VP16 targets an amino-terminal domain of HCF involved in cell cycle progression

Peroxisome proliferator-activated receptor gamma coactivator 1beta (PGC-1beta ), a novel PGC-1-related transcription coactivator associated with host cell factor

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) plays a critical role in regulating multiple aspects of energy metabolism, including adaptive thermogenesis, mitochondrial biogenesis, and fatty acid beta-oxidation. Recently, this coactivator of nuclear receptors/transcription factors has been shown to control hepatic gluconeogenesis, an important component of the pathogenesis of both type-1 and type-2 diabetes. We described here the cloning of a novel bona fide homologue of PGC-1, PGC-1beta (PGC-1 was renamed as PGC-1alpha), first identified through searches of new data base entries. Despite the fact that PGC-1alpha and -1beta share similar tissue distributions with highest levels of expression in brown fat and heart, their mRNAs are differentially regulated in the brown adipose tissue upon cold exposure and during brown fat cell differentiation. Like PGC-1alpha, PGC-1beta mRNA levels are increased significantly in the liver during fasting, suggesting a possible role for this factor in the regulation of hepatic gluconeogenesis and/or fatty acid oxidation. Consistent with this, PGC-1beta was shown to physically interact and potently coactivate hepatic nuclear factor 4 and peroxisome proliferator-activated receptor alpha, nuclear receptors that are essential for hepatic adaptation to fasting. Finally, using sequence comparisons between PGC-1alpha and -1beta, we have identified a conserved amino acid motif that serves as a docking site for host cell factor, a cellular protein implicated in cell cycle regulation and viral infection. HCF is shown to bind to both PGC-1alpha and -1beta and augment their transcriptional activity.

Stabilization but not the transcriptional activity of herpes simplex virus VP16-induced complexes is evolutionarily conserved among HCF family members

The human herpes simplex virus (HSV) protein VP16 induces formation of a transcriptional regulatory complex with two cellular factors-the POU homeodomain transcription factor Oct-1 and the cell proliferation factor HCF-1-to activate viral immediate-early-gene transcription. Although the cellular role of Oct-1 in transcription is relatively well understood, the cellular role of HCF-1 in cell proliferation is enigmatic. HCF-1 and the related protein HCF-2 form an HCF protein family in humans that is related to a Caenorhabditis elegans homolog called CeHCF. In this study, we show that all three proteins can promote VP16-induced-complex formation, indicating that VP16 targets a highly conserved function of HCF proteins. The resulting VP16-induced complexes, however, display different transcriptional activities. In contrast to HCF-1 and CeHCF, HCF-2 fails to support VP16 activation of transcription effectively. These results suggest that, along with HCF-1, HCF-2 could have a role, albeit probably a different role, in HSV infection. CeHCF can mimic HCF-1 for both association with viral and cellular proteins and transcriptional activation, suggesting that the function(s) of HCF-1 targeted by VP16 has been highly conserved throughout metazoan evolution.

Inhibition of LZIP-mediated transcription through direct interaction with a novel host cell factor-like protein

Host cell factor 1 (HCF-1) is a cellular transcriptional coactivator which coordinates the assembly of enhancer complex through direct interactions with viral and cellular trans-activators such as VP16, Oct-1, LZIP, and GA-binding protein. These interactions are mediated by the beta-propeller domain comprising the first 380 residues of HCF-1 with six kelch repeats. Here we describe the identification and characterization of a novel HCF-like kelch repeat protein, designated HCLP-1. HCLP-1 is a ubiquitously expressed nuclear protein which is composed almost entirely of a six-bladed beta-propeller. HCLP-1 selectively interacts with LZIP but not with VP16. The physical interaction between HCLP-1 and LZIP leads to the repression of the LZIP-dependent transcription. The HCLP-1-binding domain of LZIP maps to residues 109-315, which contain the bZIP DNA-binding motif. Electrophoretic mobility shift assay demonstrates that HCLP-1 indeed interferes with the binding of LZIP to its DNA target. Thus, HCLP-1 serves a transcriptional co-repressor function mediated through its inhibitory interaction with the LZIP transcription factor. Our findings suggest a new mechanism for transcriptional regulation by HCF-like proteins.

DNA recognition by the herpes simplex virus transactivator VP16: a novel DNA-binding structure

Upon infection, the herpes simplex virus (HSV) transcriptional activator VP16 directs the formation of a multiprotein-DNA complex-the VP16-induced complex-with two cellular proteins, the host cell factor HCF-1 and the POU domain transcription factor Oct-1, on TAATGARAT-containing sequences found in the promoters of HSV immediate-early genes. HSV VP16 contains carboxy-terminal sequences important for transcriptional activation and a central conserved core that is important for VP16-induced complex assembly. On its own, VP16 displays little, if any, sequence-specific DNA-binding activity. We show here that, within the VP16-induced complex, however, the VP16 core has an important role in DNA binding. Mutation of basic residues on the surface of the VP16 core reveals a novel DNA-binding surface with essential residues which are conserved among VP16 orthologs. These results illuminate how, through association with DNA, VP16 is able to interpret cis-regulatory signals in the DNA to direct the assembly of a multiprotein-DNA transcriptional regulatory complex.

Loss of HCF-1-chromatin association precedes temperature-induced growth arrest of tsBN67 cells

Human HCF-1 is a large, highly conserved, and abundant nuclear protein that plays an important but unknown role in cell proliferation. It also plays a role in activation of herpes simplex virus immediate-early gene transcription by the viral regulatory protein VP16. A single proline-to-serine substitution in the HCF-1 VP16 interaction domain causes a temperature-induced arrest of cell proliferation in hamster tsBN67 cells and prevents transcriptional activation by VP16. We show here that HCF-1 is naturally bound to chromatin in uninfected cells through its VP16 interaction domain. HCF-1 is chromatin bound in tsBN67 cells at permissive temperature but dissociates from chromatin before tsBN67 cells stop proliferating at the nonpermissive temperature, suggesting that loss of HCF-1 chromatin association is the primary cause of the temperature-induced tsBN67 cell proliferation arrest. We propose that the role of HCF-1 in cell proliferation is to regulate gene transcription by associating with a multiplicity of DNA-bound transcription factors through its VP16 interaction domain.

Human T-lymphotropic virus type 1 p30(II) functions as a transcription factor and differentially modulates CREB-responsive promoters

Human T-lymphotropic virus type 1 (HTLV-1), a complex retrovirus, causes adult T-cell lymphoma/leukemia and is linked to a variety of immune-mediated disorders. The roles of proteins encoded in the pX open reading frame (ORF) II gene region in HTLV-1 replication or in mediating virus-associated diseases remain to be defined. A nucleus-localizing 30-kDa protein, p30(II), encoded within pX ORF II has limited homology with the POU family of transcription factors. Recently, we reported that selected mutations in pX ORF II diminish the ability of HTLV-1 to maintain high viral loads in infected rabbits. Herein we have tested the transcriptional ability of p30(II) in mammalian cells by using yeast Gal4 fusion protein vectors and transfection of luciferase reporter genes driven by CREB-responsive promoters. p30(II) as a Gal4 DNA-binding domain (DBD) fusion protein transactivates Gal4-driven luciferase reporter gene activity up to 25-fold in 293 and HeLa-tat cells. We confirmed nuclear localization of p30(II) and demonstrate dose-dependent binding of p30(II)-Gal4(DBD) to Gal4 DNA-binding sites. The transcriptional activity of p30(II)-Gal4(DBD) was independent of TATA box flanking sequences, as shown by using two different Gal4 reporter systems. Studies of selected p30(II) mutants indicated that domains that mediate transcription are restricted to a central core region of the protein between amino acids 62 and 220. Transfection of a p30(II)-expressing plasmid repressed cellular CRE-driven reporter gene activity, with or without Tax expression. In contrast, p30(II) at lower concentrations enhanced HTLV-1 long terminal repeat-driven reporter gene activity independent of Tax expression. These data are the first to demonstrate a transcriptional function for p30(II) and suggest a mechanism by which this nuclear protein may influence HTLV-1 replication or cellular gene expression in vivo.

N-terminal transcriptional activation domain of LZIP comprises two LxxLL motifs and the host cell factor-1 binding motif

Host Cell Factor-1 (HCF-1, C1) was first identified as a cellular target for the herpes simplex virus transcriptional activator VP16. Association between HCF and VP16 leads to the assembly of a multiprotein enhancer complex that stimulates viral immediate-early gene transcription. HCF-1 is expressed in all cells and is required for progression through G(1) phase of the cell cycle. In addition to VP16, HCF-1 associates with a cellular bZIP protein known as LZIP (or Luman). Both LZIP and VP16 contain a four-amino acid HCF-binding motif, recognized by the N-terminal beta-propeller domain of HCF-1. Herein, we show that the N-terminal 92 amino acids of LZIP contain a potent transcriptional activation domain composed of three elements: the HCF-binding motif and two LxxLL motifs. LxxLL motifs are found in a number of transcriptional coactivators and mediate protein-protein interactions, notably recognition of the nuclear hormone receptors. LZIP is an example of a sequence-specific DNA-binding protein that uses LxxLL motifs within its activation domain to stimulate transcription. The LxxLL motifs are not required for association with the HCF-1 beta-propeller and instead interact with other regions in HCF-1 or recruit additional cofactors.

Carboxy terminus of human herpesvirus 8 latency-associated nuclear antigen mediates dimerization, transcriptional repression, and targeting to nuclear bodies

Human herpesvirus 8 (HHV-8; also known as Kaposi's sarcoma-associated herpesvirus) is the causative agent of Kaposi's sarcoma and certain B-cell lymphomas. In most infected cells, HHV-8 establishes a latent infection characterized by the expression of latency-associated nuclear antigen (LANA) encoded by open reading frame 73. Although unrelated by sequence, there are functional similarities between LANA and the EBNA-1 protein of Epstein-Barr virus. Both accumulate as subnuclear speckles and are required for maintenance of the viral episome. EBNA-1 also regulates viral gene expression and is required for cell immortalization, suggesting that LANA performs similar functions in the context of HHV-8 infection. Here we show that LANA forms stable dimers, or possibly higher-order multimers, and that this is mediated by a conserved region in the C terminus. By expressing a series of truncations, we show that both the N- and C-terminal regions localize to the nucleus, although only the C terminus accumulates as nuclear speckles characteristic of the intact protein. Lastly, we show that LANA can function as a potent transcriptional repressor when tethered to constitutively active promoters via a heterologous DNA-binding domain. Domains in both the N and C termini mediate repression. This suggests that one function of LANA is to suppress the expression of the viral lytic genes or cellular genes involved in the antiviral response.

HCF-1 amino- and carboxy-terminal subunit association through two separate sets of interaction modules: involvement of fibronectin type 3 repeats

When herpes simplex virus infects permissive cells, the viral regulatory protein VP16 forms a specific complex with HCF-1, a preexisting nuclear protein involved in cell proliferation. The majority of HCF-1 in the cell is a complex of associated amino (HCF-1(N))- and carboxy (HCF-1(C))-terminal subunits that result from an unusual proteolytic processing of a large precursor polypeptide. Here, we have characterized the structure and function of sequences required for HCF-1(N) and HCF-1(C) subunit association. HCF-1 contains two matched pairs of self-association sequences called SAS1 and SAS2. One of these matched association sequences, SAS1, consists of a short 43-amino-acid region of the HCF-1(N) subunit, which associates with a carboxy-terminal region of the HCF-1(C) subunit that is composed of a tandem pair of fibronectin type 3 repeats, a structural motif known to promote protein-protein interactions. Unexpectedly, the related protein HCF-2, which is not proteolyzed, also contains a functional SAS1 association element, suggesting that this element does not function solely to maintain HCF-1(N) and HCF-1(C) subunit association. HCF-1(N) subunits do not possess a nuclear localization signal. We show that, owing to a carboxy-terminal HCF-1 nuclear localization signal, HCF-1(C) subunits can recruit HCF-1(N) subunits to the nucleus.

Zhangfei: a second cellular protein interacts with herpes simplex virus accessory factor HCF in a manner similar to Luman and VP16

Host cell factor (HCF, C1, VCAF or CFF) is a cellular protein that is required for transcription activation of herpes simplex virus (HSV) immediate-early (IE) genes by the virion protein VP16. The biological function of HCF remains unclear. Recently we identified a cellular transcription activator, Luman. As with VP16, the transactivation function of Luman is also regulated by HCF. Here we report a second human protein, Zhangfei (ZF) that interacts with HCF in a fashion similar to Luman and VP16. Although ZF shares no significant sequence homology with Luman, the two proteins have some structural similarities. These include: a basic domain-leucine zipper (bZIP) region, an acidic activation domain and a consensus HCF-binding motif. Unlike Luman, or most other bZIP proteins, ZF by itself did not appear to bind consensus bZIP-binding sites. It was also unable to activate promoters containing these response elements. Although in transient expression assays ectopically expressed ZF was unable to block transactivation by VP16 of a HSV IE promoter, ZF could prevent the expression of several HSV proteins in cells infected with the virus. The ability of ZF to block the synthesis of the HSV IE protein ICP0 relied on its binding to HCF, since a mutant of ZF that was unable to bind HCF was also unable to prevent viral IE protein expression.

A novel 50-kilodalton fragment of host cell factor 1 (C1) in G(0) cells

Host cell factor 1 (HCF-1; also called C1) is a 230-kDa protein which is cleaved posttranslationally into separate but associated N- and C-terminal polypeptides. These polypeptides are components of the C1 complex, along with Oct-1 and the viral protein VP16. The C1 complex is formed when herpes simplex virus (HSV) infects a cell and is responsible for transcription of the HSV immediate-early genes. A temperature-sensitive mutation in the N-terminal kelch domain of HCF-1 reversibly arrests cells in a G(0)-like state when grown at the nonpermissive temperature, and the same domain interacts with VP16 in the formation of the C1 complex. The form of HCF-1 in primary G(0) cells was investigated by using peripheral blood mononucleocytes and serum-arrested human primary fibroblasts. A novel 50-kDa N-terminal fragment of HCF-1 encompassing the kelch domain was identified in the cytoplasm of these cells. This fragment arises by proteolysis of the full-length HCF-1 protein and is able to associate with VP16.

Transcriptional and post-transcriptional regulation of monocyte chemoattractant protein-3 gene expression in human endothelial cells by phorbol ester and cAMP signalling

Monocyte chemoattractant protein-3 (MCP-3) is one of the most broadly active chemokines, potentially inducing chemotaxis of all leucocytic cells. In the present study, we examined the regulation of MCP-3 mRNA and protein production in endothelial cells by protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA) and cAMP signalling. On stimulation of endothelial cells with 10 nM PMA, MCP-3 mRNA increased to 300-fold the basal level at 3 hr and rapidly declined to 0.2-fold the basal level at 24 hr. PMA-induced MCP-3 mRNA and protein production of human endothelial cells were partially inhibited by pretreatment with the adenylate cyclase activator, forskolin, or membrane-permeable cAMP derivative. The PMA-induced MCP-3 mRNA increase was almost abrogated when cells were pretreated with cycloheximide (CHX). Forskolin inhibited the transcription of PMA-induced MCP-3 gene expression. Following PMA stimulation for 3 hr, subsequent addition of actinomycin D suppressed the rapid decay of PMA-induced MCP-3 mRNA. These results suggest that PMA induces the transcriptional activation of the MCP-3 gene through de novo protein synthesis and the rapid decay of PMA-induced MCP-3 mRNA through de novo synthesis of adenosine/uridine (AU)-rich element binding proteins and cAMP signalling inhibits the PMA-induced transcriptional activation of the MCP-3 gene expression.

Roscovitine, a specific inhibitor of cellular cyclin-dependent kinases, inhibits herpes simplex virus DNA synthesis in the presence of viral early proteins

We have previously shown that two inhibitors specific for cellular cyclin-dependent kinases (cdks), Roscovitine (Rosco) and Olomoucine (Olo), block the replication of herpes simplex virus (HSV). Based on these results, we demonstrated that HSV replication requires cellular cdks that are sensitive to these drugs (L. M. Schang, J. Phillips, and P. A. Schaffer. J. Virol. 72:5626-5637, 1998). We further established that at least two distinct steps in the viral replication cycle require cdks: transcription of immediate-early (IE) genes and transcription of early (E) genes (L. M. Schang, A. Rosenberg, and P. A. Schaffer, J. Virol. 73:2161-2172, 1999). Since Rosco inhibits HSV replication efficiently even when added to infected cells at 6 h postinfection, we postulated that cdks may also be required for viral functions that occur after E gene expression. In the study presented herein, we tested this hypothesis directly by measuring the efficiency of viral replication, viral DNA synthesis, and expression of several viral genes during infections in which Rosco was added after E proteins had already been synthesized. Rosco inhibited HSV replication, and specifically viral DNA synthesis, when the drug was added at the time of release from a 12-h phosphonoacetic acid (PAA)-induced block in viral DNA synthesis. Inhibition of DNA synthesis was not a consequence of inhibition of expression of IE or E genes in that Rosco had no effect on steady-state levels of two E transcripts under the same conditions in which it inhibited viral DNA synthesis. Moreover, viral DNA synthesis was inhibited by Rosco even in the absence of protein synthesis. In a second series of experiments, the replication of four HSV mutants harboring temperature-sensitive mutations in genes essential for viral DNA replication was inhibited when Rosco was added at the time of shift-down from the nonpermissive to the permissive temperature. Viral DNA synthesis was inhibited by Rosco under these conditions, whereas expression of viral E genes was not affected. We conclude that cellular Rosco-sensitive cdks are required for replication of viral DNA in the presence of viral E proteins. This requirement may indicate that HSV DNA synthesis is functionally linked to transcription, which requires cdks, or that both viral transcription and DNA replication, independently, require viral or cellular factors activated by Rosco-sensitive cdks.

The novel coactivator C1 (HCF) coordinates multiprotein enhancer formation and mediates transcription activation by GABP

Transcription of the herpes simplex virus 1 (HSV-1) immediate early (IE) genes is determined by multiprotein enhancer complexes. The core enhancer assembly requires the interactions of the POU-homeodomain protein Oct-1, the viral transactivator alphaTIF and the cellular factor C1 (HCF). In this context, the C1 factor interacts with each protein to assemble the stable enhancer complex. In addition, the IE enhancer cores contain adjacent binding sites for other cellular transcription factors such as Sp1 and GA-binding protein (GABP). In this study, a direct interaction of the C1 factor with GABP is demonstrated, defining the C1 factor as the critical coordinator of the enhancer complex assembly. In addition, mutations that reduce the GABP transactivation potential also impair the C1-GABP interaction, indicating that the C1 factor functions as a novel coactivator of GABP-mediated transcription. The interaction and coordinated assembly of the enhancer proteins by the C1 factor may be critical for the regulation of the HSV lytic-latent cycle.

Hepatitis C virus core protein-induced loss of LZIP function correlates with cellular transformation

Hepatitis C virus (HCV) is the major etiological agent of blood-borne non-A non-B hepatitis and a leading cause of liver cirrhosis and hepatocellular carcinoma worldwide. HCV core protein is a multifunctional protein with regulatory functions in cellular transcription and virus-induced transformation and pathogenesis. Here we report on the identification of a bZIP nuclear transcription protein as an HCV core cofactor for transformation. This bZIP factor, designated LZIP, activates CRE-dependent transcription and regulates cell proliferation. Loss of LZIP function in NIH 3T3 cells triggers morphological transformation and anchorage-independent growth. We show that HCV core protein aberrantly sequesters LZIP in the cytoplasm, inactivates LZIP function and potentiates cellular transformation. Our findings suggest that LZIP might serve a novel cellular tumor suppressor function that is targeted by the HCV core.

Association of a protein phosphatase 1 activity with the human factor C1 (HCF) complex

We have screened a human cDNA expression library with a digoxygenin-labelled protein phosphatase 1 (PP1) probe to identify novel PP1 interacting proteins. Eleven cDNA clones were isolated, which included genes encoding two previously characterised and six novel PP1 binding proteins. Three of the cDNAs encoded a protein called host cell factor (HCF), which is an essential component of the cellular complex required for the transcription of the herpes simplex virus (HSV) immediate-early (IE) genes. We demonstrate that HCF and PP1 exist as a complex in nuclear extracts and that this complex is distinct from the form of HCF that associates with HSV VP16. The data suggest novel roles for HCF and PP1, which may be relevant to their functions in transcription and cell cycle progression.

Mutations in host cell factor 1 separate its role in cell proliferation from recruitment of VP16 and LZIP

Host cell factor 1 (HCF-1) is a nuclear protein required for progression through G(1) phase of the cell cycle and, via its association with VP16, transcriptional activation of the herpes simplex virus immediate-early genes. Both functions require a six-bladed beta-propeller domain encoded by residues 1 to 380 of HCF-1 as well as an additional amino-terminal region. The beta-propeller domain is well conserved in HCF homologues, consistent with a critical cellular function. To date, the only known cellular target of the beta-propeller is a bZIP transcription factor known as LZIP or Luman. Whether the interaction between HCF-1 and LZIP is required for cell proliferation remains to be determined. In this study, we used directed mutations to show that all six blades of the HCF-1 beta-propeller contribute to VP16-induced complex assembly, association with LZIP, and cell cycle progression. Although LZIP and VP16 share a common tetrapeptide HCF-binding motif, our results reveal profound differences in their interaction with HCF-1. Importantly, with several of the mutants we observe a poor correlation between the ability to associate with LZIP and promote cell proliferation in the context of the full HCF-1 amino terminus, arguing that the HCF-1 beta-propeller domain must target other cellular transcription factors in order to contribute to G(1) progression.

Analysis of HCF, the cellular cofactor of VP16, in herpes simplex virus-infected cells

Herpes simplex virus (HSV) immediate-early (IE) gene expression is initiated via the recruitment of the structural protein VP16 onto specific sites upstream of each IE gene promoter in a multicomponent complex (TRF.C) that also includes the cellular proteins Oct-1 and HCF. In vitro results have shown that HCF binds directly to VP16 and stabilizes TRF.C. Results from transfection assays have also indicated that HCF is involved in the nuclear import of VP16. However, there have been no reports on the role or the fate of HCF during HSV type 1 (HSV-1) infection. Here we show that the intracellular distribution of HCF is dramatically altered during HSV-1 infection and that the protein interacts with and colocalizes with VP16. Moreover, viral protein synthesis and replication were significantly reduced after infection of a BHK-21-derived temperature-sensitive cell line (tsBN67) which contains a mutant HCF unable to associate with VP16 at the nonpermissive temperature. Intracellular distribution of HCF and of newly synthesized VP16 in tsBN67-infected cells was similar to that observed in Vero cells, suggesting that late in infection the trafficking of both proteins was not dependent on their association. We constructed a stable cell line (tsBN67r) in which the temperature-sensitive phenotype was rescued by using an epitope-tagged wild-type HCF. In HSV-1-infected tsBN67r cells at the nonpermissive temperature, direct binding of HCF to VP16 was observed, but virus protein synthesis and replication were not restored to levels observed at the permissive temperature or in wild-type BHK cells. Together these results indicate that the factors involved in compartmentalization of VP16 alter during infection and that late in infection, VP16 and HCF may have additional roles reflected in their colocalization in replication compartments.

The kelch repeat superfamily of proteins: propellers of cell function

The kelch motif was discovered as a sixfold tandem element in the sequence of the Drosophila kelch ORF1 protein. The repeated kelch motifs predict a conserved tertiary structure, a beta-propeller. This module appears in many different polypeptide contexts and contains multiple potential protein-protein contact sites. Members of this growing superfamily are present throughout the cell and extracellularly and have diverse activities. In this review, we discuss current information concerning the structural organization of kelch repeat proteins, their biological roles and the molecular basis of their action.

Early expression of herpes simplex virus (HSV) proteins and reactivation of latent infection

During the last decade, new data accumulated describing the early events during herpes simplex virus 1 (HSV-1) replication occurring before capsid formation and virion envelopment. The HSV virion carries its own specific transcription initiation factor (alpha-TIF), which functions together with other components of the cellular transcriptase complex to mediate virus-specific immediate early (IE) transcription. The virus-coded IE proteins are the transactivator and regulatory elements modulating early transcription and subsequent translation of nonstructural virus-coded proteins needed mainly for viral DNA synthesis and for the supply of corresponding nucleoside components. They also cooperate at the late transcription and translation of the virion (capsid, tegument and envelope) proteins. In addition, the transactivator IE proteins down-regulate their own transcription, while others facilitate viral mRNA processing or interfere with the presentation of newly synthesized virus antigens. Establishment of latency is closely related to the transcription of a separate category of transcripts, termed latency-associated (LAT). Formation of LATs occurs mainly in nondividing neurons which are metabolically less active and express lower levels of cellular transcription factors (nonpermissive cells). Expression of the stable non-spliced (2 kb), and especially of stable spliced (1.5 and 1.45 kb) LATs is a prerequisite for HSV reactivation. Different HSV genomes (from various HSV strains) do not undergo IE transcription at the same rate. Restricted IE transcription and the absence of viral DNA synthesis favors LAT formation and persistence of the silenced genome. Uneven levels of LAT expression and differences in the metabolic state of carrier neurons influence the reactivation competence. Under artificial or natural activation conditions, sufficient amounts of IE transactivator proteins and proteins promoting nucleoside metabolism are synthesized even in the absence of the viral alpha-TIF facilitating reactivation.

Transactivation of herpes simplex virus type 1 immediate-early gene expression by virion-associated factors is blocked by an inhibitor of cyclin-dependent protein kinases

Initiation of productive infection by human herpes simplex virus type 1 (HSV-1) requires cell cycle-dependent protein kinase (cdk) activity. Treatment of cells with inhibitors of cdks blocks HSV-1 replication and prevents accumulation of viral transcripts, including immediate-early (IE) transcripts (26). Inhibition of IE transcript accumulation suggests that virion proteins, such as VP16, require functional cdks to activate viral transcription. In this report, we show that a cdk inhibitor, Roscovitine, blocks VP16-dependent IE gene expression. In the presence of Roscovitine, the level of virion-induced activation of a transfected reporter gene (the gene encoding chloramphenicol acetyltransferase) linked to the promoter-regulatory region of the ICP0 gene was reduced 40-fold relative to that of untreated samples. Roscovitine had little effect on the interaction of VP16 with VP16-responsive DNA sequences as measured by electrophoretic mobility shift assays. These data indicate that VP16-dependent activation of IE gene expression requires functional cdks and that this requirement is independent of the ability of VP16 to bind to DNA.

Crystal structure of the conserved core of the herpes simplex virus transcriptional regulatory protein VP16

On infection, the herpes simplex virus (HSV) virion protein VP16 (Vmw65; alphaTIF) forms a transcriptional regulatory complex-the VP16-induced complex-with two cellular proteins, HCF and Oct-1, on VP16-responsive cis-regulatory elements in HSV immediate-early promoters called TAATGARAT. Comparison of different HSV VP16 sequences reveals a conserved core region that is sufficient for VP16-induced complex formation. The crystal structure of the VP16 core has been determined at 2.1 A resolution. The results reveal a novel, seat-like protein structure. Together with the activity of mutant VP16 proteins, the structure of free VP16 suggests that it contains (1) a disordered carboxy-terminal region that associates with HCF, Oct-1, and DNA in the VP16-induced complex, and (2) a structured region involved in virion assembly and possessing a novel DNA-binding surface that differentiates among TAATGARAT VP16-response elements.

Isolated mammalian and Schizosaccharomyces pombe ran-binding domains rescue S. pombe sbp1 (RanBP1) genomic mutants

Mammalian Ran-binding protein-1 (RanBP1) and its fission yeast homologue, sbp1p, are cytosolic proteins that interact with the GTP-charged form of Ran GTPase through a conserved Ran-binding domain (RBD). In vitro, this interaction can accelerate the Ran GTPase-activating protein-mediated hydrolysis of GTP on Ran and the turnover of nuclear import and export complexes. To analyze RanBP1 function in vivo, we expressed exogenous RanBP1, sbp1p, and the RBD of each in mammalian cells, in wild-type fission yeast, and in yeast whose endogenous sbp1 gene was disrupted. Mammalian cells and wild-type yeast expressing moderate levels of each protein were viable and displayed normal nuclear protein import. sbp1(-) yeast were inviable but could be rescued by all four exogenous proteins. Two RBDs of the mammalian nucleoporin RanBP2 also rescued sbp1(-) yeast. In mammalian cells, wild-type yeast, and rescued mutant yeast, exogenous full-length RanBP1 and sbp1p localized predominantly to the cytosol, whereas exogenous RBDs localized predominantly to the cell nucleus. These results suggest that only the RBD of sbp1p is required for its function in fission yeast, and that this function may not require confinement of the RBD to the cytosol. The results also indicate that the polar amino-terminal portion of sbp1p mediates cytosolic localization of the protein in both yeast and mammalian cells.

Analysis of functional domains of the host cell factor involved in VP16 complex formation

We present biochemical analyses of the regions of the host cell factor (HCF) involved in VP16 complex formation and in the association between the N- and C-terminal domains of HCF itself. We show that the kelch repeat region of HCF (residues 1-380) is sufficient for VP16 complex formation, but that residues C-terminal to the repeats (positions 381-450) interfere with this activity. However, these latter residues are required for the interaction between the N- and C-terminal regions of HCF. The extreme C-terminal region of HCF, corresponding to an area of strong conservation with a Caenorhabditis elegans homologue, is sufficient for interaction with the N-terminal region. These results are discussed with respect to possible differences in the roles of HCF in VP16 activity versus its normal cellular function.

Herpes simplex virus transactivator VP16 discriminates between HCF-1 and a novel family member, HCF-2

Herpes simplex virus infection is initiated by VP16, a viral transcription factor that activates the viral immediate-early (IE) genes. VP16 does not recognize the IE gene promoters directly but instead forms a multiprotein complex with Oct-1 and HCF-1, a ubiquitous nuclear protein required for progression through the G1 phase of the cell cycle. The functional significance of recruiting HCF-1 to the VP16-induced complex is not understood. Here we describe the identification of a second HCF-like protein, designated HCF-2. HCF-2 is smaller than HCF-1 but shares three regions of strong amino acid sequence homology, including the beta-propeller domain required for association with VP16. HCF-2 is expressed in many tissues, especially the testis, and shows a more dynamic pattern of subcellular localization than HCF-1. Although HCF-2 associates with VP16 and can support complex assembly with Oct-1 and DNA, it is significantly less efficient than HCF-1. A similar preference is shown by LZIP, a cellular counterpart of VP16. Analysis of chimeric proteins showed that differences between the fifth and sixth kelch repeats of the beta-propeller domains from HCF-1 and HCF-2 dictate this selectivity. These results reveal an unexpected level of specificity in the recruitment of HCF-1 to the VP16-induced complex, paralleling the preferential selection of Oct-1 rather than the closely related POU domain protein Oct-2. Implications for regulation of the viral life cycle are discussed.

Transcription of herpes simplex virus immediate-early and early genes is inhibited by roscovitine, an inhibitor specific for cellular cyclin-dependent kinases

Although herpes simplex virus (HSV) replicates in noncycling as well as cycling cells, including terminally differentiated neurons, it has recently been shown that viral replication requires the activities of cellular cyclin-dependent kinases (cdks) (L. M. Schang, J. Phillips, and P. A. Schaffer, J. Virol. 72:5626-5637, 1998). Since we were unable to isolate HSV mutants resistant to two cdk inhibitors, Olomoucine and Roscovitine (Rosco), we hypothesized that cdks may be required for more than one viral function during HSV replication. In the experiments presented here, we tested this hypothesis by measuring the efficiency of (i) viral replication; (ii) expression of selected immediate-early (IE) (ICP0 and ICP4), early (E) (ICP8 and TK), and late (L) (gC) genes; and (iii) viral DNA synthesis in infected cultures to which Rosco was added after IE or IE and E proteins had already been synthesized. Rosco inhibited HSV replication, transcription of IE and E genes, and viral DNA synthesis when added at 1, 2, or 6 h postinfection or after release from a 6-h cycloheximide block. Transcription of a representative L gene, gC, was also inhibited by Rosco under all conditions examined. We conclude from these studies that cellular cdks are required for transcription of E as well as IE genes. In contrast, steady-state levels of at least one cellular housekeeping gene were not affected by Rosco. The requirement of viral IE and E transcription for cellular cdks may reflect either a requirement for specific cdk-activated cellular and/or viral transcription factors or a more global requirement for cdks in the transcriptional activation of the viral genome.

Herpes simplex virus type 1 infection stimulates p38/c-Jun N-terminal mitogen-activated protein kinase pathways and activates transcription factor AP-1

Cells respond to environmental stress and proinflammatory cytokines by stimulating the Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and the p38 mitogen-activated protein kinase cascades. Infection of eukaryotic cells with herpes simplex virus type 1 (HSV-1) resulted in stimulation of both JNK/SAPK and p38 mitogen-activated protein kinase after 3 h of infection, and activation reached a maximum of 4-fold by 9 h post-infection. By using a series of mutant viruses, we showed that the virion transactivator protein VP16 stimulates p38/JNK, whereas no immediate-early, early, or late viral expressed gene is involved. We identified the stress-activated protein kinase kinase 1 as an upstream activator of p38/JNK, and we demonstrated that activation of AP-1 binding proceeded p38/JNK stimulation. During infection, the activated AP-1 consisted mainly of JunB and JunD with a simultaneous decrease in the cellular levels of Jun protein. We suggest that activation of the stress pathways by HSV-1 infection either represents a cascade triggered by the virus to facilitate the lytic cycle or a defense mechanism of the host cell against virus invasion.

HCF-dependent nuclear import of VP16

Transactivation by VP16 requires the formation of a multicomponent complex, the TAATGAAAT recognition factor complex (TRF.C), that contains in addition to VP16, two cellular proteins, Oct-1 and HCF. HCF binds directly to VP16 and this promotes subsequent interaction of the VP16-HCF complex with the POU DNA-binding domain of Oct-1 and selective assembly onto target sites. Here we demonstrate a novel role of HCF in the intracellular compartmentalization of VP16. We show that while VP16 does not contain a consensus nuclear localization signal (NLS) and is largely cytoplasmic, co-expression with HCF resulted in VP16 nuclear accumulation. A candidate NLS within the C-terminus of HCF was identified and insertion of this motif into green fluorescent protein (GFP) promoted nuclear accumulation. Conversely, removal of this signal from HCF (HCFDeltaNLS) resulted in its cytoplasmic accumulation. Co-expression of HCFDeltaNLS with wild-type (wt) VP16, or of wt HCF with VP16 mutants lacking HCF-binding activity failed to promote the nuclear enrichment of VP16. These results indicate that in addition to its role in stabilizing TRF.C, HCF acts as a nuclear import factor for VP16.

Selected elements of herpes simplex virus accessory factor HCF are highly conserved in Caenorhabditis elegans

HCF is a mammalian nuclear protein that undergoes proteolytic processing and is required for cell proliferation. During productive herpes simplex virus (HSV) infection, the viral transactivator VP16 associates with HCF to initiate HSV gene transcription. Here, we show that the worm Caenorhabditis elegans possesses a functional homolog of mammalian HCF that can associate with and activate the viral protein VP16. The pattern of sequence conservation, however, is uneven. Sequences required for mammalian HCF processing are not present in C. elegans HCF. Furthermore, not all elements of mammalian HCF that are required for promoting cell proliferation are conserved. Nevertheless, unexpectedly, C. elegans HCF can promote mammalian cell proliferation because a region of HCF that is conserved can promote mammalian cell proliferation better than its human counterpart. These results suggest that HCF possesses a highly conserved role in metazoan cell proliferation which is targeted by VP16 to regulate HSV infection. The precise mechanisms, however, by which HCF functions in mammals and worms appear to differ.

Muskelin, a novel intracellular mediator of cell adhesive and cytoskeletal responses to thrombospondin-1

We have used an expression cloning strategy based on a cell-attachment assay screen to seek identification of molecules required in cellular responses to thrombospondin-1, a regulated macromolecular component of extracellular matrix. We report the identification and functional characterization of a novel, widely expressed, intracellular protein, named muskelin, which contains dispersed motifs with homology to the tandem repeats first identified in the Drosophila kelch ORF1 protein. In adherent C2C12 cells, muskelin localizes in the cytoplasm and at cell margins. Over-expression of muskelin in C2C12 cells promotes cell attachment to the thrombospondin-1 C-terminal domain, alters the mechanisms of attachment to intact thrombospondin-1 and correlates with decreased formation of fascin microspikes and increased assembly of focal contacts by cells adherent on thrombospondin-1. Reciprocally, cell attachment, spreading and cytoskeletal organization are specifically reduced in TSP-1-adherent cells after antisense depletion of muskelin. These results establish a requirement for muskelin in cell responses to thrombospondin-1 and demonstrate that such responses involve a novel process which is integrated into the regulation of cell-adhesive behaviour and cytoskeletal organization.

The herpesvirus transactivator VP16 mimics a human basic domain leucine zipper protein, luman, in its interaction with HCF

In human cells infected with herpes simplex virus (HSV), viral gene expression is initiated by the virion protein VP16. VP16 does not bind DNA directly but forms a multiprotein complex on the viral immediate-early gene promoters with two cellular proteins: the POU domain protein Oct-1 and host cell factor (HCF; also called C1, VCAF, and CFF). Despite its apparent role in stabilizing the VP16-induced transcription complex, the natural biological role of HCF is unclear. Only recently HCF has been implicated in control of the cell cycle. To determine the role of HCF in cells and answer why HSV has evolved an HCF-dependent mechanism for the initiation of the lytic cycle, we identified the first human ligand for HCF (R. Lu et al., Mol. Cell. Biol. 17:5117-5126, 1997). This protein, Luman, is a member of the CREB/ATF family of transcription factors that can activate transcription from promoters containing cyclic AMP response elements (CRE). Here we provide evidence that Luman and VP16 share two important structural features: an acidic activation domain and a common mechanism for binding HCF. We found that Luman, its homolog in Drosophila, dCREB-A (also known as BBF-2), and VP16 bind to HCF by a motif, (D/E)HXY(S/A), present in all three proteins. In addition, a mutation (P134S) in HCF that prevents VP16 binding also abolishes its binding to Luman and dCREB-A. We also show that while interaction with HCF is not required for the ability of Luman to activate transcription when tethered to the GAL4 promoter, it appears to be essential for Luman to activate transcription through CRE sites. These data suggest that the HCF-Luman interaction may represent a conserved mechanism for transcriptional regulation in metazoans, and HSV mimics this interaction with HCF to monitor the physiological state of the host cell.

Requirement for cellular cyclin-dependent kinases in herpes simplex virus replication and transcription

Several observations indicate that late-G1/S-phase-specific cellular functions may be required for herpes simplex virus (HSV) replication: (i) certain mutant HSV strains are replication impaired during infection of cells in the G0/G1 but not in the G1/S phase of the cell cycle, (ii) several late-G1/S-phase-specific cellular proteins and functions are induced during infection, and (iii) the activity of a cellular protein essential for expression of viral immediate-early (IE) genes, HCF, is normally required during the late G1/S phase of the cell cycle. To test the hypothesis that late-G1/S-phase-specific cellular functions are necessary for HSV replication, HEL or Vero cells were infected in the presence of the cell cycle inhibitors roscovitine (Rosco) and olomoucine (Olo). Both drugs inhibit cyclin-dependent kinase 1 (cdk-1) and cdk-2 (required for cell cycle progression into the late G1/S phase) and cdk-5 (inactive in cycling cells) but not cdk-4 or cdk-6 (active at early G1). We found that HSV replication was inhibited by Rosco and Olo but not by lovastatin (a cell cycle inhibitor that does not inhibit cdk activity), staurosporine (a broad-spectrum protein serine-threonine kinase inhibitor), PD98059 (an inhibitor specific for erk-1 and -2) or iso-Olo (a structural isomer of Olo that does not inhibit cdk activity). The concentrations of Rosco and Olo required to inhibit cell cycle progression and viral replication in both HEL and Vero cells were similar. Inhibition of viral replication was found not to be mediated by drug-induced cytotoxicity. Efforts to isolate Rosco- or Olo-resistant HSV mutants were unsuccessful, indicating that these drugs do not act by inhibiting a single viral target. Viral DNA replication and accumulation of IE and early viral RNAs were inhibited in the presence of cell cycle-inhibitory concentrations of Rosco or Olo. We therefore conclude that one or more cdks active from late G1 onward or inactive in nonneuronal cells are required for accumulation of HSV transcripts, viral DNA replication, and production of infectious virus.

OCA-B is a functional analog of VP16 but targets a separate surface of the Oct-1 POU domain

OCA-B is a B-cell-specific coregulator of the broadly expressed POU domain transcription factor Oct-1. OCA-B associates with the Oct-1 POU domain, a bipartite DNA-binding structure containing a POU-specific (POU[S]) domain joined by a flexible linker to a POU homeodomain (POU[H]). Here, we show that OCA-B alters the activity of Oct-1 in two ways. It provides a transcriptional activation domain which, unlike Oct-1, activates an mRNA-type promoter effectively, and it stabilizes Oct-1 on the Oct-1-responsive octamer sequence ATGCAAAT. These properties of OCA-B parallel those displayed by the herpes simplex virus Oct-1 coregulator VP16. OCA-B, however, interacts with a different surface of the DNA-bound Oct-1 POU domain, interacting with both the POU(S) and POU(H) domains and the center of the ATGCAAAT octamer sequence. The OCA-B and VP16 interactions with the Oct-1 POU domain are sufficiently different to permit OCA-B and VP16 to bind the Oct-1 POU domain simultaneously. These results emphasize the structural versatility of the Oct-1 POU domain in its interaction with coregulators.

Viral mimicry: common mode of association with HCF by VP16 and the cellular protein LZIP

Upon infection of human cells, the herpes simplex virus protein VP16 associates with the endogenous cell-proliferation factor HCF. VP16 can also associate with HCFs from invertebrates, suggesting that VP16 mimics a cellular protein whose interaction with HCF has been conserved. Here, we show that VP16 mimics the human basic leucine-zipper protein LZIP, which, through association with HCF, may control cell-cycle progression. VP16 and LZIP share a tetrapeptide motif-D/EHXY-used to associate with human HCF. The LZIP-related Drosophila protein BBF-2/dCREB-A contains this HCF-binding motif, indicating that the LZIP-HCF interaction has been conserved during metazoan evolution.