Herpes simplex virus 1 ICP4 forms complexes with TFIID and mediator in virus-infected cells

The N terminus and C terminus of herpes simplex virus 1 ICP4 cooperate to activate viral gene expression

Infected cell polypeptide 4 (ICP4) activates transcription from most viral promoters. Two transactivation domains, one N-terminal and one C terminal, are largely responsible for the activation functions of ICP4. A mutant ICP4 molecule lacking the C-terminal activation domain (n208) efficiently activates many early genes, whereas late genes are poorly activated, and virus growth is severely impaired. The regions within the N terminus of ICP4 (amino acids 1 to 210) that contribute to activation were investigated by analysis of deletion mutants in the presence or absence of the C-terminal activation domain. The mutants were assessed for their abilities to support viral replication and to regulate gene expression. Several deletions in regions conserved in other alphaherpesviruses resulted in impaired activation and viral growth, without affecting DNA binding. The single small deletion that had the greatest effect on activation in the absence of the C terminus corresponded to a highly conserved stretch of amino acids between 81 and 96, rendering the molecule nonfunctional. However, when the C terminus was present, the same deletion had a minimal effect on activity. The amino terminus of ICP4 was predicted to be relatively disordered compared to the DNA-binding domain and the C-terminal 500 amino acids. Moreover, the amino terminus appears to be in a relatively extended conformation as determined by the hydrodynamic properties of several mutants. The data support a model where the amino terminus is an extended and possibly flexible region of the protein, allowing it to efficiently interact with multiple transcription factors at a distance from where it is bound to DNA, thereby enabling ICP4 to function as a general activator of polymerase II transcription. The C terminus of ICP4 can compensate for some of the mutations in the N terminus, suggesting that it either specifies redundant interactions or enables the amino terminus to function more efficiently.

The Epstein-Barr virus BcRF1 gene product is a TBP-like protein with an essential role in late gene expression

That the expression of late genes is coupled to viral genome replication is well established for all herpesviruses, but the exact mechanisms of their regulation, especially by viral proteins, are poorly understood. Here, we report the identification of the Epstein-Barr virus (EBV) early protein BcRF1 as a viral factor crucial for the activation of late gene transcription following viral DNA replication during the productive cycle. In order to study the function of the BcRF1 protein, we constructed a recombinant EBV lacking this gene. In HEK293 cells, this recombinant virus underwent normal DNA replication during the productive cycle but failed to express high levels of late gene transcripts or proteins, resulting in a nonproductive infection. Interestingly, a TATT motif is present in the promoter of most EBV late genes, at the position of the TATA box. We show here that BcRF1 forms a complex with the TATT motif and that this interaction is required for activation of late viral gene expression. Moreover, our results suggest that BcRF1 acts via interaction with other viral proteins.

Contribution of MS-Based Proteomics to the Understanding of Herpes Simplex Virus Type 1 Interaction with Host Cells

Like other DNA viruses, herpes simplex virus type 1 (HSV-1) replicates and proliferates in host cells continuously modulating the host molecular environment. Following a sophisticated temporal expression pattern, HSV-1 encodes at least 89 multifunctional proteins that interplay with and modify the host cell proteome. During the last decade, advances in mass spectrometry applications coupled to the development of proteomic separation methods have allowed to partially monitor the impact of HSV-1 infection in human cells. In this review, we discuss the current use of different proteome fractionation strategies to define HSV-1 targets in two major application areas: (i) viral-protein interactomics to decipher viral-protein interactions in host cells and (ii) differential quantitative proteomics to analyze the virally induced changes in the cellular proteome. Moreover, we will also discuss the potential application of high-throughput proteomic approaches to study global proteome dynamics and also post-translational modifications in HSV-1-infected cells that will greatly improve our molecular knowledge of HSV-1 infection.

Characterization of cis-acting elements required for autorepression of the equine herpesvirus 1 IE gene

The immediate-early protein (IEP), the major regulatory protein encoded by the IE gene of equine herpesvirus 1 (EHV-1), plays a crucial role as both transcription activator and repressor during a productive lytic infection. To investigate the mechanism by which the EHV-1 IEP inhibits its own promoter, IE promoter-luciferase reporter plasmids containing wild-type and mutant IEP-binding site (IEBS) were constructed and used for luciferase reporter assays. The IEP inhibited transcription from its own promoter in the presence of a consensus IEBS (5'-ATCGT-3') located near the transcription initiation site but did not inhibit when the consensus sequence was deleted. To determine whether the distance between the TATA box and the IEBS affects transcriptional repression, the IEBS was displaced from the original site by the insertion of synthetic DNA sequences. Luciferase reporter assays revealed that the IEP is able to repress its own promoter when the IEBS is located within 26-bp from the TATA box. We also found that the proper orientation and position of the IEBS were required for the repression by the IEP. Interestingly, the level of repression was significantly reduced when a consensus TATA sequence was deleted from the promoter region, indicating that the IEP efficiently inhibits its own promoter in a TATA box-dependent manner. Taken together, these results suggest that the EHV-1 IEP delicately modulates autoregulation of its gene through the consensus IEBS that is near the transcription initiation site and the TATA box.

The histone acetyltransferase CLOCK is an essential component of the herpes simplex virus 1 transcriptome that includes TFIID, ICP4, ICP27, and ICP22

Studies published elsewhere have shown that the herpes simplex virus regulatory protein ICP0 interacts with BMAL1, a partner and regulator of circadian histone acetyltransferase CLOCK, that both proteins localize at ND10 bodies and are stabilized by viral proteins, that enzymatically active CLOCK partially complements ΔICP0 mutants, and that silencing of CLOCK suppresses the expression of viral genes. Here we report that CLOCK is a component of the transcriptional complex that includes TFIID, ICP4, ICP27, and ICP22. The results suggest that the CLOCK histone acetyltransferase is a component of the viral transcriptional machinery throughout the replicative cycle of the virus and that ICP27 and ICP22 initiate their involvement in viral gene expression as components of viral transcriptome.