Date Published: June 13, 2013
Publisher: Public Library of Science
Author(s): Gregory J. Fonseca, Michael J. Cohen, Anthony C. Nichols, John W. Barrett, Joe S. Mymryk, Matthew D. Weitzman.
Upon infection, human adenovirus (HAdV) must activate the expression of its early genes to reprogram the cellular environment to support virus replication. This activation is orchestrated in large part by the first HAdV gene expressed during infection, early region 1A (E1A). E1A binds and appropriates components of the cellular transcriptional machinery to modulate cellular gene transcription and activate viral early genes transcription. Previously, we identified hBre1/RNF20 as a target for E1A. The interaction between E1A and hBre1 antagonizes the innate antiviral response by blocking H2B monoubiquitination, a chromatin modification necessary for the interferon (IFN) response. Here, we describe a second distinct role for the interaction of E1A with hBre1 in transcriptional activation of HAdV early genes. Furthermore, we show that E1A changes the function of hBre1 from a ubiquitin ligase involved in substrate selection to a scaffold which recruits hPaf1 as a means to stimulate transcription and transcription-coupled histone modifications. By using hBre1 to recruit hPaf1, E1A is able to optimally activate viral early transcription and begin the cycle of viral replication. The ability of E1A to target hBre1 to simultaneously repress cellular IFN dependent transcription while activating viral transcription, represents an elegant example of the incredible economy of action accomplished by a viral regulatory protein through a single protein interaction.
Viruses are obligate intracellular pathogens as they require cellular machinery to replicate. Indeed, viruses often subvert the functions of cellular machinery to support their life cycle. Human adenovirus (HAdV) is no exception, and during infection must appropriate the host cellular transcriptional apparatus to begin transcription of the viral genes necessary to reprogram the cellular environment , . This is done in large part by the viral products of Early Region 1A (E1A), the first gene transcribed after infection. The E1A proteins bind and redirect the activity of transcriptional regulators to initiate transcription of the HAdV early genes , . The HAdV 5 E1A mRNA has five splice variants. The two largest isoforms, 13S and 12S, encode 289 and 243 residue (R) proteins, respectively. These proteins predominate at the early stages of virus infection. Sequence alignment of E1A from a variety of HAdVs shows four regions of conservation, and have been designated CR1-4 . The 289R and 243R E1A proteins of HAdV 5 are identical except for the presence of an additional 46 amino acid sequence within the 289R . This unique 46 amino acid region encompasses CR3 . Both the CR3 region and N-terminal 82 residues of E1A are sufficient to activate transcription when fused to a heterologous DNA binding domain , . Although each region can separately recruit a plethora of transcriptional activators , –, they function together to recruit cellular transcriptional complexes for the activation of viral transcription , , , . CR3, specifically, activates transcription through interactions with the mediator complex component Med23 (mediator complex subunit 23) , , . CR3 activity is further modulated by pCaf (CREBBP-associated factor), Gcn5 (general control of amino-acid synthesis, yeast, homolog), p300 (E1A binding protein p300), BS69 (bone morphogenetic protein receptor-associated molecule 1) and Sug1 (26S proteasome AAA-ATPase subunit RPT6) , , –. Likewise, the N-terminus of E1A interacts with transcriptional activators, such as p300, CBP (CREB-binding protein), p400 (E1A binding protein p400), pCaf, TBP (TATA binding protein), and TRAAP (transformation/transcription domain-associated protein) . Although there exists a large body of research focusing on the role that CR3 plays in virus transcription, the requirement for the N-terminus, which is conserved in both the 289R and 243R E1A proteins, and the mechanisms through which it cooperates with CR3 to activate viral transcription, are poorly understood.
HAdV E1A is an unusually strong and multifarious regulator of gene expression. E1A is the first protein expressed during viral infection and acts to reprogram cellular gene expression, as well as activate viral early gene expression. As such, E1A serves as a paradigm of eukaryotic transcriptional control. As one particular example, the ability of E1A to bind and sequester the p300/CBP acetyltransferases is a well established mechanism by which E1A can repress cellular transcription , , . We previously demonstrated that HAdV 5 E1A targets the cellular hBre1 complex. E1A disrupts the interaction between the hBre1 ligase and the Ube2b conjugase, leading to a global reduction in H2B-ub, decreased occupancy by hPaf1 and a consequent abrogation of type I IFN dependent gene expression  (Figure S3 in Text S1). The interaction of E1A with hBre1 provides a mechanism by which E1A antagonizes expression of cellular genes required for the innate immune response to viral infection in addition to the sequestration of p300/CBP. In the present work, we have determined that the interaction of E1A with hBre1 is further exploited by the virus to activate expression of the viral E2e, E3 and E4 transcription units. Thus, subversion of the hBre1 complex by E1A results in two distinct and opposite effects: inhibition of transcription from cellular IFN responsive genes and activation of viral gene expression (Figure 8).