Interaction of the 89K murine cytomegalovirus immediate-early protein with core histones

Human cytomegalovirus major immediate early 1 protein targets host chromosomes by docking to the acidic pocket on the nucleosome surface

The 72-kDa immediate early 1 (IE1) protein encoded by human cytomegalovirus (hCMV) is a nuclearly localized promiscuous regulator of viral and cellular transcription. IE1 has long been known to associate with host mitotic chromatin, yet the mechanisms underlying this interaction have not been specified. In this study, we identify the cellular chromosome receptor for IE1. We demonstrate that the viral protein targets human nucleosomes by directly binding to core histones in a nucleic acid-independent manner. IE1 exhibits two separable histone-interacting regions with differential binding specificities for H2A-H2B and H3-H4. The H2A-H2B binding region was mapped to an evolutionarily conserved 10-amino-acid motif within the chromatin-tethering domain (CTD) of IE1. Results from experimental approaches combined with molecular modeling indicate that the IE1 CTD adopts a β-hairpin structure, docking with the acidic pocket formed by H2A-H2B on the nucleosome surface. IE1 binds to the acidic pocket in a way similar to that of the latency-associated nuclear antigen (LANA) of the Kaposi's sarcoma-associated herpesvirus. Consequently, the IE1 and LANA CTDs compete for binding to nucleosome cores and chromatin. Our work elucidates in detail how a key viral regulator is anchored to human chromosomes and identifies the nucleosomal acidic pocket as a joint target of proteins from distantly related viruses. Based on the striking similarities between the IE1 and LANA CTDs and the fact that nucleosome targeting by IE1 is dispensable for productive replication even in "clinical" strains of hCMV, we speculate that the two viral proteins may serve analogous functions during latency of their respective viruses.

Ablation of the regulatory IE1 protein of murine cytomegalovirus alters in vivo pro-inflammatory TNF-alpha production during acute infection

Little is known about the role of viral genes in modulating host cytokine responses. Here we report a new functional role of the viral encoded IE1 protein of the murine cytomegalovirus in sculpting the inflammatory response in an acute infection. In time course experiments of infected primary macrophages (MΦs) measuring cytokine production levels, genetic ablation of the immediate-early 1 (ie1) gene results in a significant increase in TNFα production. Intracellular staining for cytokine production and viral early gene expression shows that TNFα production is highly associated with the productively infected MΦ population of cells. The ie1- dependent phenotype of enhanced MΦ TNFα production occurs at both protein and RNA levels. Noticeably, we show in a series of in vivo infection experiments that in multiple organs the presence of ie1 potently inhibits the pro-inflammatory cytokine response. From these experiments, levels of TNFα, and to a lesser extent IFNβ, but not the anti-inflammatory cytokine IL10, are moderated in the presence of ie1. The ie1- mediated inhibition of TNFα production has a similar quantitative phenotype profile in infection of susceptible (BALB/c) and resistant (C57BL/6) mouse strains as well as in a severe immuno-ablative model of infection. In vitro experiments with infected macrophages reveal that deletion of ie1 results in increased sensitivity of viral replication to TNFα inhibition. However, in vivo infection studies show that genetic ablation of TNFα or TNFRp55 receptor is not sufficient to rescue the restricted replication phenotype of the ie1 mutant virus. These results provide, for the first time, evidence for a role of IE1 as a regulator of the pro-inflammatory response and demonstrate a specific pathogen gene capable of moderating the host production of TNFα in vivo.

The suppression of the production rather than the blockage of action of the potent inflammatory mediator TNFα is a particular hallmark of anti-TNFα mechanisms associated with microbial and parasitic infections. Whether this mode of counter-regulation is an important feature of infection by viruses is not clear. Also, it remains to be determined whether a specific pathogen gene in the context of an infection in vivo is capable of modulating levels of TNFα production. In this study we disclose a virus-mediated moderation of TNFα production, dependent on the ie1 gene of murine cytomegalovirus (MCMV). The ie1 gene product IE1 is a well-characterized nuclear protein capable of altering levels of host and viral gene expression although its biological role in the context of a natural infection is to date unknown. We provide evidence showing that ie1 is associated with a moderated pro-inflammatory cytokine response, in particular with TNFα production. Further, we show that the viral moderation of this cytokine is not only readily apparent in vitro but also in the natural host. The identification of a viral gene responsible for this mode of regulation in vivo may have therapeutic potential in the future in both anti-viral and anti-inflammatory strategies.

H2B homology region of major immediate-early protein 1 is essential for murine cytomegalovirus to disrupt nuclear domain 10, but is not important for viral replication in cell culture

Cytomegalovirus (CMV) major immediate-early protein 1 (IE1) has multiple functions and is important for efficient viral infection. As does its counterpart in human CMV, murine CMV (MCMV) IE1 also functions as a disruptor of mouse-cell nuclear domain 10 (ND10), where many different gene-regulation proteins congregate. It still remains unclear how MCMV IE1 disperses ND10 and whether this dispersion could have any effect on viral replication. MCMV IE1 is 595 aa long and has multiple functional domains that have not yet been fully analysed. In this study, we dissected the IE1 molecule by truncation and/or deletion and found that the H2B homology domain (amino acid sequence NDIFERI) is required for the dispersion of ND10 by IE1. Furthermore, we made additional deletions and point mutations and found that the minimal truncation in the H2B homology domain required for IE1 to lose the ability to disperse ND10 is just 3 aa (IFE). Surprisingly, the mutated IE1 still interacted with PML and co-localized with ND10 but failed to disperse ND10. This suggests that binding to ND10 key protein is essential to, but not sufficient for, the dispersal of ND10, and that some other unknown mechanism must be involved in this biological procedure. Finally, we generated MCMV with IFE-deleted IE1 (MCMVdlIFE) and its revertant (MCMVIFERQ). Although MCMVdlIFE lost the ability to disperse ND10, plaque assays and viral gene production assays showed that the deletion of IFE did not increase viral replication in cell culture. We conclude that the dispersion of ND10 appears not to be important for MCMV replication in a mouse-cell culture.

Chromatinisation of herpesvirus genomes

The double-stranded DNA genomes of herpesviruses exist in at least three alternative global chromatin states characterised by distinct nucleosome content. When encapsidated in virus particles, the viral DNA is devoid of any nucleosomes. In contrast, within latently infected nuclei herpesvirus genomes are believed to form regular nucleosomal structures resembling cellular chromatin. Finally, during productive infection nuclear viral DNA appears to adopt a state of intermediate chromatin formation with irregularly spaced nucleosomes. Nucleosome occupancy coupled with posttranslational histone modifications and other epigenetic marks may contribute significantly to the extent and timing of transcription from the viral genome and, consequently, to the outcome of infection. Recent research has provided first insights into the viral and cellular mechanisms that either maintain individual herpesvirus chromatin states or mediate transition between them. Here, we summarise and discuss both early work and new developments pointing towards common principles pertinent to the dynamic structure and epigenetic regulation of herpesvirus chromatin. Special emphasis is given to the emerging similarities in nucleosome assembly and disassembly processes on herpes simplex virus type 1 and human cytomegalovirus genomes over the course of the viral productive replication cycle and during the switch between latent and lytic infectious stages.

Structural Organization of the Spliced Immediate-Early Gene Complex that Encodes the Major Acidic Nuclear (IE1) and Transactivator (IE2) Proteins of African Green Monkey Cytomegalovirus

Total immediate-early (IE) RNA synthesized after infection with African green monkey cytomegalovirus (SCMV) in the presence of cycloheximide contained a major 2.3-kb mRNA species that acted as template for in vitro synthesis of a single 94-kD nuclear protein. The same IE RNA hybridized predominantly to a 1.8-kb subregion of the 220-kb genome which mapped 1.5 kb to the left of the in vitro transcription start site and TATATAA motif previously associated with the powerful MIE (IE94) enhancer region. However, DNA sequence and S1-mapping analysis of a 5-kb region downstream from the promoter revealed the existence of a far upstream noncoding first exon and four additional spliced exons capable of encoding two alternative protein products with shared N-terminal domains. This region is similar in structure to that of the MIE gene complex of human cytomegalovirus (HCMV), including being highly CpG suppressed. Exons 2, 3, and 4 encode an acidic protein equivalent to the 68-kD IE1 protein (UL123) of HCMV and exons 2, 3, and 5 encode a protein equivalent to the 80-kD IE2 (UL122) DNA-binding protein of HCMV. Transcripts from across the IE2 region were detected within the cycloheximide RNA, but they were present at 10- to 20-fold lower abundance than IE1 transcripts. The proposed 547-codon IE1 (IE94) acidic phosphoprotein of SCMV displays minimal residual homology with the IE1 protein of HCMV, but both associate with metaphase chromosomes and have large C-terminal glutamic-acid-rich domains. In contrast, the proposed 583-codon IE2 protein of SCMV displays extensive amino acid similarity to the HCMV IE2 transcriptional regulatory protein especially within C-terminal domains that are known to play a major role in promoter targeting for both transactivation and negative autoregulation functions. Copyright 1995 S. Karger AG, Basel

Semen alloantigens and lymphocytotoxic antibodies in AIDS and ICL

More than 90% of people with AIDS develop circulating immune complexes (CICs) and lymphocytotoxic antibodies (LCTAs). Animals infected with HIV, however, never display CICs or LCTAs, and remain healthy. Similarly, HIV-infected people who do not develop CICs or LCTAs also do not progress to AIDS. The appearance of CICs and LCTAs is, however, highly prognostic for AIDS and death. Since HIV infection does not, per se, lead to the development of CICs and LCTAs, other causes are likely. One such cause, for which both epidemiologic and experimental evidence exists, is semen. Semen components include sperm, seminal fluid, lymphocytes, and sometimes infectious agents, including HIV, mycoplasmas, and herpes and hepatitis viruses, all of which independently cause immune suppression. Extensive evidence demonstrates sperm (and various viruses) contains many proteins mimicking the CD4 protein of T-helper cells, while HIV, mycoplasmas, and seminal fluid mimic class II MHC proteins of other lymphocytes. We identify a large number of protein sequences that display such mimicry using computer homology searching, and demonstrate experimentally that sperm antibodies specifically precipitate antibodies against class II MHC mimics such as mycoplasmas, which in turn precipitate antibodies to lymphocyte antigens. These data prove that immunologic exposure to sperm and lymphocytes (as may occur in receptive anal intercourse, needle sharing, or blood transfusions) is theoretically capable of initiating lymphocytotoxic autoimmunity. Such autoimmunity may play a significant role in the pathogenesis of AIDS, and will need to be addressed clinically in high risk individuals regardless of HIV status and regardless of the success of anti-HIV prophylaxis and treatment.

Effect of interferon-alpha on immediate early gene expression of murine cytomegalovirus

Interferon-alpha (IFN-alpha) significantly reduced the replication of murine cytomegalovirus (MCMV) in mouse embryo fibroblasts derived from the susceptible mouse strain C3H/HeJ. When infectious virus production was measured, a strong decrease in virus titer was observed in IFN-treated cells at a multiplicity of infection (moi) of 1 and 0.5 pfu/cell. Analysis of virus-specified mRNAs by Northern blot assay revealed that IFN-alpha had a significant effect on the expression of viral mRNAs at 48h. In particular, the mRNAs of the major immediate early (IE) transcription units, IE1, IE2, and IE3, were impaired by IFN-alpha. In addition, decrease of IE1 mRNA synthesis was accompanied by a reduction of the major IE product (pp89), as revealed by Western blot assay. These results suggest that IFN-alpha may inhibit MCMV replication by directly impairing IE gene transcription.

Expression of the acidic nuclear immediate-early protein (IE1) of human cytomegalovirus in stable cell lines and its preferential association with metaphase chromosomes

Stable DNA-transfected Vero cell lines that express the major immediate-early nuclear antigen (IE68) of HCMV-(Towne) have been established. Immunofluorescence staining with monoclonal antibodies revealed that the protein was distributed either in a uniform diffuse nuclear pattern or as punctate nuclear granules in up to 80% of the cells in these cultures. In addition, 1 to 2% of the positive nuclei gave a distinctive staining pattern suggesting an association with the chromosomes of mitotic cells. Colcemid-blocking studies confirmed that most of the IE antigen was localized in the vicinity of condensed chromosomes in all metaphase cells after methanol fixation. In contrast, the SV40 large T-antigen protein was found to be preferentially excluded from metaphase chromosomes in a similar colcemid-treated human cell line. In transient expression assays, 1 to 2% of IE antigen-positive Vero, 293, or Balb/c3T3 cells also displayed a metaphase chromosome association pattern. Mapping studies using deletion and truncation mutants revealed that the monoclonal antibodies recognized epitopes encoded within the small NH2-terminal exons that are common to both the IE1 and IE2 gene products. However, an intact exon-4 (IE1) region, but not the exon-5 (IE2) region of the HCMV IE gene complex, was required for conferring both the normal diffuse nuclear localization pattern and the chromosome-association properties. Furthermore, removal of the glutamic acid-rich COOH-terminal coding portions of exon-4 resulted in aberrant staining patterns with production of large, phase-dense nuclear globules in all positive cells. An association between the IE68 IE1 protein and metaphase chromosomes was also detected after HCMV-(Towne) infection in a small proportion of both nonpermissive Balb/c3T3 cells and permissive HF cells. We conclude that the IE1 acidic nuclear phosphoprotein displays some properties similar to those of the EBNA-1 protein of Epstein-Barr virus and suggest that it may potentially play a role in maintenance of the latent state of HCMV DNA.

The 89,000-Mr murine cytomegalovirus immediate-early protein stimulates c-fos expression and cellular DNA synthesis

The immediate-early (IE) genes of murine cytomegalovirus (MCMV) are expressed in the absence of prior viral protein synthesis and regulate the transcription of MCMV early genes. The effect of MCMV IE genes on growth induction was studied. Different plasmids containing MCMV IE genes were microinjected into arrested NIH 3T3 mouse fibroblasts. Plasmids containing the ieI gene coding for the 89,000-Mr major IE protein pp89 were found to stimulate the expression of the c-fos protooncogene. Synthesis of pp89 and its transport to the nucleus appeared to be required for c-fos expression. DNA synthesis occurred in cells that were injected with MCMV IE genes and in neighboring cells that were not injected. The results suggest that the phosphoprotein pp89 stimulates cells to enter the cell cycle.