Recruitment of activated IRF-3 and CBP/p300 to herpes simplex virus ICP0 nuclear foci: Potential role in blocking IFN-beta induction

Epigenetic modulation of gene expression from quiescent herpes simplex virus genomes

The ability of herpes simplex virus to persist in cells depends on the extent of viral-gene expression, which may be controlled by epigenetic mechanisms. We used quiescent infection with the viral mutants d109 and d106 to explore the effects of cell type and the presence of the viral protein ICP0 on the expression and chromatin structure of the human cytomegalovirus (HCMV) tk and gC promoters on the viral genome. Expression from the HCMV promoter on the d109 genome decreased with time and was considerably less in HEL cells than in Vero cells. Expression from the HCMV promoter in d106 was considerably more abundant than in d109, and this increased with time in both cell types. The same pattern of expression was seen on the tk and gC genes on the viral genomes, although the levels of tk and gC RNA were approximately 10(2)- and 10(5)-fold lower than those of wild-type virus in d106 and d109, respectively. In micrococcal-nuclease digestion experiments, nucleosomes were evident on the d109 genome, and the amount of total H3 as determined by chromatin immunoprecipitation was considerably greater on d109 than d106 genomes. The acetylation of histone H3 on the d106 genomes was evident at early and late times postinfection in Vero cells, but only at late times in HEL cells. The same pattern was observed for H3 acetylated on lysine 9. Trimethylation of H3K9 on d109 genomes was evident only at late times postinfection in Vero cells, while it was observed both early and late in HEL cells. Heterochromatin protein 1gamma (HP1gamma) was generally present only on d109 genomes at late times postinfection of HEL cells. The observations of chromatin structure correlate with the expression patterns of the three analyzed genes on the quiescent genomes. Therefore, several mechanisms generally affect the expression and contribute to the silencing of persisting genomes. These are the abundance of nucleosomes, the acetylation state of the histones, and heterochromatin. The extents to which these different mechanisms contribute to repression vary in different cell types and are counteracted by the presence of ICP0.

Infection with herpes simplex type 1-based amplicon vectors results in an IRF3/7-dependent, TLR-independent activation of the innate antiviral response in primary human fibroblasts

Amplicons are helper-dependent herpes simplex virus type 1 (HSV-1)-based vectors that can deliver very large, foreign DNA sequences and, as such, are good candidates for both gene delivery and vaccine development. However, many studies have shown that innate immune responses induced by virus vectors can play a significant role in the control of transgenic expression and in the induction of inflammatory responses. Furthermore, amplicons are very interesting tools to study innate cellular responses elicited by entry of HSV-1 particles in the absence of any virus gene expression. For these reasons, in this study we characterized the innate antiviral response established in human fibroblasts of limited passage (HFFF-2) infected by amplicons. Our results indicate that infection with amplicons triggered an interferon (IFN)-regulatory factors 3 and 7 (IRF3/7)-dependent antiviral response, rendered the cells resistant to vesicular stomatitis virus infection and induced significant changes in the pattern of cellular gene expression, including the upregulation of Toll-like receptor 3 (TLR3), IRF7 and IFN-stimulated genes (ISGs). In contrast, we observed only a mild and contained type I IFN response in infected cells. Amplicon infection induced nuclear translocation and subsequent degradation of IRF3, without hyperphosphorylation of the protein. Inhibition of endosome-resident TLR signalling by blocking lysosome maturation or the knockdown of TLR3 and 4 did not abolish the cellular response to amplicons, whereas knockdown of IRF3 and 7 inhibited ISG and IFN-beta expression severely. Therefore, our results confirm the existence of TLR-independent, IRF3/7-dependent activation pathways triggered by HSV-1 particles in human fibroblasts.

Role of chromatin during herpesvirus infections

DNA viruses have long served as model systems to elucidate various aspects of eukaryotic gene regulation, due to their ease of manipulation and relatively low complexity of their genomes. In some cases, these viruses have revealed mechanisms that are subsequently recognized to apply also to cellular genes. In other cases, viruses adopt mechanisms that prove to be exceptions to the more general rules. The double-stranded DNA viruses that replicate in the eukaryotic nucleus typically utilize the host cell RNA polymerase II (RNAP II) for viral gene expression. As a consequence, these viruses must reckon with the impact of chromatin on active transcription and replication. Unlike the small DNA tumor viruses, such as polyomaviruses and papillomaviruses, the relatively large genomes of herpesviruses are not assembled into nucleosomes in the virion and stay predominantly free of histones during lytic infection. In contrast, during latency, the herpesvirus genomes associate with histones and become nucleosomal, suggesting that regulation of chromatin per se may play a role in the switch between the two stages of infection, a long-standing puzzle in the biology of herpesviruses. In this review we will focus on how chromatin formation on the herpes simplex type-1 (HSV-1) genome is regulated, citing evidence supporting the hypothesis that the switch between the lytic and latent stages of HSV-1 infection might be determined by the chromatin state of the HSV-1.

The gamma 1 34.5 protein of herpes simplex virus 1 is required to interfere with dendritic cell maturation during productive infection

The gamma(1)34.5 protein of herpes simplex virus 1 is an essential factor for viral virulence. In infected cells, this viral protein prevents the translation arrest mediated by double-stranded RNA-dependent protein kinase R. Additionally, it associates with and inhibits TANK-binding kinase 1, an essential component of Toll-like receptor-dependent and -independent pathways that activate interferon regulatory factor 3 and cytokine expression. Here, we show that gamma(1)34.5 is required to block the maturation of conventional dendritic cells (DCs) that initiate adaptive immune responses. Unlike wild-type virus, the gamma(1)34.5 null mutant stimulates the expression of CD86, major histocompatibility complex class II (MHC-II), and cytokines such as alpha/beta interferon in immature DCs. Viral replication in DCs inversely correlates with interferon production. These phenotypes are also mirrored in a mouse ocular infection model. Further, DCs infected with the gamma(1)34.5 null mutant effectively activate naive T cells whereas DCs infected with wild-type virus fail to do so. Type I interferon-neutralizing antibodies partially reverse virus-induced upregulation of CD86 and MHC-II, suggesting that gamma(1)34.5 acts through interferon-dependent and -independent mechanisms. These data indicate that gamma(1)34.5 is involved in the impairment of innate immunity by inhibiting both type I interferon production and DC maturation, leading to defective T-cell activation.

Herpes simplex virus type 1 regulatory protein ICP0 aids infection in cells with a preinduced interferon response but does not impede interferon-induced gene induction

Several independent lines of evidence indicate that interferon-mediated innate responses are involved in controlling herpes simplex virus type 1 (HSV-1) infection and that the viral immediate-early regulatory protein ICP0 augments HSV-1 replication in interferon-treated cells. However, this is a complex situation in which the experimental outcome is determined by the choice of multiplicity of infection and cell type and by whether cultured cells or animal models are used. It is now known that neither STAT1 nor interferon regulatory factor 3 (IRF-3) play essential roles in the replication defect of ICP0-null mutant HSV-1 in cultured cells. This study set out to investigate the specific role of ICP0 in HSV-1 resistance to the interferon defense. We have used a cell line in which ICP0 expression can be induced at levels similar to those during the early stages of a normal infection to determine whether ICP0 by itself can interfere with interferon or IRF-3-dependent signaling and whether ICP0 enables the virus to circumvent the effects of interferon-stimulated genes (ISGs). We found that the presence of ICP0 was unable to compromise ISG induction by either interferon or double-stranded RNA. On the other hand, ICP0 preexpression reduced but did not eliminate the inhibitory effects of ISGs on HSV-1 infection, with the extent of the relief being highly dependent on multiplicity of infection. The results are discussed in terms of the relationships between ICP0 and intrinsic and innate antiviral resistance mechanisms.

Analysis of the functions of herpes simplex virus type 1 regulatory protein ICP0 that are critical for lytic infection and derepression of quiescent viral genomes

Herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP0 is important for stimulating the initiation of the lytic cycle and efficient reactivation of latent or quiescent infection. Extensive investigation has suggested several potential functions for ICP0, including interference in the interferon response, disruption of functions connected with PML nuclear bodies (ND10), and inhibition of cellular histone deacetylase (HDAC) activity through an interaction with the HDAC-1 binding partner CoREST. Analysis of the significance of these potential functions and whether they are direct or indirect effects of ICP0 is complicated because HSV-1 mutants expressing mutant forms of ICP0 infect cells with widely differing efficiencies. On the other hand, transfection approaches for ICP0 expression do not allow studies of whole cell populations because of their limited efficiency. To overcome these problems, we have established a cell line in which ICP0 expression can be induced at levels pertaining during the early stages of HSV-1 infection in virtually all cells in the culture. Such cells enable 100% complementation of ICP0-null mutant HSV-1. Using cells expressing the wild type and a variety of mutant forms of ICP0, we have used this system to analyze the role of defined domains of the protein in stimulating lytic infection and derepression from quiescence. Activity in these core functions correlated well the ability of ICP0 to disrupt ND10 and inhibit the recruitment of ND10 proteins to sites closely associated with viral genomes at the onset of infection, whereas the CoREST binding region was neither sufficient nor necessary for ICP0 function in lytic and reactivating infections.

ICP0 inhibits the decrease of HSV amplicon-mediated transgene expression

The herpes simplex virus (HSV) amplicon vector produces an initial host response that limits transgene expression. In this study, we hypothesized that restoration of the HSV gene infected cell protein (ICP0) into the amplicon could circumvent this host response and thus overcome silencing of encoded transgenes. To test this, we constructed an amplicon vector that encodes the ICP0 under control of its native promoter (ICP0+ amplicon). Expression of ICP0 was transient and, at a multiplicity of infection (MOI) of 1, did not significantly alter interferon (IFN)-based responses against the vector or cell kinetics/apoptosis of infected cells. Chromatin immunoprecipitation (ChIP) PCR analysis revealed that conventional amplicon DNA became associated with histone deacetylase 1 (HDAC1) immediately after infection, whereas ICP0+ amplicon DNA remained relatively unbound by HDAC1 for at least 72 hours after infection. Mice administered systemic ICP0+ amplicon exhibited significantly greater and more sustained transgene expression in their livers than did those receiving conventional amplicon, likely due to increased transcriptional or post-transcriptional activity rather than increased copy numbers of vector DNA. These findings indicate that restoration of ICP0 expression may be employed within HSV amplicon constructs to decrease transgene silencing in vitro and in vivo.

Transcriptional coactivators are not required for herpes simplex virus type 1 immediate-early gene expression in vitro

Virion protein 16 (VP16) of herpes simplex virus type 1 (HSV-1) is a potent transcriptional activator of viral immediate-early (IE) genes. The VP16 activation domain can recruit various transcriptional coactivators to target gene promoters. However, the role of transcriptional coactivators in HSV-1 IE gene expression during lytic infection had not been fully defined. We showed previously that transcriptional coactivators such as the p300 and CBP histone acetyltransferases and the BRM and Brg-1 chromatin remodeling complexes are recruited to viral IE gene promoters in a manner dependent mostly on the presence of the activation domain of VP16. In this study, we tested the hypothesis that these transcriptional coactivators are required for viral IE gene expression during infection of cultured cells. The disrupted expression of the histone acetyltransferases p300, CBP, PCAF, and GCN5 or the BRM and Brg-1 chromatin remodeling complexes did not diminish IE gene expression. Furthermore, IE gene expression was not impaired in cell lines that lack functional p300, or BRM and Brg-1. We also tested whether these coactivators are required for the VP16-dependent induction of IE gene expression from transcriptionally inactive viral genomes associated with high levels of histones in cultured cells. We found that the disruption of coactivators also did not affect IE gene expression in this context. Thus, we conclude that the transcriptional coactivators that can be recruited by VP16 do not contribute significantly to IE gene expression during lytic infection or the induction of IE gene expression from nucleosomal templates in vitro.

Viral evasion and subversion of pattern-recognition receptor signalling

The expression of pattern-recognition receptors (PRRs) by immune and tissue cells provides the host with the ability to detect and respond to infection by viruses and other microorganisms. Significant progress has been made from studying this area, including the identification of PRRs, such as Toll-like receptors and RIG-I-like receptors, and the description of the molecular basis of their signalling pathways, which lead to the production of interferons and other cytokines. In parallel, common mechanisms used by viruses to evade PRR-mediated responses or to actively subvert these pathways for their own benefit are emerging. Accumulating evidence on how viral infection and PRR signalling pathways intersect is providing further insights into the function of the pathways involved, their constituent proteins and ways in which they could be manipulated therapeutically.

Control of TANK-binding kinase 1-mediated signaling by the gamma(1)34.5 protein of herpes simplex virus 1

TANK-binding kinase 1 (TBK1) is a key component of Toll-like receptor-dependent and -independent signaling pathways. In response to microbial components, TBK1 activates interferon regulatory factor 3 (IRF3) and cytokine expression. Here we show that TBK1 is a novel target of the gamma(1)34.5 protein, a virulence factor whose expression is regulated in a temporal fashion. Remarkably, the gamma(1)34.5 protein is required to inhibit IRF3 phosphorylation, nuclear translocation, and the induction of antiviral genes in infected cells. When expressed in mammalian cells, the gamma(1)34.5 protein forms complexes with TBK1 and disrupts the interaction of TBK1 and IRF3, which prevents the induction of interferon and interferon-stimulated gene promoters. Down-regulation of TBK1 requires the amino-terminal domain. In addition, unlike wild type virus, a herpes simplex virus mutant lacking gamma(1)34.5 replicates efficiently in TBK1(-/-) cells but not in TBK1(+/+) cells. Addition of exogenous interferon restores the antiviral activity in both TBK1(-/-) and TBK(+/+) cells. Hence, control of TBK1-mediated cell signaling by the gamma(1)34.5 protein contributes to herpes simplex virus infection. These results reveal that TBK1 plays a pivotal role in limiting replication of a DNA virus.

HSV ICP0 recruits USP7 to modulate TLR-mediated innate response

Pattern recognition receptors represent the first line of defense against invading pathogens. Herpes simplex virus (HSV) encodes multiple ligands detected by these receptors, yet persists in the majority of infected individuals indicating a breakdown in host defense against the virus. Here we identify a novel mechanism through which HSV immediate-early protein ICP0 inhibits TLR-dependent inflammatory response by blocking NF-kappaB and JNK activation downstream of TLR signal activation. This process depends on ICP0-mediated translocation of USP7 (HAUSP) from the nucleus to cytoplasm. We show that nuclear USP7 migrates to the cytoplasm in response to TLR engagement, a process that contributes to termination of TLR response. Cytoplasmic USP7 binds to and deubiquitinates TRAF6 and IKKgamma, thus terminating TLR-mediated NF-kappaB and JNK activation. These findings suggest that USP7 is part of a negative feedback loop regulating TLR signaling and that ICP0 exploits this physiologic process to attenuate innate response to HSV. ICP0 inhibition of the TLR response serves to uncouple the innate and adaptive immune response, thereby playing a key role in HSV pathogenesis and persistence.

STAT-1- and IRF-3-dependent pathways are not essential for repression of ICP0-null mutant herpes simplex virus type 1 in human fibroblasts

Efficient herpes simplex virus type 1 (HSV-1) infection of human fibroblasts (HFs) is highly dependent on the viral immediate-early regulatory protein ICP0 unless the infection is conducted at a high multiplicity. ICP0-null mutant HSV-1 exhibits a plaque-forming defect of up to 3 orders of magnitude in HFs, whereas in many other cell types, this defect varies between 10- and 30-fold. The reasons for the high ICP0 requirement for HSV-1 infection in HFs have not been established definitively. Previous studies using other cell types suggested that ICP0-null mutant HSV-1 is hypersensitive to interferon and that this sensitivity is dependent on the cellular promyelocytic leukemia (PML) protein. To investigate the roles of two important aspects of interferon signaling in the phenotype of ICP0-null mutant HSV-1, we isolated HFs depleted of STAT-1 or interferon regulatory factor 3 (IRF-3). Surprisingly, plaque formation by the mutant virus was not improved in either cell type. We found that the sensitivity to interferon pretreatment of both ICP0-null mutant and wild-type (wt) HSV-1 was highly dependent on the multiplicity of infection. At a low multiplicity in virus yield experiments, both viruses were extremely susceptible to interferon pretreatment of HFs, but the sensitivity of the wild type but not the mutant could be overcome at higher multiplicities. We found that both wt and ICP0-null mutant HSV-1 remained sensitive to interferon in PML-depleted HFs albeit to an apparently lesser extent than in control cells. The data imply that the substantial reduction in ICP0-null HSV-1 infectivity at a low multiplicity in HFs does not occur through the activities of STAT-1- and IRF-3-dependent pathways and cannot be explained solely by enhanced sensitivity to interferon. We suggest that antiviral activities induced by interferon may be separable from and additive to those resulting from PML-related intrinsic resistance mechanisms.

Targeting IRFs by ubiquitination: regulating antiviral responses

The IRF [IFN (interferon) regulatory factor] family of transcription factors control many cellular processes, including induction of key antiviral cytokines, type I IFNs, following viral infection. Recent studies have revealed several endogenous and viral proteins involved in ubiquitin-mediated regulation of IRF activity and thus having an impact on type I IFN signalling. Through the ubiquitin pathway, these proteins can manipulate the antiviral response either by initiating proteasomal degradation of the IRFs or, in contrast, by promoting activation of the IRFs.

Expression of gamma interferon-dependent genes is blocked independently by virion host shutoff RNase and by US3 protein kinase

Gamma interferon receptor alpha (IFN-gammaR alpha) is stable but posttranslationally modified in herpes simplex virus 1(F) [HSV-1(F)]-infected cells. Studies with antibody directed to the phosphorylation site indicate that IFN-gammaR alpha is phosphorylated by the U(S)3 kinase. The modification is abolished in cells infected with DeltaU(S)3, DeltaU(L)13, or Delta(U(S)3/U(L)13) mutant virus. Transcripts of the IFN-gamma-dependent genes do not accumulate in cells transduced with the U(S)3 protein kinase and treated with IFN-gamma. In contrast, the accumulation of IFN-gamma-dependent gene transcripts is suppressed in cells infected with the wild-type virus, in cells infected with the DeltaU(S)3 mutant virus, and to a lesser extent in the DeltaU(L)41 virus-infected cells. The accumulation of IFN-gamma-dependent gene transcripts in DeltaU(L)41-infected cells could be due at least in part to a significant delay and reduction in the accumulation of the U(S)3 protein. The results suggest that the expression of IFN-gamma-dependent genes is blocked independently by the degradation of IFN-gamma-dependent gene transcripts--a function of the virion host shutoff RNase--and by posttranslational modification of the IFN-gammaR alpha protein.

Interferons and viruses: an interplay between induction, signalling, antiviral responses and virus countermeasures

The interferon (IFN) system is an extremely powerful antiviral response that is capable of controlling most, if not all, virus infections in the absence of adaptive immunity. However, viruses can still replicate and cause disease in vivo, because they have some strategy for at least partially circumventing the IFN response. We reviewed this topic in 2000 [Goodbourn, S., Didcock, L. & Randall, R. E. (2000). J Gen Virol 81, 2341-2364] but, since then, a great deal has been discovered about the molecular mechanisms of the IFN response and how different viruses circumvent it. This information is of fundamental interest, but may also have practical application in the design and manufacture of attenuated virus vaccines and the development of novel antiviral drugs. In the first part of this review, we describe how viruses activate the IFN system, how IFNs induce transcription of their target genes and the mechanism of action of IFN-induced proteins with antiviral action. In the second part, we describe how viruses circumvent the IFN response. Here, we reflect upon possible consequences for both the virus and host of the different strategies that viruses have evolved and discuss whether certain viruses have exploited the IFN response to modulate their life cycle (e.g. to establish and maintain persistent/latent infections), whether perturbation of the IFN response by persistent infections can lead to chronic disease, and the importance of the IFN system as a species barrier to virus infections. Lastly, we briefly describe applied aspects that arise from an increase in our knowledge in this area, including vaccine design and manufacture, the development of novel antiviral drugs and the use of IFN-sensitive oncolytic viruses in the treatment of cancer.

Role for herpes simplex virus 1 ICP27 in the inhibition of type I interferon signaling

Host cells respond to viral infection by many mechanisms, including the production of type I interferons which act in a paracrine and autocrine manner to induce the expression of antiviral interferon-stimulated genes (ISGs). Viruses have evolved means to inhibit interferon signaling to avoid induction of the innate immune response. Herpes simplex virus 1 (HSV-1) has several mechanisms to inhibit type I interferon production, the activities of ISGs, and the interferon signaling pathway itself. We report that the inhibition of the Jak/STAT pathway by HSV-1 requires viral gene expression and that viral immediate-early protein ICP27 plays a role in downregulating STAT-1 phosphorylation and in preventing the accumulation of STAT-1 in the nucleus. We also show that expression of ICP27 by transfection causes an inhibition of IFN-induced STAT-1 nuclear accumulation. Therefore, ICP27 is necessary and sufficient for at least some of the effects of HSV infection on STAT-1.

The interplay between viruses and innate immune signaling: recent insights and therapeutic opportunities

The immediate response to viral infection relies on pattern-recognition receptors (PRRs), most prominently the Toll-like receptors (TLRs) and the RNA helicases RIG-I and MDA5, as well as double stranded RNA-dependent protein kinase (PKR) and the DNA receptor, DAI. These PRRs recognize pathogen-associated molecular patterns (PAMPs) such as viral proteins and nucleic acids. The engagement of these receptors then initiates intracellular signaling cascades which ultimately cause the activation of transcription factors and the expression of type I interferons and pro-inflammatory cytokines. This innate response establishes an anti-viral state in the infected cell and its neighbours and alerts immune cells to the danger. In order to establish a productive infection, viruses need to overcome this initial anti-viral response. Evasion of innate immune defences is achieved by means of viral proteins that inhibit the signaling cascades emanating from the PRRs. The same innate signal transduction pathways have been implicated in conditions of sterile inflammation, such as rheumatoid arthritis and multiple sclerosis, and in autoimmunity. Because viral proteins target crucial host proteins involved in these pathways, they can point the way to key drug targets. Further, the viral proteins themselves or derivatives of them may be of use therapeutically to curtail inflammation and autoimmunity.

Herpes simplex virus-infected cell protein 0 blocks the silencing of viral DNA by dissociating histone deacetylases from the CoREST-REST complex

A preeminent phenotype of the infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is that it acts as a promiscuous transactivator. In most cell lines exposed to DeltaICP0 mutant virus at low ratios of virus per cell infection, alpha genes are expressed but the transition to beta and gamma gene expression does not ensue, but can be enhanced by inhibitors of histone deacetylases (HDACs). Earlier studies have shown that ICP0 interacts with CoREST and displaces HDAC1 from the CoREST-REST-HDAC1/2 complex. HDAC1 and CoREST are then independently translocated to the cytoplasm. Here, we test the hypothesis that ICP0 blocks the silencing of HSV DNA by displacing HDAC1 from the CoREST-REST complex. Specifically, first, mapping studies led us to construct a truncated CoREST (CoREST(146-482)) that in transfected cells displaced HDAC1 from the CoREST-REST complex. Second, we constructed two viruses. In BACs encoding the entire HSV-1, we replaced the gene encoding ICP0 with AmpR to yield a DeltaICP0 mutant R8501. We also replaced ICP0 with CoREST(146-482) to yield recombinant R8502. The yield of R8502 mutant virus in Vero, HEp-2, and human embryonic lung cells exposed to 0.1 pfu of virus per cell was 100-, 10-, and 10-fold higher, respectively, than those of R8501 mutant virus. In Vero cells, the yield of R8502 was identical with that of wild-type virus. We conclude that CoREST(146-482) functionally replaced ICP0 and that, by extension, ICP0 acts to block the silencing of viral DNA by displacing HDAC1/2 from the CoREST-REST complex.

Histone modifications induced by a family of bacterial toxins

Upon infection, pathogens reprogram host gene expression. In eukaryotic cells, genetic reprogramming is induced by the concerted activation/repression of transcription factors and various histone modifications that control DNA accessibility in chromatin. We report here that the bacterial pathogen Listeria monocytogenes induces a dramatic dephosphorylation of histone H3 as well as a deacetylation of histone H4 during early phases of infection. This effect is mediated by the major listerial toxin listeriolysin O in a pore-forming-independent manner. Strikingly, a similar effect also is observed with other toxins of the same family, such as Clostridium perfringens perfringolysin and Streptococcus pneumoniae pneumolysin. The decreased levels of histone modifications correlate with a reduced transcriptional activity of a subset of host genes, including key immunity genes. Thus, control of epigenetic regulation emerges here as an unsuspected function shared by several bacterial toxins, highlighting a common strategy used by intracellular and extracellular pathogens to modulate the host response early during infection.

Regulation of protein turnover by acetyltransferases and deacetylases

Lysine acetylation was first discovered as a post-translational modification of histones and has long been considered as a direct regulator of chromatin structure and function. Histone acetyltransferases (HATs) and histone deacetylases (HDACs) are the enzymes involved in this modification and they were thought to act as critical gene silencers or activators. Further investigations indicated that lysine acetylation can also occur in non-histone proteins and pointed to HATs and HDACs as multifunctional factors, acting not only on transcription but also on a variety of other cellular processes. One of these processes is the regulation of protein stability. Indeed, at least four independent HATs, namely CBP, p300, PCAF and TAF1, and one HDAC, HDAC6, possess intrinsic ubiquitin-linked functions in addition to their regular HAT/HDAC activities. Furthermore HATs and HDACs can be found in multi-subunit complexes with enzymes of the ubiquitination machinery. Moreover, lysine acetylation itself was found to directly or indirectly affect protein stability. These observations reveal therefore a tight link between protein lysine acetylation and ubiquitination and designate the acetylation machinery as a determinant element in the control of cellular proteolytic activities.