A deep mutational scan of an acidic activation domain

Transcriptional activation domains are intrinsically disordered peptides with little primary sequence conservation. These properties have made it difficult to identify the sequence features that define activation domains. For example, although acidic activation domains were discovered 30 years ago, we still do not know what role, if any, acidic residues play in these peptides. To address this question we designed a rational mutagenesis scheme to independently test four sequence features theorized to control the strength of activation domains: acidity (negative charge), hydrophobicity, intrinsic disorder, and short linear motifs. To test enough mutants to deconvolve these four features we developed a method to quantify the activities of thousands of activation domain variants in parallel. Our results with Gcn4, a classic acidic activation domain, suggest that acidic residues in particular regions keep two hydrophobic motifs exposed to solvent. We also found that the specific activity of the Gcn4 activation domain increases during amino acid starvation. Our results suggest that Gcn4 may have evolved to have low activity but high inducibility. Our results also demonstrate that high-throughput rational mutation scans will be powerful tools for unraveling the properties that control how intrinsically disordered proteins function.

Varicella-Zoster Virus IE62 Protein Utilizes the Human Mediator Complex in Promoter Activation

ABSTRACT The varicella-zoster virus (VZV) major transactivator, IE62, is involved in the expression of all kinetic classes of VZV genes and can also activate cellular promoters, promoters from heterologous viruses, and artificial promoters containing only TATA elements. A key component of the mechanism of IE62 transactivation is an acidic activation domain comprising the N-terminal 86 amino acids of IE62. However, the cellular target of this N-terminal acidic activation is unknown. In the work presented here, we show that the IE62 activation domain targets the human Mediator complex via the Med25 (ARC92) subunit and that this interaction appears to be fundamental for transactivation by the IE62 activation domain. In contrast, the Med23 subunit (Sur2/TRAP150β/DRIP130/CRSP130) of the Mediator complex is not essential for IE62-mediated activation. Further, the IE62 activation domain appears to selectively interact with a form of the Mediator complex lacking CDK8. Chromatin immunoprecipitation experiments showed that IE62 stimulates recruitment of Mediator to an IE62-responsive model promoter. Finally, immunofluorescence microscopy of VZV-infected cells demonstrated intranuclear translocation of the Mediator complex to viral replication compartments. These studies suggest that Mediator is an essential component for efficient VZV gene expression.

ICP0 Is Not Required for Efficient Stress-Induced Reactivation of Herpes Simplex Virus Type 1 from Cultured Quiescently Infected Neuronal Cells

ABSTRACT Viral genes sufficient and required for herpes simplex virus type 1 (HSV-1) reactivation were identified using neuronally differentiated PC12 cells (ND-PC12 cells) in which quiescent infections with wild-type and recombinant strains were established. In this model, the expression of ICP0, VP16, and ICP4 from adenovirus vectors was sufficient to reactivate strains 17+ and KOS. The transactivators induced similar levels of reactivation with KOS; however, 17+ responded more efficiently to ICP0. To identify viral transactivators required for reactivation, we examined quiescently infected PC12 cell cultures (QIF-PC12 cell cultures) established with HSV-1 deletion mutants R7910 (ΔICP0), KD6 (ΔICP4), and in1814, a virus containing an insertion mutation in VP16. Although growth of these mutant viruses was impaired in ND-PC12 cells, R7910 and in1814 reactivated at levels equivalent to or better than their respective parental controls following stress (i.e., heat or forskolin) treatment. After treatment with trichostatin A, in1814 and 17+ reactivated efficiently, whereas the F strain and R7910 reactivated inefficiently. In contrast, KD6 failed to reactivate. In experiments with the recombinant KM100, which contains the in1814 mutation in VP16 and the n212 mutation in ICP0, spontaneous and stress-induced reactivation was observed. However, two strains, V422 and KM110, which lack the acidic activation domain of VP16, did not reactivate above low spontaneous levels after stress. These results demonstrate that in QIF-PC12 cells ICP0 is not required for efficient reactivation of HSV-1, the acidic activation domain of VP16 is essential for stress-induced HSV-1 reactivation, and HSV-1 reactivation is modulated uniquely by different treatment constraints and phenotypes.

Four EBNA2 Domains Are Important for EBNALP Coactivation

ABSTRACT EBNA2 transcriptional activation and regulated EBNALP coactivation are critical for Epstein-Barr virus-infected primary B-lymphocyte growth transformation. EBNALP coactivation requires the EBNA2 acidic activation domain (E2AD); EBNALP can bind to E2AD. EBNALP has now been found to bind less well to EBNA2 amino acids 1 to 58, which has been identified to be a second transcriptional activation domain, E2AD2. E2AD2 was specifically coactivated by EBNALP. Moreover, E2AD, E2AD2, EBNA2 RG domain, and the intermediate domain between RG and E2AD had significant roles in EBNA2-mediated activation and EBNALP coactivation.

The Acidic Activation Domain of the Baculovirus Transactivator IE1 Contains a Virus-Specific Domain Essential for DNA Replication

ABSTRACT IE1 is a potent transcriptional transactivator of the baculovirus Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV) and has been shown to be essential for viral DNA replication. IE1 contains an acidic activation domain (AAD) at the N terminus that is essential for transcriptional transactivation, but its role in viral DNA replication is unknown. In this study the role of the IE1 AAD in DNA replication is investigated. We have determined that deletion of the AAD eliminates the ability of IE1 to support DNA replication, showing that the AAD is essential for DNA replication as well as transcriptional transactivation. Replacement of the AAD with the archetype domain from herpesvirus VP16 and the evolutionarily related domain from Autographa californica MNPV (AcMNPV) IE1 produces chimeric proteins that are potent transactivators. Surprisingly, however, these chimeric proteins were unable to support DNA replication, indicating that there is a host- or virus-specific replication subdomain in the AAD that was not functionally replaced by the VP16 or AcMNPV AAD. Using N- and C-terminal deletion mutants, the region of the AAD that was essential for DNA replication was mapped to amino acids 1 to 65. AAD deletion mutants also showed that an IE1 that is functional for transcriptional transactivation is not required for viral DNA replication. The IE1 AAD therefore contains an essential replication domain that is separable from the transcriptional activation domains. Our results suggest that IE1 specifically interacts with a component of the viral replication complex, supporting the view that it acts as a nucleating factor by binding to the viral replication origins.

Large-Scale Chromatin Unfolding and Remodeling Induced by VP16 Acidic Activation Domain

Analysis of the relationship between transcriptional activators and chromatin organization has focused largely on lower levels of chromatin structure. Here we describe striking remodeling of large-scale chromatin structure induced by a strong transcriptional activator. A VP16-lac repressor fusion protein targeted the VP16 acidic activation domain to chromosome regions containing lac operator repeats. Targeting was accompanied by increased transcription, localized histone hyperacetylation, and recruitment of at least three different histone acetyltransferases. Observed effects on large-scale chromatin structure included unfolding of a 90-Mbp heterochromatic chromosome arm into an extended 25–40-μm chromonema fiber, remodeling of this fiber into a novel subnuclear domain, and propagation of large-scale chromatin unfolding over hundreds of kilobase pairs. These changes in large-scale chromatin structure occurred even with inhibition of ongoing transcription by α-amanitin. Our results suggest a functional link between recruitment of the transcriptional machinery and changes in large-scale chromatin structure. Based on the observed long-range propagation of changes in large-scale chromatin structure, we suggest a possible rationale for the observed clustering of housekeeping genes within Mbp-sized chromosome bands.

Concerted activity of host cell factor subregions in promoting stable VP16 complex assembly and preventing interference by the acidic activation domain.

In contrast to our understanding of the roles of Oct-1 and VP16 in VP16-mediated transcriptional activation, virtually nothing is known of the role of the second cellular component, termed host cell factor (HCF), or of its structure-function relationships. We show that the majority of the internal region of HCF, including the repeats involved in HCF cleavage, is dispensable for complex assembly with VP16 and Oct-1. The N-terminal domain of HCF (HCF.N) had only weak VP16 binding and complex promoting activity, while the C-terminal region (HCF.C) had no intrinsic activity. However, the C-terminal region strongly enhanced complex formation and reduced dissociation kinetics when linked to the N-terminal domain (HCF.NC). The potent activity of the HCF.NC fusion in complex assembly was recapitulated in vivo in yeast and mammalian cells. Moreover, HCF.N could promote increased complex formation when the acidic activation domain of VP16 was deleted. Restoration of the activation domain strongly inhibited complex formation with HCF.N, but the addition of the C-terminal domain of HCF restored strong stable complex formation with intact VP16. The results indicate that this C-terminal domain is critically required to alter the presentation of the acidic domain of VP16. Additional results are consistent with the interpretation that this alteration in acidic domain presentation for complex assembly also facilitates the activation function in VP16. The sequence of an HCF homolog from Caenorhabditis elegans shows it to be a natural HCF.NC construct, reinforcing the conclusions from our functional analysis.