Structural and functional characterization of a complex between the acidic transactivation domain of EBNA2 and the Tfb1/p62 subunit of TFIIH

High-throughput discovery and characterization of viral transcriptional effectors in human cells

Viruses encode transcriptional regulatory proteins critical for controlling viral and host gene expression. Given their multifunctional nature and high sequence divergence, it is unclear which viral proteins can affect transcription and which specific sequences contribute to this function. Using a high-throughput assay, we measured the transcriptional regulatory potential of over 60,000 protein tiles across ∼1,500 proteins from 11 coronaviruses and all nine human herpesviruses. We discovered hundreds of transcriptional effector domains, including a conserved repression domain in all coronavirus Spike homologs, dual activation-repression domains in viral interferon regulatory factors (VIRFs), and an activation domain in six herpesvirus homologs of the single-stranded DNA-binding protein that we show is important for viral replication and late gene expression in Kaposi's sarcoma-associated herpesvirus (KSHV). For the effector domains we identified, we investigated their mechanisms via high-throughput sequence and chemical perturbations, pinpointing sequence motifs essential for function. This work massively expands viral protein annotations, serving as a springboard for studying their biological and health implications and providing new candidates for compact gene regulation tools.

Dynamic structures of intrinsically disordered proteins related to the general transcription factor TFIIH, nucleosomes, and histone chaperones

Advances in structural analysis by cryogenic electron microscopy (cryo-EM) and X-ray crystallography have revealed the tertiary structures of various chromatin-related proteins, including transcription factors, RNA polymerases, nucleosomes, and histone chaperones; however, the dynamic structures of intrinsically disordered regions (IDRs) in these proteins remain elusive. Recent studies using nuclear magnetic resonance (NMR), together with molecular dynamics (MD) simulations, are beginning to reveal dynamic structures of the general transcription factor TFIIH complexed with target proteins including the general transcription factor TFIIE, the tumor suppressor p53, the cell cycle protein DP1, the DNA repair factors XPC and UVSSA, and three RNA polymerases, in addition to the dynamics of histone tails in nucleosomes and histone chaperones. In complexes of TFIIH, the PH domain of the p62 subunit binds to an acidic string formed by the IDR in TFIIE, p53, XPC, UVSSA, DP1, and the RPB6 subunit of three RNA polymerases by a common interaction mode, namely extended string-like binding of the IDR on the positively charged surface of the PH domain. In the nucleosome, the dynamic conformations of the N-tails of histones H2A and H2B are correlated, while the dynamic conformations of the N-tails of H3 and H4 form a histone tail network dependent on their modifications and linker DNA. The acidic IDRs of the histone chaperones of FACT and NAP1 play important roles in regulating the accessibility to histone proteins in the nucleosome.

Thrombocytopenia in COVID‑19 and vaccine‑induced thrombotic thrombocytopenia

The highly heterogeneous symptomatology and unpredictable progress of COVID-19 triggered unprecedented intensive biomedical research and a number of clinical research projects. Although the pathophysiology of the disease is being progressively clarified, its complexity remains vast. Moreover, some extremely infrequent cases of thrombotic thrombocytopenia following vaccination against SARS-CoV-2 infection have been observed. The present study aimed to map the signaling pathways of thrombocytopenia implicated in COVID-19, as well as in vaccine-induced thrombotic thrombocytopenia (VITT). The biomedical literature database, MEDLINE/PubMed, was thoroughly searched using artificial intelligence techniques for the semantic relations among the top 50 similar words (>0.9) implicated in COVID-19-mediated human infection or VITT. Additionally, STRING, a database of primary and predicted associations among genes and proteins (collected from diverse resources, such as documented pathway knowledge, high-throughput experimental studies, cross-species extrapolated information, automated text mining results, computationally predicted interactions, etc.), was employed, with the confidence threshold set at 0.7. In addition, two interactomes were constructed: i) A network including 119 and 56 nodes relevant to COVID-19 and thrombocytopenia, respectively; and ii) a second network containing 60 nodes relevant to VITT. Although thrombocytopenia is a dominant morbidity in both entities, three nodes were observed that corresponded to genes (AURKA, CD46 and CD19) expressed only in VITT, whilst ADAM10, CDC20, SHC1 and STXBP2 are silenced in VITT, but are commonly expressed in both COVID-19 and thrombocytopenia. The calculated average node degree was immense (11.9 in COVID-19 and 6.43 in VITT), illustrating the complexity of COVID-19 and VITT pathologies and confirming the importance of cytokines, as well as of pathways activated following hypoxic events. In addition, PYCARD, NLP3 and P2RX7 are key potential therapeutic targets for all three morbid entities, meriting further research. This interactome was based on wild-type genes, revealing the predisposition of the body to hypoxia-induced thrombosis, leading to the acute COVID-19 phenotype, the 'long-COVID syndrome', and/or VITT. Thus, common nodes appear to be key players in illness prevention, progression and treatment.

PLK1-dependent phosphorylation restrains EBNA2 activity and lymphomagenesis in EBV-infected mice

While Epstein-Barr virus (EBV) establishes a life-long latent infection in apparently healthy human immunocompetent hosts, immunodeficient individuals are at particular risk to develop lymphoproliferative B-cell malignancies caused by EBV. A key EBV protein is the transcription factor EBV nuclear antigen 2 (EBNA2), which initiates B-cell proliferation. Here, we combine biochemical, cellular, and in vivo experiments demonstrating that the mitotic polo-like kinase 1 (PLK1) binds to EBNA2, phosphorylates its transactivation domain, and thereby inhibits its biological activity. EBNA2 mutants that impair PLK1 binding or prevent EBNA2 phosphorylation are gain-of-function mutants. They exhibit enhanced transactivation capacities, accelerate the proliferation of infected B cells, and promote the development of monoclonal B-cell lymphomas in infected mice. Thus, PLK1 coordinates the activity of EBNA2 to attenuate the risk of tumor incidences in favor of the establishment of latency in the infected but healthy host.

Phase separation of Epstein-Barr virus EBNA2 protein reorganizes chromatin topology for epigenetic regulation

Epstein-Barr virus nuclear antigen 2 (EBNA2) is a transactivator of viral and cellular gene expression, which plays a critical role in the Epstein-Barr virus-associated diseases. It was reported that EBNA2 regulates gene expression by reorganizing chromatin and manipulating epigenetics. Recent studies showed that liquid-liquid phase separation plays an essential role in epigenetic and transcriptional regulation. Here we show that EBNA2 reorganized chromatin topology to form accessible chromatin domains (ACDs) of the host genome by phase separation. The N-terminal region of EBNA2, which is necessary for phase separation, is sufficient to induce ACDs. The C-terminal domain of EBNA2 promotes the acetylation of accessible chromatin regions by recruiting histone acetylase p300 to ACDs. According to these observations, we proposed a model of EBNA2 reorganizing chromatin topology for its acetylation through phase separation to explain the mechanism of EBNA2 hijacking the host genome by controlling its epigenetics.

Yang et al find that phase separation of the Epstein-Barr virus nuclear antigen 2 (EBNA2) is involved in the formation of accessible chromatin domains of the host genome. They also find that EBNA2 recruits histone acetyltransferase to promote histone acetylation on accessible chromatin regions and regulate gene expression and that these two functions are performed by the N- and Cterminus respectively.

Three human RNA polymerases interact with TFIIH via a common RPB6 subunit

In eukaryotes, three RNA polymerases (RNAPs) play essential roles in the synthesis of various types of RNA: namely, RNAPI for rRNA; RNAPII for mRNA and most snRNAs; and RNAPIII for tRNA and other small RNAs. All three RNAPs possess a short flexible tail derived from their common subunit RPB6. However, the function of this shared N-terminal tail (NTT) is not clear. Here we show that NTT interacts with the PH domain (PH-D) of the p62 subunit of the general transcription/repair factor TFIIH, and present the structures of RPB6 unbound and bound to PH-D by nuclear magnetic resonance (NMR). Using available cryo-EM structures, we modelled the activated elongation complex of RNAPII bound to TFIIH. We also provide evidence that the recruitment of TFIIH to transcription sites through the p62–RPB6 interaction is a common mechanism for transcription-coupled nucleotide excision repair (TC-NER) of RNAPI- and RNAPII-transcribed genes. Moreover, point mutations in the RPB6 NTT cause a significant reduction in transcription of RNAPI-, RNAPII- and RNAPIII-transcribed genes. These and other results show that the p62–RPB6 interaction plays multiple roles in transcription, TC-NER, and cell proliferation, suggesting that TFIIH is engaged in all RNAP systems.

Structural and dynamical insights into the PH domain of p62 in human TFIIH

TFIIH is a crucial transcription and DNA repair factor consisting of the seven-subunit core. The core subunit p62 contains a pleckstrin homology domain (PH-D), which is essential for locating TFIIH at transcription initiation and DNA damage sites, and two BSD (BTF2-like transcription factors, synapse-associated proteins and DOS2-like proteins) domains. A recent cryo-electron microscopy (cryo-EM) structure of human TFIIH visualized most parts of core, except for the PH-D. Here, by nuclear magnetic resonance spectroscopy we have established the solution structure of human p62 PH-D connected to the BSD1 domain by a highly flexible linker, suggesting the flexibility of PH-D in TFIIH. Based on this dynamic character, the PH-D was modeled in the cryo-EM structure to obtain the whole human TFIIH core structure, which indicates that the PH-D moves around the surface of core with a specific but limited spatial distribution; these dynamic structures were refined by molecular dynamics (MD) simulations. Furthermore, we built models, also refined by MD simulations, of TFIIH in complex with five p62-binding partners, including transcription factors TFIIEα, p53 and DP1, and nucleotide excision repair factors XPC and UVSSA. The models explain why the PH-D is crucially targeted by these factors, which use their intrinsically disordered acidic regions for TFIIH recruitment.

Assays and technologies for developing proteolysis targeting chimera degraders

Targeted protein degradation by small-molecule degraders represents an emerging mode of action in drug discovery. Proteolysis targeting chimeras (PROTACs) are small molecules that can recruit an E3 ligase and a protein of interest (POI) into proximity, leading to induced ubiquitination and degradation of the POI by the proteasome system. To date, the design and optimization of PROTACs remain empirical due to the complicated mechanism of induced protein degradation. Nevertheless, it is increasingly appreciated that profiling step-by-step along the ubiquitin-proteasome degradation pathway using biochemical and biophysical assays are essential in understanding the structure-activity relationship and facilitating the rational design of PROTACs. This review aims to summarize these assays and to discuss the potential of expanding the toolbox with other new techniques.

TFIIH: A multi-subunit complex at the cross-roads of transcription and DNA repair

Transcription factor IIH (TFIIH) is a multiprotein complex involved in both eukaryotic transcription and DNA repair, revealing a tight connection between these two processes. Composed of 10 subunits, it can be resolved into a 7-subunits core complex with the XPB translocase and the XPD helicase, and the 3-subunits kinase complex CAK, which also exists as a free complex with a distinct function. Initially identified as basal transcription factor, TFIIH also participates in transcription regulation and plays a key role in nucleotide excision repair (NER) for opening DNA at damaged sites, lesion verification and recruitment of additional repair factors. Our understanding of TFIIH function in eukaryotic cells has greatly benefited from studies of the genetic rare diseases xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD), that are not only characterized by cancer and aging predispositions but also by neurological and developmental defects. Although much remains unknown about TFIIH function, significant progresses have been done regarding the structure of the complex, the functions of its catalytic subunits and the multiple roles of the regulatory core-TFIIH subunits. This review provides a non-exhaustive survey of key discoveries on the structure and function of this pivotal factor, which can be considered as a promising target for therapeutic strategies.

'Nonlinear' Biochemistry of Nucleosome Detergents

The transcriptional activation domains (TADs) are critical for life, yet intrinsically disordered polypeptides with no specific consensus sequence, interacting with multiple targets via low-specificity fuzzy contacts. The recent integration of machine learning approaches in biochemistry allows analysis of large experimental datasets of functional TADs as a whole and clear observation of TAD features. The emerging picture describes TADs as sequences without consensus but with a variety of detergent-like mini-motifs enriched in negatively charged and aromatic amino acids. Comparison of the canonical direct coactivator recruitment model and a new model describing TADs as nucleosome detergents that trigger chromatin remodeling during gene activation helps solve a fundamental enigma of molecular biology spanning 30 years.

Common TFIIH recruitment mechanism in global genome and transcription-coupled repair subpathways

Nucleotide excision repair is initiated by two different damage recognition subpathways, global genome repair (GGR) and transcription-coupled repair (TCR). In GGR, XPC detects DNA lesions and recruits TFIIH via interaction with the pleckstrin homology (PH) domain of TFIIH subunit p62. In TCR, an elongating form of RNA Polymerase II detects a lesion on the transcribed strand and recruits TFIIH by an unknown mechanism. Here, we found that the TCR initiation factor UVSSA forms a stable complex with the PH domain of p62 via a short acidic string in the central region of UVSSA, and determined the complex structure by NMR. The acidic string of UVSSA binds strongly to the basic groove of the PH domain by inserting Phe408 and Val411 into two pockets, highly resembling the interaction mechanism of XPC with p62. Mutational binding analysis validated the structure and identified residues crucial for binding. TCR activity was markedly diminished in UVSSA-deficient cells expressing UVSSA mutated at Phe408 or Val411. Thus, a common TFIIH recruitment mechanism is shared by UVSSA in TCR and XPC in GGR.

Structural characterization of interactions between transactivation domain 1 of the p65 subunit of NF-κB and transcription regulatory factors

p65 is a member of the NF-κB family of transcriptional regulatory proteins that functions as the activating component of the p65-p50 heterodimer. Through its acidic transactivation domain (TAD), p65 has the capacity to form interactions with several different transcriptional regulatory proteins, including TFIIB, TFIIH, CREB-binding protein (CBP)/p300 and TAFII31. Like other acidic TADs, the p65 TAD contains two subdomains (p65TA1 and p65TA2) that interact with different regulatory factors depending on the target gene. Despite its role in controlling numerous NF-κB target genes, there are no high-resolution structures of p65TA1 bound to a target transcriptional regulatory factor. In this work, we characterize the interaction of p65TA1 with two factors, the Tfb1/p62 subunit of TFIIH and the KIX domain of CBP. In these complexes, p65TA1 transitions into a helical conformation that includes its characteristic ΦXXΦΦ motif (Φ = hydrophobic amino acid). Structural and functional studies demonstrate that the two binding interfaces are primarily stabilized by three hydrophobic amino acids within the ΦXXΦΦ motif and these residues are also crucial to its ability to activate transcription. Taken together, the results provide an atomic level description of how p65TA1 is able to bind different transcriptional regulatory factors needed to activate NF-κB target genes.

Nucleosome distortion as a possible mechanism of transcription activation domain function

After more than three decades since the discovery of transcription activation domains (ADs) in gene-specific activators, the mechanism of their function remains enigmatic. The widely accepted model of direct recruitment by ADs of co-activators and basal transcriptional machinery components, however, is not always compatible with the short size yet very high degree of sequence randomness and intrinsic structural disorder of natural and synthetic ADs. In this review, we formulate the basis for an alternative and complementary model, whereby sequence randomness and intrinsic structural disorder of ADs are necessary for transient distorting interactions with promoter nucleosomes, triggering promoter nucleosome translocation and subsequently gene activation.

Applications of isothermal titration calorimetry - the research and technical developments from 2011 to 2015

Isothermal titration calorimetry is a widely used biophysical technique for studying the formation or dissociation of molecular complexes. Over the last 5 years, much work has been published on the interpretation of isothermal titration calorimetry (ITC) data for single binding and multiple binding sites. As over 80% of ITC papers are on macromolecules of biological origin, this interpretation is challenging. Some researchers have attempted to link the thermodynamics constants to events at the molecular level. This review highlights work carried out using binding sites characterized using x-ray crystallography techniques that allow speculation about individual bond formation and the displacement of individual water molecules during ligand binding and link these events to the thermodynamic constants for binding. The review also considers research conducted with synthetic binding partners where specific binding events like anion-π and π-π interactions were studied. The revival of assays that enable both thermodynamic and kinetic information to be collected from ITC data is highlighted. Lastly, published criticism of ITC research from a physical chemistry perspective is appraised and practical advice provided for researchers unfamiliar with thermodynamics and its interpretation. Copyright © 2016 John Wiley & Sons, Ltd.

Epstein-Barr Virus EBNA-2 Polymorphic Patterns in Nasopharyngeal Carcinoma in Southern China

OBJECTIVES

The aim of this study was to characterize the specific Epstein-Barr virus (EBV) polymorphisms from nasopharyngeal carcinoma (NPC) patients and healthy donors in Southern China, and to explore the relationship between EBV genotypes and NPC.

METHODS

The EBNA-2 gene of 32 EBV-positive NPCs and 39 EBV-positive throat washing (TW) samples from healthy individuals from Southern China was analyzed using PCR and sequencing.

RESULTS

We found E2-A to be the predominant subtype in Southern China. The distribution of EBNA-2 subtypes between NPCs and TWs was significantly different (p < 0.01) and the E2-A subtype was overly represented in NPCs.

CONCLUSIONS

E2-A is the predominant subtype in Southern China, and the distribution of EBNA-2 subtypes between NPCs and TWs is significantly different. In Southern China, the E2-A subtype is significantly more frequent in NPCs than TWs, and is also more common than in Northern China. This suggests that the E2-A subtype may play an important role in the pathogenesis of NPC and that EBNA-2 gene variations are tumor-specific polymorphisms.

Structural Insight into the Mechanism of TFIIH Recognition by the Acidic String of the Nucleotide Excision Repair Factor XPC

In global genome repair (GGR), XPC detects damaged nucleotides and recruits TFIIH complex. The small acidic region of XPC binds to the pleckstrin homology (PH) domain of TFIIH subunit p62; however, the recognition mechanism remains elusive. Here, we use nuclear magnetic resonance to present the tertiary structure of XPC bound to the PH domain. The XPC acidic region forms a long string stabilized by insertion of Trp133 and Val136 into two separate hollows of the PH domain, coupled with extensive electrostatic contacts. Analysis of several XPC mutants revealed that particularly Trp133 is essential for binding to the PH domain. In cell lines stably expressing mutant XPC, alanine substitution at Trp133 or Trp133/Val136 compromised UV resistance, recruitment of TFIIH to DNA damage, and removal of UV-induced photoproducts from genomic DNA. These findings show how TFIIH complex is recruited by XPC to damaged DNA, advancing our understanding of the early stage of GGR.

Characterization of ERM transactivation domain binding to the ACID/PTOV domain of the Mediator subunit MED25

The N-terminal acidic transactivation domain (TAD) of ERM/ETV5 (ERM_38–68), a PEA3 group member of Ets-related transcription factors, directly interacts with the ACID/PTOV domain of the Mediator complex subunit MED25. Molecular details of this interaction were investigated using nuclear magnetic resonance (NMR) spectroscopy. The TAD is disordered in solution but has a propensity to adopt local transient secondary structure. We show that it folds upon binding to MED25 and that the resulting ERM–MED25 complex displays characteristics of a fuzzy complex. Mutational analysis further reveals that two aromatic residues in the ERM TAD (F47 and W57) are involved in the binding to MED25 and participate in the ability of ERM TAD to activate transcription. Mutation of a key residue Q451 in the VP16 H1 binding pocket of MED25 affects the binding of ERM. Furthermore, competition experiments show that ERM and VP16 H1 share a common binding interface on MED25. NMR data confirms the occupancy of this binding pocket by ERM TAD. Based on these experimental data, a structural model of a functional interaction is proposed. This study provides mechanistic insights into the Mediator–transactivator interactions.

The EBNA-2 N-Terminal Transactivation Domain Folds into a Dimeric Structure Required for Target Gene Activation

Epstein-Barr virus (EBV) is a γ-herpesvirus that may cause infectious mononucleosis in young adults. In addition, epidemiological and molecular evidence links EBV to the pathogenesis of lymphoid and epithelial malignancies. EBV has the unique ability to transform resting B cells into permanently proliferating, latently infected lymphoblastoid cell lines. Epstein-Barr virus nuclear antigen 2 (EBNA-2) is a key regulator of viral and cellular gene expression for this transformation process. The N-terminal region of EBNA-2 comprising residues 1-58 appears to mediate multiple molecular functions including self-association and transactivation. However, it remains to be determined if the N-terminus of EBNA-2 directly provides these functions or if these activities merely depend on the dimerization involving the N-terminal domain. To address this issue, we determined the three-dimensional structure of the EBNA-2 N-terminal dimerization (END) domain by heteronuclear NMR-spectroscopy. The END domain monomer comprises a small fold of four β-strands and an α-helix which form a parallel dimer by interaction of two β-strands from each protomer. A structure-guided mutational analysis showed that hydrophobic residues in the dimer interface are required for self-association in vitro. Importantly, these interface mutants also displayed severely impaired self-association and transactivation in vivo. Moreover, mutations of solvent-exposed residues or deletion of the α-helix do not impair dimerization but strongly affect the functional activity, suggesting that the EBNA-2 dimer presents a surface that mediates functionally important intra- and/or intermolecular interactions. Our study shows that the END domain is a novel dimerization fold that is essential for functional activity. Since this specific fold is a unique feature of EBNA-2 it might provide a novel target for anti-viral therapeutics.

Epstein-Barr virus is an oncogenic γ-herpesvirus that may cause infectious mononucleosis in young adults and fatal lymphoproliferative disorders in immunocompromised patients and is associated with the pathogenesis of Burkitt's lymphoma, nasopharyngeal and gastric carcinoma. Epstein-Barr virus nuclear antigen 2 (EBNA-2) is a key regulator of viral and cellular gene expression which initiates and maintains a specific transcription program that promotes proliferation and differentiation of the infected B cell. EBNA-2 is a transcriptional activator that is recruited to DNA by cellular adaptor proteins, carries two transactivation domains, and has the capacity to form dimers or multimers. This study provides the first three-dimensional structure of the EBNA-2 N-terminal Dimerization (END) domain. Two END domain monomers, each consisting of four β-strands and a single α-helix, assemble into a dimer by interaction of two β-strands from each monomer in a parallel fashion. The dimer surface exposes residues that are critical for transactivation of target genes by EBNA-2. The dimeric fold of the EBNA-2 END domain has not been observed for any cellular protein and thus could provide a novel target for anti-viral therapeutics.

A ΩXaV motif in the Rift Valley fever virus NSs protein is essential for degrading p62, forming nuclear filaments and virulence

Rift Valley fever virus (RVFV) is a single-stranded RNA virus capable of inducing fatal hemorrhagic fever in humans. A key component of RVFV virulence is its ability to form nuclear filaments through interactions between the viral nonstructural protein NSs and the host general transcription factor TFIIH. Here, we identify an interaction between a ΩXaV motif in NSs and the p62 subunit of TFIIH. This motif in NSs is similar to ΩXaV motifs found in nucleotide excision repair (NER) factors and transcription factors known to interact with p62. Structural and biophysical studies demonstrate that NSs binds to p62 in a similar manner as these other factors. Functional studies in RVFV-infected cells show that the ΩXaV motif is required for both nuclear filament formation and degradation of p62. Consistent with the fact that the RVFV can be distinguished from other Bunyaviridae-family viruses due to its ability to form nuclear filaments in infected cells, the motif is absent in the NSs proteins of other Bunyaviridae-family viruses. Taken together, our studies demonstrate that p62 binding to NSs through the ΩXaV motif is essential for degrading p62, forming nuclear filaments and enhancing RVFV virulence. In addition, these results show how the RVFV incorporates a simple motif into the NSs protein that enables it to functionally mimic host cell proteins that bind the p62 subunit of TFIIH.

Structural basis of transcription initiation by RNA polymerase II

Transcription of eukaryotic protein-coding genes commences with the assembly of a conserved initiation complex, which consists of RNA polymerase II (Pol II) and the general transcription factors, at promoter DNA. After two decades of research, the structural basis of transcription initiation is emerging. Crystal structures of many components of the initiation complex have been resolved, and structural information on Pol II complexes with general transcription factors has recently been obtained. Although mechanistic details await elucidation, available data outline how Pol II cooperates with the general transcription factors to bind to and open promoter DNA, and how Pol II directs RNA synthesis and escapes from the promoter.

EBNA2 and Its Coactivator EBNA-LP

While all herpesviruses can switch between lytic and latent life cycle, which are both driven by specific transcription programs, a unique feature of latent EBV infection is the expression of several distinct and well-defined viral latent transcription programs called latency I, II, and III. Growth transformation of B-cells by EBV in vitro is based on the concerted action of Epstein-Barr virus nuclear antigens (EBNAs) and latent membrane proteins(LMPs). EBV growth-transformed B-cells express a viral transcriptional program, termed latency III, which is characterized by the coexpression of EBNA2 and EBNA-LP with EBNA1, EBNA3A, -3B, and -3C as well as LMP1, LMP2A, and LMP2B. The focus of this review will be to discuss the current understanding of how two of these proteins, EBNA2 and EBNA-LP, contribute to EBV-mediated B-cell growth transformation.

Chromatin Structure of Epstein-Barr Virus Latent Episomes

EBV latent infection is characterized by a highly restricted pattern of viral gene expression. EBV can establish latent infections in multiple different tissue types with remarkable variation and plasticity in viral transcription and replication. During latency, the viral genome persists as a multi-copy episome, a non-integrated-closed circular DNA with nucleosome structure similar to cellular chromosomes. Chromatin assembly and histone modifications contribute to the regulation of viral gene expression, DNA replication, and episome persistence during latency. This review focuses on how EBV latency is regulated by chromatin and its associated processes.

A single amino acid in EBNA-2 determines superior B lymphoblastoid cell line growth maintenance by Epstein-Barr virus type 1 EBNA-2

Sequence differences in the EBNA-2 protein mediate the superior ability of type 1 Epstein-Barr virus (EBV) to transform human B cells into lymphoblastoid cell lines compared to that of type 2 EBV. Here we show that changing a single amino acid (S442D) from serine in type 2 EBNA-2 to the aspartate found in type 1 EBNA-2 confers a type 1 growth phenotype in a lymphoblastoid cell line growth maintenance assay. This amino acid lies in the transactivation domain of EBNA-2, and the S442D change increases activity in a transactivation domain assay. The superior growth properties of type 1 EBNA-2 correlate with the greater induction of EBV LMP-1 and about 10 cell genes, including CXCR7. In chromatin immunoprecipitation assays, type 1 EBNA-2 is shown to associate more strongly with EBNA-2 binding sites near the LMP-1 and CXCR7 genes. Unbiased motif searching of the EBNA-2 binding regions of the differentially regulated cell genes identified an ETS-interferon regulatory factor composite element motif that closely corresponds to the sequences known to mediate EBNA-2 regulation of the LMP-1 promoter. It appears that the superior induction by type 1 EBNA-2 of the cell genes contributing to cell growth is due to their being regulated in a manner different from that for most EBNA-2-responsive genes and in a way similar to that for the LMP-1 gene.

IMPORTANCE

The EBNA-2 transcription factor plays a key role in B cell transformation by EBV and defines the two EBV types. Here we identify a single amino acid (Ser in type 1 EBV, Asp in type 2 EBV) of EBNA-2 that determines the superior ability of type 1 EBNA-2 to induce a key group of cell genes and the EBV LMP-1 gene, which mediate the growth advantage of B cells infected with type 1 EBV. The EBNA-2 binding sites in these cell genes have a sequence motif similar to the sequence known to mediate regulation of the EBV LMP-1 promoter. Further detailed analysis of transactivation and promoter binding provides new insight into the physiological regulation of cell genes by EBNA-2.