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

Characteristics of herpes simplex virus infection and pathogenesis suggest a strategy for vaccine development

Herpes simplex virus (HSV) can cause oral or genital ulcerative lesions and even encephalitis in various age groups with high infection rates. More seriously, HSV may lead to a wide range of recurrent diseases throughout a lifetime. No vaccines against HSV are currently available. The accumulated clinical research data for HSV vaccines reveal that the effects of HSV interacting with the host, especially the host immune system, may be important for the development of HSV vaccines. HSV vaccine development remains a major challenge. Thus, we focus on the research data regarding the interactions of HSV and host immune cells, including dendritic cells (DCs), innate lymphoid cells (ILCs), macrophages, and natural killer (NK) cells, and the related signal transduction pathways involved in immune evasion and cytokine production. The aim is to explore possible strategies to develop new effective HSV vaccines.

The mutated tegument protein UL7 attenuates the virulence of herpes simplex virus 1 by reducing the modulation of α-4 gene transcription

BACKGROUND

UL7, a tegument protein of Herpes Simplex Virus type I (HSV-1), is highly conserved in viral infection and proliferation and has an unknown mechanism of action.

METHODS

A HSV-1 UL7 mutant (UL7-MU) was constructed using the CRISPR-cas9 system. The replication rate and plaque morphology were used to analyze the biological characteristics of the wild-type (WT), UL7-MU and MU-complemented P1 viruses. The virulence of the viruses was evaluated in mice. Real-time RT-qPCR and ChIP assays were used to determine the expression levels of relevant genes.

RESULTS

The replication capacity of a recombinant virus (UL7-MU strain) was 10-fold lower than that of the WT strain. The neurovirulence and pathologic effect of the UL7-MU strain were attenuated in infected mice compared with the WT strain. In the latency model, the expression of latency-associated transcript (LAT) in the central nervous system (CNS) and trigeminal nerve was lower in UL7-MU-infected mice than in WT strain-infected mice. The transcription level of the immediate-early gene α-4 in UL7-MU-infected cells was reduced by approximately 2-fold compared with the clear transcriptional peak identified in WT strain-infected Vero cells within 7 h post-infection (p.i.).

CONCLUSION

By modulating the transcription of the α-4 gene, UL7 may be involved in transcriptional regulation through its interaction with the transcript complex structure of the viral genome during HSV-1 infection.

HSV-1 tegument protein and the development of its genome editing technology

Herpes simplex virus 1 (HSV-1) is composed of complex structures primarily characterized by four elements: the nucleus, capsid, tegument and envelope. The tegument is an important viral component mainly distributed in the spaces between the capsid and the envelope. The development of viral genome editing technologies, such as the identification of temperature-sensitive mutations, homologous recombination, bacterial artificial chromosome, and the CRISPR/Cas9 system, has been shown to largely contribute to the rapid promotion of studies on the HSV-1 tegument protein. Many researches have demonstrated that tegument proteins play crucial roles in viral gene regulatory transcription, viral replication and virulence, viral assembly and even the interaction of the virus with the host immune system. This article briefly reviews the recent research on the functions of tegument proteins and specifically elucidates the function of tegument proteins in viral infection, and then emphasizes the significance of using genome editing technology in studies of providing new techniques and insights into further studies of HSV-1 infection in the future.

Control of alpha-herpesvirus IE gene expression by HCF-1 coupled chromatin modification activities

The immediate early genes of the alpha-herpesviruses HSV and VZV are transcriptionally regulated by viral and cellular factors in a complex combinatorial manner. Despite this complexity and the apparent redundancy of activators, the expression of the viral IE genes is critically dependent upon the cellular transcriptional coactivator HCF-1. Although the role of HCF-1 had remained elusive, recent studies have demonstrated that the protein is a component of multiple chromatin modification complexes including the Set1/MLL1 histone H3K4 methyltransferases. Studies using model viral promoter-reporter systems as well as analyses of components recruited to the viral genome during the initiation of infection have elucidated the significance of HCF-1 chromatin modification complexes in contributing to the final state of modified histones assembled on the viral IE promoters. Strikingly, the absence of HCF-1 results in the accumulation of nucleosomes bearing repressive marks on the viral IE promoters and silencing of viral gene expression.

The neuronal host cell factor-binding protein Zhangfei inhibits herpes simplex virus replication

During lytic infection in epithelial cells the expression of herpes simplex virus type 1 (HSV-1) immediate-early (IE) genes is initiated by a multiprotein complex comprising the virion-associated protein VP16 and two cellular proteins, host cellular factor (HCF) and Oct-1. Oct-1 directly recognizes TAATGARAT elements in promoters of IE genes. The role of HCF is not clear. HSV-1 also infects sensory neurons innervating the site of productive infection and establishes a latent infection in these cells. It is likely that some VP16 is retained by the HSV-1 nucleocapsid as it reaches the neuronal nucleus. Its activity must therefore be suppressed for successful establishment of viral latency. Recently, we discovered an HCF-binding cellular protein called Zhangfei. Zhangfei, in an HCF-dependent manner, inhibits Luman/LZIP/CREB3, another cellular HCF-binding transcription factor. Here we show that Zhangfei is selectively expressed in human neurons. When delivered to cultured cells that do not normally express the protein, Zhangfei inhibited the ability of VP16 to activate HSV-1 IE expression. The inhibition was specific for HCF-dependent transcriptional activation by VP16, since a Gal4-VP16 chimeric protein was inhibited only on a TAATGARAT-containing promoter and not a on a Gal4-containing promoter. Zhangfei associated with VP16 and inhibited formation of the VP16-HCF-Oct-1 complex on TAATGARAT motifs. Zhangfei also suppressed HSV-1-induced expression of several cellular genes including topoisomerase IIalpha, suggesting that in addition to suppressing IE expression Zhangfei may have an inhibitory effect on HSV-1 DNA replication and late gene expression.

Molecular analysis of muskelin identifies a conserved discoidin-like domain that contributes to protein self-association

Muskelin is an intracellular protein with a C-terminal kelch-repeat domain that was initially characterized as having functional involvement in cell spreading on the extracellular matrix glycoprotein thrombospondin-1. As one approach to understanding the functional properties of muskelin, we have combined bioinformatic and biochemical studies. Through analysis of a new dataset of eight animal muskelins, we showed that the N-terminal region of the polypeptide corresponds to a predicted discoidin-like domain. This domain architecture is conserved in fungal muskelins and reveals a structural parallel between the muskelins and certain extracellular fungal galactose oxidases, although the phylogeny of the two groups appears distinct. In view of the fact that a number of kelch-repeat proteins have been shown to self-associate, co-immunoprecipitation, protein pull-down assays and studies of cellular localization were carried out with wild-type, deletion mutant and point mutant muskelins to investigate the roles of the discoidin-like and kelch-repeat domains. We obtained evidence for cis- and trans-interactions between the two domains. These studies provide evidence that muskelin self-associates through a head-to-tail mechanism involving the discoidin-like domain.

A single amino acid substitution in herpes simplex virus type 1 VP16 inhibits binding to the virion host shutoff protein and is incompatible with virus growth

In addition to its well-established role in the activation of herpes simplex virus immediate-early gene transcription, VP16 interacts with and downregulates the function of the virion host shutoff protein (vhs), thereby attenuating vhs-mediated destruction of viral mRNAs and translational arrest at late times of infection. We have carried out two-hybrid analysis in vivo and protein-protein interaction assays in vitro to identify determinants in VP16 necessary for interaction with vhs. The minimal amino-terminal subfragment of VP16 capable of binding to vhs encompassed residues 1 to 345. Alteration of a single leucine at position 344 to alanine (L344A) in the context of the amino-terminal fragment of VP16 containing residues 1 to 404 was sufficient to abolish interaction with vhs in vitro and in vivo. Leu344 could be replaced with hydrophobic amino acids (Ile, Phe, Met, or Val) but not by Asn, Lys, or Pro, indicating that hydrophobicity is an important property of binding to vhs. VP16 harboring a loss-of-function mutation at L344 was not compromised in its ability to interact with host cell factor (HCF-1) or to activate transcription of viral immediate-early genes in transient-transfection assays. Virus complementation assays using the VP16-null virus 8MA and the VP16/vhs double-mutant virus 8MAdeltaSma showed that VP16(L344A) was able to complement the growth of 8MAdeltaSma but not 8MA. Thus, a single point mutation in VP16 uncouples binding to vhs from other functions of VP16 required for virus growth and indicates that direct physical association between VP16 and vhs is necessary to sustain a productive infection.

Primary structure and compartmentalization of Drosophila melanogaster host cell factor

Human host cell factor-1 (HCF-1) is a large, 2035-residue nuclear protein that interacts with cellular and viral transcription factors. It contains an N-terminal kelch domain, C-terminal fibronectin type III (FnIII) domain, and a central region including tandem repeats which act as cleavage sites. A second human HCF-1 related gene encodes a protein with a high degree of homology in both the N-terminal kelch domain and C-terminal FnIII domain, but lacks the central portion and as a result is considerably smaller at 792 residues. A unique HCF orthologue has been found in Caenorhabditis elegans which is structurally more related to HCF-2 than HCF-1. Here we report the cloning and expression of the single Drosophila melanogaster host cell factor orthologue (dHCF). The dHCF is 1500 residues in size, intermediate between HCF-1 and HCF-2 and contains an N-terminal kelch domain, and C-terminal FnIII domain both of which show a very high degree of identity, and a central region of some 700 residues with more limited homology. Despite containing a central region no repeat-related motifs were apparent. The dHCF is expressed as a single unprocessed polypeptide consistent with the lack of the internal HCF-1 processing sites, and exhibits a predominantly nuclear localization. We show that this nuclear localization is dependent on a bipartite nuclear localization signal at the C-terminus of the protein, which contains a long spacer of 20 amino acids between two basic clusters. Finally, we also show that dHCF is unable to rescue the tsBN67 cell cycle arrest phenotype. These results indicate that dHCF is an orthologue of HCF-1, although both proteins might not be functionally exchangeable.

Molecular cloning of Drosophila HCF reveals proteolytic processing and self-association of the encoded protein

HCF-1 functions as a coactivator for herpes simplex virus VP16 and a number of mammalian transcription factors. Mature HCF-1 is composed of two subunits generated by proteolytic cleavage of a larger precursor at six centrally-located HCF(PRO) repeats. The resulting N- and C-terminal subunits remain tightly associated via two complementary pairs of self-association domains: termed SAS1N-SAS1C and SAS2N-SAS2C. Additional HCF proteins have been identified in mammals (HCF-2) and Caenorhabditis elegans (CeHCF). Both contain well-conserved SAS1 domains but do not undergo proteolytic processing. Thus, the significance of the cleavage and self-association of HCF-1 remains enigmatic. Here, we describe the isolation of the Drosophila HCF homologue (dHCF) using a genetic screen based on conservation of the SAS1 interaction. The N-terminal beta-propeller domain of dHCF supports VP16-induced complex formation and is more similar to mammalian HCF-1 than other homologues. We show that full-length dHCF expressed in Drosophila cells undergoes proteolytic cleavage giving rise to tightly associated N- and C-terminal subunits. As with HCF-1, the SAS1N and SAS1C elements of dHCF are separated by a large central region, however, this sequence lacks obvious homology to the HCF(PRO) repeats required for HCF-1 cleavage. The conservation of HCF processing in insect cells argues that formation of separate N- and C-terminal subunits is important for HCF function.

Role of HIV-1 Vpr in AIDS pathogenesis: relevance and implications of intravirion, intracellular and free Vpr

Vpr, a 14-kDa, 96 amino acid protein, is conserved among the primate lentiviruses HIV-1, HIV-2 and Simian Immunodeficiency virus supporting the notion that it plays an important role in virus life cycle in vivo. Vpr appears to have several functions including cell cycle arrest at G2 stage, apoptosis, nuclear localization, nuclear import of the pre-integration complex, cation selective channel activity and transcriptionally activate HIV-1 LTR and other heterologous promoters. Over the years, we have addressed several issues pertaining to Vpr including the amount of Vpr present in the virus particles and structure-function relationship of Vpr. Here, we have reviewed the sources of Vpr that may potentially contribute to the cytopathic features observed in the context of HIV-1 infection. There are three different sources of Vpr available in the infected individuals to initiate the pathogenic effects. These include cell-associated, virion-associated (infectious, infectious-non productive, and non-infectious defective viruses) and free Vpr (cell-free and virus-free). A potential role of Vpr in neuropathogenesis of HIV infection in CNS was also suggested by early studies demonstrating neurotoxicity of recombinant Vpr protein. Interestingly, free Vpr (cell-free and virus-free) has been demonstrated in the serum of HIV-1 infected individuals and in the CSF of AIDS patients with neurological dysfunctions. Based on the toxic effects of extra-cellular Vpr on cells noted in several studies, it is likely that free Vpr could contribute to the bystander cell depletion in lymphoid tissues, peripheral blood, and the CNS. These results led us to propose a model for the role of Vpr in AIDS pathogenesis.

Interaction of HCF-1 with a cellular nuclear export factor

HCF-1 is a cellular protein required by VP16 to activate the herpes simplex virus (HSV) immediate-early genes. VP16 is a component of the viral tegument and, after release into the cell, binds to HCF-1 and translocates to the nucleus to form a complex with the POU domain protein Oct-1 and a VP16-responsive DNA sequence. This VP16-induced complex boosts transcription of the viral immediate-early genes and initiates lytic replication. In uninfected cells, HCF-1 functions as a coactivator for the cellular transcription factors LZIP and GABP and also plays an essential role in cell proliferation. VP16 and LZIP share a tetrapeptide HCF-binding motif recognized by the beta-propeller domain of HCF-1. Here we describe a new cellular HCF-1 beta-propeller domain binding protein, termed HPIP, which contains a functional HCF-binding motif and a leucine-rich nuclear export sequence. We show that HPIP shuttles between the nucleus and cytoplasm in a CRM1-dependent manner and that overexpression of HPIP leads to accumulation of HCF-1 in the cytoplasm. These data suggest that HPIP regulates HCF-1 activity by modulating its subcellular localization. Furthermore, HPIP-mediated export may provide the pool of cytoplasmic HCF-1 required for import of virion-derived VP16 into the nucleus.

Inactivation of the retinoblastoma protein family can bypass the HCF-1 defect in tsBN67 cell proliferation and cytokinesis

Owing to a single missense mutation in the cell proliferation factor HCF-1, the temperature-sensitive tsBN67 hamster cell line arrests proliferation at nonpermissive temperatures, primarily in a G(0)/G(1) state, and displays temperature-sensitive cytokinesis defects. The HCF-1 mutation in tsBN67 cells also causes a temperature-sensitive dissociation of HCF-1 from chromatin prior to cell proliferation arrest, suggesting that HCF-1-chromatin association is important for mammalian-cell proliferation. Here, we report that the simian virus 40 (SV40) early region, in particular, large T antigen (Tag), and the adenovirus oncoprotein E1A can rescue the tsBN67 cell proliferation defect at nonpermissive temperatures. The SV40 early region rescues the tsBN67 cell proliferation defect without restoring the HCF-1-chromatin association, indicating that these oncoproteins bypass a requirement for HCF-1 function. The SV40 early region also rescues the tsBN67 cytokinesis defect, suggesting that the roles of HCF-1 in cell proliferation and proper cytokinesis are intimately linked. The ability of SV40 Tag and adenovirus E1A to inactivate members of the pRb protein family-pRb, p107, and p130-is important for the bypass of HCF-1 function. These results suggest that HCF-1 regulates mammalian-cell proliferation and cytokinesis, at least in part, by either directly or indirectly opposing pRb family member function.

An activation domain in the C-terminal subunit of HCF-1 is important for transactivation by VP16 and LZIP

In herpes simplex virus, lytic replication is initiated by the viral transactivator VP16 acting with cellular cofactors Oct-1 and HCF-1. Although this activator complex has been studied in detail, the role of HCF-1 remains elusive. Here, we show that HCF-1 contains an activation domain (HCF-1(AD)) required for maximal transactivation by VP16 and its cellular counterpart LZIP. Expression of the VP16 cofactor p300 augments HCF-1(AD) activity, suggesting a mechanism of synergy. Infection of cells lacking the HCF-1(AD) leads to reduced viral immediate-early gene expression and lowered viral titers. These findings underscore the importance of HCF-1 to herpes simplex virus replication and VP16 transactivation.

Host cell factor-1 interacts with and antagonizes transactivation by the cell cycle regulatory factor Miz-1

Human host cell factor-1 (HCF-1) is essential for cell cycle progression and is required, in conjunction with the herpes simplex virus transactivator VP16, for induction of viral immediate-early gene expression. We show here that HCF-1 directly binds to the Myc-interacting protein Miz-1, a transcription factor that induces cell cycle arrest at G(1), in part by directly stimulating expression of the cyclin-dependent kinase inhibitor p15(INK4b). A domain encompassing amino acids 750-836, contained within a subregion of HCF-1 required for cell cycle progression, was sufficient to bind Miz-1. Conversely, HCF-1 interacted with two separate regions in Miz-1: the N-terminal POZ domain and a C-terminal domain (residues 637-803) previously shown to harbor determinants for interaction with c-Myc and the coactivator p300. The latter functioned as a potent transactivation domain when tethered to DNA, indicating that HCF-1 targets a transactivation function in Miz-1. HCF-1 or a Miz-1-binding fragment of HCF-1 repressed transactivation by Gal4-Miz-1 in transfection assays. Moreover, HCF-1 repressed Miz-1-mediated transactivation of a reporter gene linked to the p15(INK4b) promoter. Protein/protein interaction studies and transient transfection assays demonstrated that HCF-1 interferes with recruitment of p300 to Miz-1, similar to what has been reported with c-Myc. Our findings identify Miz-1 as a novel HCF-1-interacting partner and illustrate cross-talk between these two proteins that may be of consequence to their respective functions in gene regulation and their opposing effects on the cell cycle.

Herpes simplex virus tegument protein US11 interacts with conventional kinesin heavy chain

Little is known about the mechanisms of transport of neurotropic herpesviruses, such as herpes simplex virus (HSV), varicella-zoster virus, and pseudorabies virus, within neurons. For these viruses, which replicate in the nucleus, anterograde transport from the cell body of dorsal root ganglion (DRG) neurons to the axon terminus occurs over long distances. In the case of HSV, unenveloped nucleocapsids in human DRG neurons cocultured with autologous skin were observed by immunoelectron microscopy to colocalize with conventional ubiquitous kinesin, a microtubule-dependent motor protein, in the cell body and axon during anterograde axonal transport. Subsequently, four candidate kinesin-binding structural HSV proteins were identified (VP5, VP16, VP22, and US11) using oligohistidine-tagged human ubiquitous kinesin heavy chain (uKHC) as bait. Of these viral proteins, a direct interaction between uKHC and US11 was identified. In vitro studies identified residues 867 to 894 as the US11-binding site in uKHC located within the proposed heptad repeat cargo-binding domain of uKHC. In addition, the uKHC-binding site in US11 maps to the C-terminal RNA-binding domain. US11 is consistently cotransported with kinetics similar to those of the capsid protein VP5 into the axons of dissociated rat neurons, unlike the other tegument proteins VP16 and VP22. These observations suggest a major role for the uKHC-US11 interaction in anterograde transport of unenveloped HSV nucleocapsids in axons.

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.

Regulation of herpes simplex virus gene expression

Expression of the more than 80 individual genes of herpes simplex virus 1 (HSV-1) takes place in a tightly regulated sequential manner that was first described over 20 years ago. Investigations since that time have focused on understanding the mechanisms that regulate this orderly and efficient expression of viral genes. This review examines recent findings that have shed light on how this process is regulated during productive infection of the cell. Although the story is still not complete, several aspects of HSV gene expression are now clearer as a result of these findings. In particular, several new functions have recently been ascribed to some of the known viral regulatory proteins. The results indicate that the viral gene expression is regulated through transcriptional as well as post-transcriptional mechanisms. In addition, it has become increasingly clear that the virus has evolved specific functions to interact with the host cell in order to divert and redirect critical host functions for its own needs. Understanding the interactions of HSV and the host cell during infection will be essential for a complete understanding of how viral gene expression is regulated. Future challenges in the field will be to develop a complete understanding of the mechanisms that temporally regulate virus gene expression, and to identify and characterize the relevant interactions between the virus and the distinctive cell types normally infected by the virus.

Differences in determinants required for complex formation and transactivation in related VP16 proteins

VP16-H is an essential structural protein of herpes simplex virus type 1 (HSV-1) and is also a potent activator of virus immediate-early (IE) gene expression. Current models of functional determinants within VP16-H indicate that it consists of two domains, an N-terminal domain involved in recruiting VP16-H to a multicomponent DNA binding complex with two host proteins, Oct-1 and host cell factor (HCF), and an acidic C-terminal domain exclusively involved in transactivation. VP16-E, from equine herpesvirus 1 (EHV-1), exhibits strong conservation with the N-terminal domain of VP16-H but, with the exception of a short segment at the extreme C terminus, lacks almost the entire acidic C-terminal domain. Studies of key activation determinants within the C terminus of VP16-H would predict that VP16-E may activate poorly, if at all. However, VP16-E is a potent activator of both EHV-1 and HSV-1 IE gene transcription. We show that VP16-E does not follow the simple two-domain model of VP16-H. Thus, despite the conservation in the N-terminal domains, this region in VP16-E is not sufficient for assembly into the DNA binding complex with Oct-1 and HCF. The short conserved determinant close to the C terminus is completely dispensable in VP16-H but is absolutely required in VP16-E. In activation studies, the potency of intact VP16-E was not recapitulated in chimeric proteins in which it was fused with a GAL4 DNA binding domain. Furthermore, a chimeric protein consisting of the C-terminal region of VP16-E fused to the N-terminal domain of VP16-H, while able to promote complex formation, nevertheless exhibited very weak activation. These results indicate that the mode of recruitment of the activation domain, i.e., through complex formation with Oct-1 and HCF, may be crucial for activation and that key determinants required for activation in VP16-E, and possibly VP16-H, may involve interactions between regions of the C terminus and the N terminus rather than discrete domains with independent functions.

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.

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.

Autocatalytic proteolysis of the transcription factor-coactivator C1 (HCF): a potential role for proteolytic regulation of coactivator function

Site-specific proteolysis is an important biological mechanism for the regulation of cellular processes such as gene expression, cell signaling, development, and apoptosis. In transcriptional regulation, specific proteolysis regulates the localization and activity of many regulatory factors. The C1 factor (HCF), a cellular transcription factor and coactivator, undergoes site-specific proteolytic processing at a series of unusual amino acid reiterations to generate a family of amino- and carboxyl-terminal polypeptides that remain tightly associated. Expression and purification of bacterially expressed domains of the C1 factor identifies an autocatalytic activity that is responsible for the specific cleavage of the reiterations. In addition, coexpression of the autocatalytic domain with a heterologous protein containing a target cleavage site demonstrates that the C1 protease may also function in trans.

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.

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.