degrading PML and therefore are retained in the nucleus. In the second step, the products of the initial cleavage of wild-type virus-infected cells are themselves subject to proteasome-dependent degradation. The average half life of intact ICP0 during the nuclear phase is approximately 1 h. The proteasome-independent cleavage products are no longer detected at late times corresponding to the cytoplasmic phase of ICP0. The results are consistent with the hypothesis that the cleavage products of ICP0 function in topologically distinct domains during its nuclear phase.

ICP0 enables and monitors the function of D cyclins in herpes simplex virus 1 infected cells

The herpes simplex virus 1 ICP0 is a regulatory protein. Early in infection ICP0 localizes in ND10 bodies and performs two functions: As an E3 ligase in conjunction with E2 UbcH5a conjugating enzyme, it degrades the ND10 components PML and SP100. Concurrently, it suppresses the silencing of viral DNA by dispersing the HDAC1/CoREST/REST/LSD1 repressor complex. Subsequently, ICP0 is exported to the cytoplasm. In cells treated with HDAC inhibitors or transfected with irrelevant DNA, the export is delayed in a DNA dose-dependent fashion. Here, we follow up an observation that ICP0 binds cyclin D3 and that ICP0 mutants unable to bind cyclin D3 are not exported. Moreover, in infected cells cdk4 is activated, but cdk2 is not. We report that (i) cyclin D1, D2, or D3 colocalize with ND10 bodies and ICP0 early in infection and ultimately become incorporated into viral replication compartments, (ii) each of the D cyclins partially rescues DeltaICP0 mutants, and (iii) inhibition of cdk4 by inhibitor I sequesters ICP0 in the nucleus. A key finding is that overexpression of cyclin D3 enables the transport of ICP0 to the cytoplasm. We conclude that (i) ICP0 facilitates the recruitment of cyclin D3 to the sites of viral DNA synthesis, (ii) until its functions are completed, ICP0 is retained in the nucleus, and (iii) a common signal that results in the export of ICP0 to the cytoplasm is the accumulation of a viral DNA-synthesis-dependent late protein.

Translocation and colocalization of ICP4 and ICP0 in cells infected with herpes simplex virus 1 mutants lacking glycoprotein E, glycoprotein I, or the virion host shutoff product of the UL41 gene

In wild-type herpes simplex virus 1-infected cells, the major regulatory protein ICP4 resides in the nucleus whereas ICP0 becomes dynamically associated with proteasomes and late in infection is translocated and dispersed in the cytoplasm. Inhibition of proteasomal function results in retention or transport of ICP0 to the nucleus. We report that in cells infected with mutants lacking glycoprotein E (gE), glycoprotein I (gI), or the product of the U(L)41 gene, both ICP4 and ICP0 are translocated to the cytoplasm and coaggregate in small dense structures that, in the presence of proteasomal inhibitor MG132, also contain proteasomal components. Gold particle-conjugated antibody to ICP0 reacted in thin sections with dense protein aggregates in the cytoplasm of mutant virus-infected cells. Similar aggregates were present in the nuclei but not in the cytoplasm of wild-type virus-infected cells. Exposure of cells early in infection to MG132 does not result in retention of ICP0 as in wild-type virus-infected cells. The results suggest that the retention of ICP4 and ICP0 in the nucleus is a dynamic process that involves the function of other viral proteins that may include the Fc receptor formed by the gE/gI complex and is not merely the consequence of expression of a nuclear localization signal. It is noteworthy that in DeltaU(L)41-infected cells gE is retained in the trans-Golgi network and is not widely dispersed in cellular membranes.

Herpes simplex virus type 1 immediate-early protein ICP27 is required for efficient incorporation of ICP0 and ICP4 into virions

Early in infection, herpes simplex virus type 1 (HSV-1) immediate-early (IE) proteins ICP0 and ICP4 localize to the nucleus, where they stimulate viral transcription. Later in infection, ICP0 and to a lesser extent ICP4 accumulate in the cytoplasm, but their biological role there is unknown. Previously, it was shown that the cytoplasmic localization of ICP0/4 requires the multifunctional IE protein ICP27, which is itself an activator of viral gene expression. Here, we identify a viral ICP27 mutant, d3-4, which is unable to efficiently localize ICP0 and ICP4 to the cytoplasm but which otherwise resembles wild-type HSV-1 in its growth and viral gene expression phenotypes. These results genetically separate the function of ICP27 that affects ICP0/4 localization from its other functions, which affect viral growth and gene expression. As both ICP0 and ICP4 are known to be minor virion components, we used d3-4 to test the hypothesis that the cytoplasmic localization of these proteins is required for their incorporation into viral particles. Consistent with this conjecture, d3-4 virions were found to lack ICP0 in their tegument and to have greatly reduced levels of ICP4. Thus, the cytoplasmic localization of ICP0 and ICP4 appears to be a prerequisite for the assembly of these important transcriptional regulatory proteins into viral particles. Furthermore, our results show that ICP27 plays a previously unrecognized role in determining the composition of HSV-1 virions.

Relaxed repression of herpes simplex virus type 1 genomes in Murine trigeminal neurons

The expression of herpes simplex virus (HSV) genomes in the absence of viral regulatory proteins in sensory neurons is poorly understood. Previously, our group reported an HSV immediate early (IE) mutant (d109) unable to express any of the five IE genes and encoding a model human cytomegalovirus immediate early promoter-green fluorescent protein (GFP) transgene. In cultured cells, GFP expressed from this mutant was observed in only a subset of infected cells. The subset exhibited cell type dependence, as the fractions of GFP-expressing cells varied widely among the cell types examined. Herein, we characterize this mutant in murine embryonic trigeminal ganglion (TG) cultures. We found that d109 was nontoxic to neural cultures and persisted in the cultures throughout their life spans. Unlike with some of the cultured cell lines and strains, expression of the GFP transgene was observed in a surprisingly large subset of neurons. However, very few nonneuronal cells expressed GFP. The abilities of ICP0 and an inhibitor of histone deacetylase, trichostatin A (TSA), to activate GFP expression from nonexpressing cells were also compared. The provision of ICP0 by infection with d105 reactivated quiescent genomes in nearly every cell, whereas reactivation by TSA was much more limited and restricted to the previously nonexpressing neurons. Moreover, we found that d109, which does not express ICP0, consistently reactivated HSV type 1 (KOS) in latently infected adult TG cultures. These results suggest that the state of persisting HSV genomes in some TG neurons may be more dynamic and more easily activated than has been observed with nonneuronal cells.

The herpes simplex virus type 1 BgKL variant, unlike the BgOL variant, shows a higher association with orolabial infection than with infections at other sites, supporting the variant-dispersion-replacement hypothesis

The identification and geographic distribution of the herpes simplex virus type 1 (HSV-1) BglII restriction fragment length polymorphism (RFLP) variants named BgK(L) and BgO(L) in clinical isolates from orolabial and cutaneous sites were described in our previous reports, in which the dispersion and replacement of HSV-1 variants were proposed. The base substitution sites deduced from the BgK(L) multiple RFLP variations were mapped to the U(L)12 (DNase), R(L)2 (alpha0 transactivator), and latency-associated transcript genes in the present study. The results show that the relative frequencies (RFs) of BgK(L) are significantly higher in orolabial and cutaneous HSV-1 infections than in ocular infections. For the BgO(L) variant, the opposite was found; i.e., the RF of BgO(L) was significantly lower in orolabial and cutaneous infections than in ocular infections. No significant differences in the RFs of non-BgK(L):non-BgO(L) isolates were observed. The ratio of the BgK(L) RF to the BgO(L) RF was much higher for the orolabial and cutaneous infection groups than for the ocular infection group, whereas the BgK(L) RF-to-non-BgK(L):non-BgO(L) RF ratios for the former groups were slightly higher than those for the latter group. The higher efficiency of orolabial and cutaneous infections caused by BgK(L) compared to the efficiency of infections caused by BgO(L) allows BgK(L) to spread more efficiently in human populations and to displace BgO(L), because the mouth and lips are the most common HSV-1 infection sites in children. The present study supports our HSV-1 dispersion-and-replacement hypothesis and suggests that HSV-1, the latency-reactivation of which allows variants to accumulate in human populations, has evolved under competitive conditions, providing a new perspective on the polymorphism or variation of HSV-1.

ICP27-dependent resistance of herpes simplex virus type 1 to leptomycin B is associated with enhanced nuclear localization of ICP4 and ICP0

It was previously shown that herpes simplex virus type 1 (HSV-1) is sensitive to leptomycin B (LMB), an inhibitor of nuclear export factor CRM1, and that a single methionine to threonine change at residue 50 (M50T) of viral immediate-early (IE) protein ICP27 can confer LMB resistance. In this work, we show that deletion of residues 21-63 from ICP27 can also confer LMB resistance. We further show that neither the M50T mutation nor the presence of LMB affects the nuclear shuttling activity of ICP27, suggesting that another function of ICP27 determines LMB resistance. A possible clue to this function emerged when it was discovered that LMB treatment of HSV-1-infected cells dramatically enhances the cytoplasmic accumulation of two other IE proteins, ICP0 and ICP4. This effect is completely dependent on ICP27 and is reversed in cells infected with LMB-resistant mutants. Moreover, LMB-resistant mutations in ICP27 enhance the nuclear localization of ICP0 and ICP4 even in the absence of LMB, and this effect can be discerned in transfected cells. Thus, the same amino (N)-terminal region of ICP27 that determines sensitivity to LMB also enhances ICP27's previously documented ability to promote the cytoplasmic accumulation of ICP4 and ICP0. We speculate that ICP27's effects on ICP4 and ICP0 may contribute to HSV-1 LMB sensitivity.

Degradation of cyclins D in pseudorabies virus (PRV) infected proliferating cells

The pseudorabies virus code for an ICP0 protein which is half the size of the HSV1 ICP0 protein. In this work, we made the assumption that some function might have been lost in the ICP0 from PRV. One function attributed to the ICP0 from HSV1 was the stabilization of cyclins D. We then looked at the stability of these cyclins during the lytic infection with the PRV. Our results show that cyclins D are not stabilized during infection with the PRV. These results are in accord with recent data from the literature.

Herpes simplex virus type 1 regulatory protein ICP0 does not protect cyclins D1 and D3 from degradation during infection

Previous reports have suggested that herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP0 stabilizes cyclins D1 and D3 during infection by inducing the degradation of cdc34, the E2-conjugating enzyme that is responsible for regulating the stability of these cyclins. Since ICP0 has complex effects on the progress of viral infection that vary greatly with cell type and viral dose, it can be difficult to distinguish between direct effects caused by ICP0 itself and indirect effects caused by the rate of the progression of infection in the absence of ICP0 at the chosen multiplicity of infection. This report describes the fates of cdc34 and cyclins D1 and D3 during HSV-1 infection under conditions that ensured that viral infection and gene expression were proceeding at equivalent rates in the presence and absence of ICP0. It was confirmed that both D-type cyclins were unstable during HSV-1 infection of a variety of cell types, but no effect on cdc34 was observed, even when high levels of ICP0 were expressed. Furthermore, there was no evidence that ICP0 protected either cyclin D1 or cyclin D3 from degradation. Reconstruction of the conditions of the experiments in the previous studies, using the stated cell type and multiplicities of infection, indicated that the original results could be explained by differences in the rate of progression of infection rather than by the presence or absence of ICP0. The data presented in this report are incompatible with the hypothesis that ICP0 induces the degradation of cdc34 and thereby stabilizes cyclins D1 and D3 during HSV-1 infection.

Herpes simplex virus protein kinase US3 activates and functionally overlaps protein kinase A to block apoptosis

Herpes simplex virus 1 encodes at least four genes whose functions include blocking apoptosis induced by exogenous agents (e.g., sorbitol, Fas ligand, and BAD protein) or replication-incompetent mutants (e.g., the d120 mutant lacking both copies of the alpha 4 gene). U(S)3, one of these four genes, encodes a serine-threonine kinase that has been demonstrated to block apoptosis induced by proapoptotic cellular proteins or by the d120 mutant. The amino acid context of serine-threonine phosphorylated by U(S)3 is similar to that of the cAMP-dependent protein kinase PKA. We report that (i) the pattern of proteins phosphorylated by U(S)3 in transduced cells or in cells infected with WT virus overlaps that of phosphoproteins targeted by PKA, (ii) activation of PKA blocks apoptosis induced by d120 mutant or by BAD protein independently of U(S)3, (iii) U(S)3 protein kinase phosphorylates peptides containing the serine or threonine targeted by PKA including that present in the regulatory type II alpha subunit of PKA, and (iv) in WT virus-infected cells the regulatory type II alpha subunit is phosphorylated in a U(S)3-dependent manner. We conclude that a major determinant of the antiapoptotic activity of the U(S)3 protein kinase is the phosphorylation of PKA substrates by either or both enzymes.

Effects of pharmacological cyclin-dependent kinase inhibitors on viral transcription and replication

Cyclin-dependent kinases (CDKs) are required for replication of adeno-, papilloma- and other viruses that replicate only in dividing cells. Surprisingly, CDKs are also required for replication of HIV-1, HSV-1, and other viruses that can replicate in non-dividing cells. Since two low-molecular weight pharmacological CDK inhibitors (PCIs), flavopiridol (Flavo) and roscovitine (Rosco), appear to be non-toxic in human clinical trials against cancer, these drugs have been proposed as potential antiviral drugs. Rosco preferentially inhibits CDKs involved in cell cycle regulation (CDK1, 2, and 7) or neuronal functions (CDK5), whereas Flavo preferentially inhibits CDKs involved in cell cycle (CDK1, 2, 4, 7) or transcription (CDK7, and 9). As potential antivirals, PCIs display several advantages: (i) they are active against many different viruses, including drug-resistant strains of HIV-1 and HSV-1; (ii) PCI-resistant mutants of HIV-1 or HSV-1 have not been identified; and (iii) the antiviral effects of PCIs and conventional antivirals appear to be additive (as expected from drugs that target independent pathways). Moreover, PCIs target both the etiological agents (i.e., the virus) and the pathogenic mechanisms (i.e., unrestricted cell division) of the many diseases that include both a CDK-requiring virus and unrestricted cell division (e.g., Kaposi's sarcoma, cervical carcinoma, HIV-associated nephropathy-HIVAN). This is nicely illustrated in a recent study which demonstrated the efficacy of Flavo in a mouse model of HIVAN. Herein, we will review the involvement of CDKs in viral replication and the antiviral properties of the most extensively characterized PCIs, with special emphasis on the mechanisms of inhibition of viral transcription.

Herpes simplex virus 1 gene expression is accelerated by inhibitors of histone deacetylases in rabbit skin cells infected with a mutant carrying a cDNA copy of the infected-cell protein no. 0

An earlier report showed that the expression of viral genes by a herpes simplex virus 1 mutant [HSV-1(vCPc0)] in which the wild-type, spliced gene encoding infected-cell protein no. 0 (ICP0) was replaced by a cDNA copy is dependent on both the cell type and multiplicity of infection. At low multiplicities of infection, viral gene expression in rabbit skin cells was delayed by many hours, although ultimately virus yield was comparable to that of the wild-type virus. This defect was rescued by replacement of the cDNA copy with the wild-type gene. To test the hypothesis that the delay reflected a dysfunction of ICP0 in altering the structure of host protein-viral DNA complexes, we examined the state of histone deacetylases (HDACs) (HDAC1, HDAC2, and HDAC3). We report the following. (i) HDAC1 and HDAC2, but not HDAC3, were modified in infected cells. The modification was mediated by the viral protein kinase U(S)3 and occurred between 3 and 6 h after infection with wild-type virus but was delayed in rabbit skin cells infected with HSV-1(vCPc0) mutant, concordant with a delay in the expression of viral genes. (ii) Pretreatment of rabbit skin cells with inhibitors of HDAC activity (e.g., sodium butyrate, Helminthosporium carbonum toxin, or trichostatin A) accelerated the expression of HSV-1(vCPc0) but not that of wild-type virus. We conclude the following. (i) In the interval in which HSV-1(vCPc0) DNA is silent, its DNA is in chromatin-like structures amenable to modification by inhibitors of histone deacetylases. (ii) Expression of wild-type virus genes in these cells precluded the formation of DNA-protein structures that would be affected by either the HDACs or their inhibitors. (iii) Since the defect in HSV-1(vCPc0) maps to ICP0, the results suggest that this protein initiates the process of divestiture of viral DNA from tight chromatin structures but could be replaced by other viral proteins in cells infected with a large number of virions.

Herpes simplex virus 1 mutant in which the ICP0 HUL-1 E3 ubiquitin ligase site is disrupted stabilizes cdc34 but degrades D-type cyclins and exhibits diminished neurotoxicity

Herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0) is a multifunctional protein that functions as a promiscuous transactivator and promotes the degradation of multiple cellular proteins. In vitro studies indicated that it encodes two physically separated functional E3 ubiquitin ligase domains. One, designated herpesvirus ubiquitin ligase 1 (HUL-1), maps to a region encoded by exon 3 and is contained between residues 543 and 680. Deletion of amino acids 621 to 625 abolishes this activity. The second, designated HUL-2, maps to the RING finger domain present in ICP0 encoded by exon 2. Earlier studies have shown that ICP0 stabilizes cyclins D1 and D3, and several lines of investigation led to the hypothesis that this function of ICP0 is the consequence of degradation of the E2 enzyme cdc34, known to be involved in the proteasome-dependent degradation of D-type cyclins. Consistent with this hypothesis, we have previously shown that cdc34 physically interacts with ICP0 at or near aspartate 199 and at amino acids 621 to 625 and that the former site is required for effective ubiquitylation and degradation of cdc34. Furthermore, the ICP0 HUL-1 domain promotes the polyubiquitination of cdc34 in vitro. If the mechanism by which D-type cyclins are salvaged in wild-type-infected cells is dependent on polyubiquitination and consequent destruction of cdc34, than the mutant virus R6701, which was constructed for these studies and lacks ICP0 residues 621 to 625, should destabilize the D cyclins and preclude the degradation of cdc34. We report that ICP0 residues 621 to 625 are essential for degradation of cdc34 in infected cells and for the ICP0-mediated stabilization of D-type cyclins, that a mutation that specifically disrupted the ring finger domain of the HUL-2 site had no effect on the degradation of cdc34 in infected cells, and that deletion of ICP0 residues 621 to 625 decreased the replicative capacity of the virus in growth-arrested but not in dividing cells and resulted in diminished pathogenicity on intracerebral inoculation of mice. We conclude that the ICP0 HUL-1 domain acts in infected cells to degrade cdc34 and that this function requires the interaction of cdc34 with sequences in exons 2 and 3 but does not involve the HUL-2 RING finger E3 domain.

Viruses and the 26S proteasome: hacking into destruction

The discovery that the human papillomavirus E6 oncoprotein could direct the ubiquitination and degradation of the p53 tumour suppressor at the 26S proteasome was the beginning of a new view on virus-host interactions. A decade later, a plethora of viral proteins have been shown to direct host-cell proteins for proteolytic degradation. These activities are required for various aspects of the virus life-cycle from entry, through replication and enhanced cell survival, to viral release. As with oncogenes and cell-cycle control, the study of apparently simple viruses has provided a wealth of information on the function of a whole class of cellular proteins whose function is arguably as important as that of the kinases: the ubiquitin-protein ligases.

The degradation of promyelocytic leukemia and Sp100 proteins by herpes simplex virus 1 is mediated by the ubiquitin-conjugating enzyme UbcH5a

Infected cell protein 0 (ICP0) of herpes simplex virus 1 expresses two E3 ubiquitin (Ub) ligase activities mapping in the domains encoded by exons 2 and 3, respectively. Site 1 (exon 3) is responsible for the degradation of the E2 Ub-conjugating enzyme cdc34 whereas site 2 (exon 2) is associated with a ring finger and has been shown to mediate the degradation of promyelocytic leukemia (PML) and Sp100 proteins and the dispersal of nuclear domain 10 (ND10). In in vitro assays site 2 polyubiquitylates the E2 enzymes UbcH5a and UbcH6 but not other (e.g., UbcH7) enzymes. In this article, we show that ectopic expression of dominant negative UbcH5a carrying the substitution C85A delayed or blocked the degradation of PML and Sp100 and dispersal of ND10 whereas ectopic expression of wild-type UbcH5a or dominant negative UbcH6 and UbcH7 carrying the substitutions C131A and C86A, respectively, had no effect. These results link the degradation of PML and Sp100 and the dispersal of ND10 to the E3 activities of ICP0 associated with the UbcH5a E2 enzyme.

The promyelocytic leukemia nuclear body: sites of activity?

The promyelocytic leukemia (PML) nuclear body is one of many subnuclear domains in the eukaryotic cell nucleus. It has received much attention in the past few years because it accumulates the promyelocytic leukemia protein called PML. This protein is implicated in many nuclear events and is found as a fusion with the retinoic acid receptor RARalpha in leukemic cells. The importance of PML bodies in cell differentiation and growth is implicated in acute promyelocitic leukemia cells, which do not contain PML bodies. Treatment of patients with drugs that reverse the disease phenotype also causes PML bodies to reform. In this review, we discuss the structure, composition, and dynamics that may provide insights into the function of PML bodies. We also discuss the repsonse of PML bodies to cellular stresses, such as virus infection and heat shock. We interpret the changes that occur as evidence for a role of these structures in gene transcription. We also examine the role of the posttranslational modification. SUMO-1 addition, in directing proteins to this nuclear body. Characterization of the mobility of PML body associated proteins further supports a role in specific nuclear events, rather than the bodies resulting from random accumulations of proteins.

Overexpression of promyelocytic leukemia protein precludes the dispersal of ND10 structures and has no effect on accumulation of infectious herpes simplex virus 1 or its proteins

A key early event in the replication of herpes simplex virus 1 (HSV-1) is the localization of infected-cell protein no. 0 (ICP0) in nuclear structures knows as ND10 or promyelocytic leukemia oncogenic domains (PODs). This is followed by dispersal of ND10 constituents such as the promyelocytic leukemia protein (PML), CREB-binding protein (CBP), and Daxx. Numerous experiments have shown that this dispersal is mediated by ICP0. PML is thought to be the organizing structural component of ND10. To determine whether the virus targets PML because it is inimical to viral replication, telomerase-immortalized human foreskin fibroblasts and HEp-2 cells were transduced with wild-type baculovirus or a baculovirus expressing the M(r) 69,000 form of PML. The transduced cultures were examined for expression and localization of PML in mock-infected and HSV-1-infected cells. The results obtained from studies of cells overexpressing PML were as follows. (i) Transduced cells accumulate large amounts of unmodified and SUMO-I-modified PML. (ii) Mock-infected cells exhibited enlarged ND10 structures containing CBP and Daxx in addition to PML. (iii) In infected cells, ICP0 colocalized with PML in ND10 early in infection, but the two proteins did not overlap or were juxtaposed in orderly structures. (iv) The enlarged ND10 structures remained intact at least until 12 h after infection and retained CBP and Daxx in addition to PML. (v) Overexpression of PML had no effect on the accumulation of viral proteins representative of alpha, beta, or gamma groups and had no effect on the accumulation of infectious virus in cells infected with wild-type virus or a mutant (R7910) from which the alpha 0 genes had been deleted. These results indicate the following: (i) PML overexpressed in transduced cells cannot be differentiated from endogenous PML with respect to sumoylation and localization in ND10 structures. (ii) PML does not affect viral replication or the changes in the localization of ICP0 through infection. (iii) Disaggregation of ND10 structures is not an obligatory event essential for viral replication.

Characterization of the novel E3 ubiquitin ligase encoded in exon 3 of herpes simplex virus-1-infected cell protein 0

Infected cell protein 0 (ICP0) of herpes simplex virus-1 is a 775-aa residue multifunctional protein that acts as a promiscuous transactivator linked to the degradation of several proteins. ICP0 is the only protein known which encodes two physically separated E3 ubiquitin (Ub) ligase domains, one, designated herpes virus Ub ligase 1 (HUL-1) located in a domain encoded in exon 3 and one designated herpes virus Ub ligase 2 (HUL-2) associated with the really interesting new gene (RING) finger domain encoded by exon 2. We report the following: (i) ICP0 residues 543-680 are sufficient for HUL-1 E3 activity and necessary determinants are encoded between residues 616 and 680. (ii) In substrate independent in vitro ubiquitylation reactions, a chimeric protein containing the HUL-1 domain promotes the ubiquitylation of itself and the ubiquitin conjugating enzyme (E2) cdc34 and interacts with cdc34. (iii) The mechanism of HUL-1 E3 function does not involve formation of a thioester between the HUL-1 domain and Ub. (iv) Residues 621-625 are essential for in vitro HUL-1 E3 activity and interaction between the HUL-1 domain and cdc34, suggesting that this interaction is required for HUL-1 E3 function.

A protein encoded by the herpes simplex virus (HSV) type 1 2-kilobase latency-associated transcript is phosphorylated, localized to the nucleus, and overcomes the repression of expression from exogenous promoters when inserted into the quiescent HSV genome

Herpes simplex virus (HSV) is characterized by its ability to establish a latent infection in sensory neurons, from which it can periodically reactivate. The mechanisms of latency, however, remain unclear. The HSV genome is quiescent during latency except for the expression of the latency-associated transcripts (LATs). Although the exact function of the LATs remains obscure, current evidence suggests they are multifunctional and are involved in both establishment of latency and reactivation from latency. The LATs contain several open reading frames (ORFs). One or more of the functions of the LATs could therefore be protein mediated. We have previously reported that deregulated expression of the largest of the HSV type 1 (HSV-1) LAT ORFs ( approximately 274 amino acids) greatly enhances virus growth in cell types that are normally relatively nonpermissive for HSV replication and also that it complements mutations to the immediate-early (IE) gene ICP0 (S. K. Thomas, G. Gough, D. S. Latchman, and R. S. Coffin, J. Virol. 73:6618-6625, 1999). Here we show that LAT ORF expression overcomes the repression of expression from exogenous promoters introduced into the HSV-1 genome which normally occurs in the absence of IE gene expression. To further explore LAT ORF function, we have generated an epitope-tagged LAT ORF, LATmycHis, which forms punctate structures in the infected-cell nucleus reminiscent of the structures formed by ICP0. These are associated with the appearance of a phosphorylated form of the protein and are formed adjacent to, or around the edges of, viral replication compartments. These results provide further evidence that the HSV-1 LAT ORF protein is biologically functional and that the tightly regulated expression of this protein may be important in the wild-type latency phenotype in vivo.

Herpes simplex virus 1-infected cell protein 0 contains two E3 ubiquitin ligase sites specific for different E2 ubiquitin-conjugating enzymes

Infected cell protein 0 (ICP0) of herpes simplex virus 1, a multifunctional ring finger protein, enhances the expression of genes introduced into cells by infection or transfection, interacts with numerous cellular and viral proteins, and is associated with the degradation of several cellular proteins. Sequences encoded by exon 2 of ICP0 (residues 20-241) bind the UbcH3 (cdc34) ubiquitin-conjugating enzyme, and its carboxy terminus expresses a ubiquitin ligase activity demonstrable by polyubiquitylation of cdc34 in vitro. We report that: (i) The physical interaction of cdc34 and ICP0 leads to its degradation. Thus, substitution of ICP0 aspartate 199 with alanine attenuates the degradation of cdc34 and its binding to the ICP0 ring finger domain. (ii) Substitution of residue 620 reported to abolish the interaction with a ubiquitin-specific protease has no effect on the function of ubiquitin ligase. (iii) ICP0 contains an additional distinct E3 ligase activity specific for the UbcH5a- and UbcH6 E2-conjugating enzymes mapping to the ring finger domain. This is, to our knowledge, the first identification of a viral protein with at least two physically separated E3 ligase activities with different E2 specificities. The results suggest that each activity may target different proteins.

Herpes simplex virus type 1 immediate-early protein ICP0 and is isolated RING finger domain act as ubiquitin E3 ligases in vitro

Proteasome-dependent degradation of ubiquitinated proteins plays a key role in many important cellular processes. Ubiquitination requires the E1 ubiquitin activating enzyme, an E2 ubiquitin conjugating enzyme, and frequently a substrate-specific ubiquitin protein ligase (E3). One class of E3 ubiquitin ligases has been shown to contain a common zinc-binding RING finger motif. We have previously shown that herpes simplex virus type 1 ICP0, itself a RING finger protein, induces the proteasome-dependent degradation of several cellular proteins and induces the accumulation of colocalizing conjugated ubiquitin in vivo. We now report that both full-length ICP0 and its isolated RING finger domain induce the accumulation of polyubiquitin chains in vitro in the presence of E1 and the E2 enzymes UbcH5a and UbcH6. Mutations within the RING finger region that abolish the in vitro ubiquitination activity also cause severe reductions in ICP0 activity in other assays. We conclude that ICP0 has the potential to act as an E3 ubiquitin ligase during viral infection and to target specific cellular proteins for destruction by the 26S proteasome.

cdc2 cyclin-dependent kinase binds and phosphorylates herpes simplex virus 1 U(L)42 DNA synthesis processivity factor

Earlier studies have shown that cdc2 kinase is activated during herpes simplex virus 1 infection and that its activity is enhanced late in infection even though the levels of cyclin A and B are decreased below levels of detection. Furthermore, activation of cdc2 requires the presence of infected cell protein no. 22 and the U(L)13 protein kinase, the same gene products required for optimal expression of a subset of late genes exemplified by U(S)11, U(L)38, and U(L)41. The possibility that the activation of cdc2 and expression of this subset may be connected emerged from the observation that dominant negative cdc2 specifically blocked the expression of U(S)11 protein in cells infected and expressing dominant negative cdc2. Here we report that in the course of searching for a putative cognate partner for cdc2 that may have replaced cyclins A and B, we noted that the DNA polymerase processivity factor encoded by the U(L)42 gene contains a degenerate cyclin box and has been reported to be structurally related to proliferating cell nuclear antigen, which also binds cdk2. Consistent with this finding, we report that (i) U(L)42 is able to physically interact with cdc2 at both the amino-terminal and carboxyl-terminal domains, (ii) the carboxyl-terminal domain of U(L)42 can be phosphorylated by cdc2, (iii) immunoprecipitates obtained with anti U(L)42 antibody contained a roscovitine-sensitive kinase activity, (iv) kinase activity associated with U(L)42 could be immunodepleted by antibody to cdc2, and (v) U(L)42 transfected into cells associates with a nocodazole-enhanced kinase. We conclude that U(L)42 can associate with cdc2 and that the kinase activity has the characteristic traits of cdc2 kinase.

Posttranslational processing of infected cell proteins 0 and 4 of herpes simplex virus 1 is sequential and reflects the subcellular compartment in which the proteins localize

The herpes simplex virus 1 (HSV-1) infected cell proteins 0 and 4 (ICP0 and ICP4) are multifunctional proteins extensively posttranscriptionally processed by both cellular and viral enzymes. We examined by two-dimensional separations the posttranslational forms of ICP0 and ICP4 in HEp-2 cells and in human embryonic lung (HEL) fibroblasts infected with wild-type virus, mutant R325, lacking the sequences encoding the U(S)1.5 protein and the overlapping carboxyl-terminal domain of ICP22, or R7914, in which the aspartic acid 199 of ICP0 was replaced by alanine. We report the following (i) Both ICP0 and ICP4 were sequentially posttranslationally modified at least until 12 h after infection. In HEL fibroblasts, the processing of ICP0 shifted from A+B forms at 4 h to D+G forms at 8 h and finally to G, E, and F forms at 12 h. The ICP4 progression was from the A' form noted at 2 h to B' and C' forms noted at 4 h to the additional D' and E' forms noted at 12 h. The progression tended to be toward more highly charged forms of the proteins. (ii) Although the overall patterns were similar, the mobility of proteins made in HEp-2 cells differed from those made in HEL fibroblasts. (iii) The processing of ICP0 forms E and F was blocked in HEL fibroblasts infected with R325 or with wild-type virus and treated with roscovitine, a specific inhibitor of cell cycle-dependent kinases cdc2, cdk2, and cdk5. R325-infected HEp-2 cells lacked the D' form of ICP4, and roscovitine blocked the appearance of the most highly charged E' form of ICP4. (iv) A characteristic of ICP0 is that it is translocated into the cytoplasm of HEL fibroblasts between 5 and 9 h after infection. Addition of MG132 to the cultures late in infection resulted in rapid relocation of cytoplasmic ICP0 back into the nucleus. Exposure of HEL fibroblasts to MG132 late in infection resulted in the disappearance of the highly charged ICP0 G isoform. The G form of ICP0 was also absent in cells infected with R7914 mutant. In cells infected with this mutant, ICP0 is not translocated to the cytoplasm. (v) Last, cdc2 was active in infected cells, and this activity was inhibited by roscovitine. In contrast, the activity of cdk2 exhibited by immunoprecipitated protein was reduced and resistant to roscovitine and may represent a contaminating kinase activity. We conclude from these results that the ICP0 G isoform is the cytoplasmic form, that it may be phosphorylated by cdc2, consistent with evidence published earlier (S. J., Advani, R. R. Weichselbaum, and B. Roizman, Proc. Natl. Acad. Sci. USA 96:10996-11001, 2000), and that the processing is reversed upon relocation of the G isoform from the cytoplasm into the nucleus. The processing of ICP4 is also affected by R325 and roscovitine. The latter result suggests that ICP4 may also be a substrate of cdc2 late in infection. Last, additional modifications are superimposed by cell-type-specific enzymes.

The infected cell protein 0 of herpes simplex virus 1 dynamically interacts with proteasomes, binds and activates the cdc34 E2 ubiquitin-conjugating enzyme, and possesses in vitro E3 ubiquitin ligase activity

The infected cell protein 0 (ICP0) of herpes simplex virus 1, a promiscuous transactivator shown to enhance the expression of genes introduced into cells by infection or transfection, interacts with numerous cellular proteins and has been linked to the disruption of ND10 and degradation of several proteins. ICP0 contains a RING finger domain characteristic of a class of E3 ubiquitin ligases. We report that: (i) in infected cells, ICP0 interacts dynamically with proteasomes and is bound to proteasomes in the presence of the proteasome inhibitor MG132. Also in infected cells, cdc34, a polyubiquitinated E2 ubiquitin-conjugating enzyme, exhibits increased ICP0-dependent dynamic interaction with proteasomes. (ii) In an in vitro substrate-independent ubiquitination system, the RING finger domain encoded by exon 2 of ICP0 binds cdc34, whereas the carboxyl-terminal domain of ICP0 functions as an E3 ligase independent of the RING finger domain. The results indicate that ICP0 can act as a unimolecular E3 ubiquitin ligase and that it promotes ubiquitin-protein ligation and binds the E2 cdc34. It differs from other unimolecular E3 ligases in that the domain containing the RING finger binds E2, whereas the ligase activity maps to a different domain of the protein. The results also suggest that ICP0 shuttles between nucleus and cytoplasm as a function of its dynamic interactions with proteasomes.

The U(S)3 protein kinase blocks apoptosis induced by the d120 mutant of herpes simplex virus 1 at a premitochondrial stage

Earlier studies have shown that the d120 mutant of herpes simplex virus 1, which lacks both copies of the alpha4 gene, induces caspase-3-dependent apoptosis in HEp-2 cells. Apoptosis was also induced by the alpha4 rescuant but was blocked by the complementation of rescuant with a DNA fragment encoding the U(S)3 protein kinase (R. Leopardi and B. Roizman, Proc. Natl. Acad. Sci. USA 93:9583-9587, 1996, and R. Leopardi, C. Van Sant, and B. Roizman, Proc. Natl. Acad. Sci. USA 94:7891-7896, 1997). To investigate its role in the apoptotic cascade, the U(S)3 open reading frame was cloned into a baculovirus (Bac-U(S)3) under the control of the human cytomegalovirus immediate-early promoter. We report the following. (i) Bac-U(S)3 blocks processing of procaspase-3 to active caspase. Procaspase-3 levels remained unaltered if superinfected with Bac-U(S)3 at 3 h after d120 mutant infection, but significant amounts of procaspase-3 remained in cells superinfected with Bac-Us3 at 9 h postinfection with d120 mutant. (ii) The U(S)3 protein kinase blocks the proapoptotic cascade upstream of mitochondrial involvement inasmuch as Bac-U(S)3 blocks release of cytochrome c in cells infected with the d120 mutant. (iii) Concurrent infection of HEp-2 cells with Bac-U(S)3 and the d120 mutant did not alter the pattern of accumulation or processing of ICP0, -22, or -27, and therefore U(S)3 does not appear to block apoptosis by targeting these proteins.

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.