The LIM protein Ajuba promotes adipogenesis by enhancing PPARγ and p300/CBP interaction.

Adipocytes play a vital role in energy homeostasis and adipogenesis is a hierarchically regulated cellular differentiation process, in which the precursor mesenchymal stem cells are differentiated into mature adipocytes. Here, we report Ajuba is an important regulator of adipocyte differentiation by functioning as an obligate co-activator of PPARγ. Ajuba binds the DNA-binding domain of PPARγ via its preLIM region in a ligand-independent manner. Depletion of Ajuba in 3T3-L1 cells decreases PPARγ target gene expression and results in delayed adipogenic differentiation. Conversely, stable overexpression of Ajuba in 3T3-L1 cells increases PPARγ target gene expression and accelerates adipogenic differentiation. Mechanistic investigations demonstrate that Ajuba recruits p300/CBP via its LIM domain and facilitates p300/CBP binding to PPARγ. Moreover, Ajuba, PPARγ, p300/CBP can cooperatively occupy the PPARγ target promoters and concomitantly increases histone acetylation at these loci. Collectively, these data suggest that Ajuba is a co-activator constitutively associated with PPARγ and may be a potential therapeutic target for PPARγ-mediated metabolic disorders.

Targeting histone deacetylase complexes via KRAB-zinc finger proteins: the PHD and bromodomains of KAP-1 form a cooperative unit that recruits a novel isoform of the Mi-2alpha subunit of NuRD.

Macromolecular complexes containing histone deacetylase and ATPase activities regulate chromatin dynamics and are vitally responsible for transcriptional gene silencing in eukaryotes. The mechanisms that target these assemblies to specific loci are not as well understood. We show that the corepressor KAP-1, via its PHD (plant homeodomain) and bromodomain, links the superfamily of Krüppel associated box (KRAB) zinc finger proteins (ZFP) to the NuRD complex. We demonstrate that the tandem PHD finger and bromodomain of KAP-1, an arrangement often found in cofactor proteins but functionally ill-defined, form a cooperative unit that is required for transcriptional repression. Substitution of highly related PHD fingers or bromodomains failed to restore repression activity, suggesting high specificity in their cooperative function. Moreover, single amino acid substitutions in either the bromodomain or PHD finger, including ones that mimic disease-causing mutations in the hATRX PHD finger, abolish repression. A search for effectors of this repression function yielded a novel isoform of the Mi-2alpha protein, an integral component of the NuRD complex. Endogenous KAP-1 is associated with Mi-2alpha and other components of NuRD, and KAP-1-mediated silencing requires association with NuRD and HDAC activity. These data suggest the KRAB-ZFP superfamily of repressors functions to target the histone deacetylase and chromatin remodeling activities of the NuRD complex to specific gene promoters in vivo.

Regulating the neoplastic phenotype using engineered transcriptional repressors.

We have applied engineered transcriptional repressors to specifically inhibit disease gene-activated pathways in oncogenesis. We have demonstrated that synthetic repressors combining PAX3 DNA binding domains with different repression domains, KRAB or SNAG, are able to specifically inhibit malignant growth and suppress tumorigenesis in alveolar rhabdomyosarcoma tumor cells transformed by the translocation-derived chimeric transcriptional activator, PAX3-FKHR. We discuss the potential applications of the engineered repressor strategy that relate to target gene analysis, mechanisms of repression, cell regulation, and possible anti-viral and cancer therapy.

Hormone-dependent tumor regression in vivo by an inducible transcriptional repressor directed at the PAX3-FKHR oncogene.

In alveolar rhabdomyosarcomas (ARMSs), a specific chromosomal translocation creates a fusion transcription factor, PAX3-FKHR, that is oncogenic due to transcriptional activation. As a strategy for down-regulation of PAX3-FKHR target genes, we created conditional PAX3 repressors by fusing the PAX3 DNA-binding motifs to the hormone binding domain (HBD) of the estrogen receptor and to the KRAB repression domain. We validated proper expression, specific DNA binding, corepressor interaction, and nuclear localization for the KRAB-PAX3-HBD protein and showed it to be a 4-hydroxytamoxifen-dependent transcriptional repressor of transiently transfected and integrated PAX3 reporters in ARMS cells. We established ARMS cell lines that exhibited stable expression of the conditional PAX3 repressor proteins and used them to down-regulate the malignant growth under low serum or anchorage-independent conditions in a hormone-dependent manner. Terminal deoxynucleotidyl transferase-mediated nick end labeling assays revealed that hormonal activation of the PAX3 repressors induced extensive apoptosis that correlated with down-regulation of BCL-X(L) expression. SCID mice that were engrafted with the KRAB-PAX3-HBD ARMS cell lines and were implanted with 4-hydroxytamoxifen timed-release pellets exhibited suppression of tumor growth and an altered vascularity that was not observed in the control mice. These observations strongly suggest that we have directly repressed the PAX3 target genes that are deregulated by the PAX3-FKHR oncogene in ARMS.

Molecular determinants for targeting heterochromatin protein 1-mediated gene silencing: direct chromoshadow domain-KAP-1 corepressor interaction is essential.

The KRAB domain is a highly conserved transcription repression module commonly found in eukaryotic zinc finger proteins. KRAB-mediated repression requires binding to the KAP-1 corepressor, which in turn recruits members of the heterochromatin protein 1 (HP1) family. The HP1 proteins are nonhistone chromosomal proteins, although it is unclear how they are targeted to unique chromosomal domains or promoters. In this report, we have reconstituted and characterized the HP1-KAP-1 interaction using purified proteins and have compared KAP-1 to three other known HP1 binding proteins: SP100, lamin B receptor (LBR), and the p150 subunit from chromatin assembly factor (CAF-1 p150). We show that the chromoshadow domain (CSD) of HP1 is a potent repression domain that binds directly to all four previously described proteins. For KAP-1, we have mapped the CSD interaction region to a 15-amino-acid segment, termed the HP1BD, which is also present in CAF-1 p150 but not SP100 or LBR. The region of KAP-1 harboring the HP1BD binds as a monomer to a dimer of the CSD, as revealed by gel filtration, analytical ultracentrifugation, and optical biosensor analyses. The use of a spectrum of amino acid substitutions in the human HP1alpha CSD revealed a strong correlation between CSD-mediated repression and binding to KAP-1, CAF-1 p150, and SP100 but not LBR. Differences among the HP1 binding partners could also be discerned by fusion to a heterologous DNA binding domain and by the potential to act as dominant negative molecules. Together, these results strongly suggest that KAP-1 is a physiologically relevant target for HP1 function.

An engineered PAX3-KRAB transcriptional repressor inhibits the malignant phenotype of alveolar rhabdomyosarcoma cells harboring the endogenous PAX3-FKHR oncogene.

The t(2;13) chromosomal translocation in alveolar rhabdomyosarcoma tumors (ARMS) creates an oncogenic transcriptional activator by fusion of PAX3 DNA binding motifs to a COOH-terminal activation domain derived from the FKHR gene. The dominant oncogenic potential of the PAX3-FKHR fusion protein is dependent on the FKHR activation domain. We have fused the KRAB repression module to the PAX3 DNA binding domain as a strategy to suppress the activity of the PAX3-FKHR oncogene. The PAX3-KRAB protein bound PAX3 target DNA sequences and repressed PAX3-dependent reporter plasmids. Stable expression of the PAX3-KRAB protein in ARMS cell lines resulted in loss of the ability of the cells to grow in low-serum or soft agar and to form tumors in SCID mice. Stable expression of a PAX3-KRAB mutant, which lacks repression function, or a KRAB protein alone, lacking a PAX3 DNA binding domain, failed to suppress the ARMS malignant phenotype. These data suggest that the PAX3-KRAB repressor functions as a DNA-binding-dependent suppressor of the transformed phenotype of ARMS cells, probably via competition with the endogenous PAX3-FKHR oncogene and repression of target genes required for ARMS tumorigenesis. The engineered repressor approach that directs a transcriptional repression domain to target genes deregulated by the PAX3-FKHR oncogene may be a useful strategy to identify the target genes critical for ARMS tumorigenesis.

Biochemical analysis of the Kruppel-associated box (KRAB) transcriptional repression domain.

The Kruppel-associated box (KRAB) domain is a 75-amino acid transcriptional repressor module commonly found in eukaryotic zinc finger proteins. KRAB-mediated gene silencing requires binding to the RING-B box-coiled-coil domain of the corepressor KAP-1. Little is known about the biochemical properties of the KRAB domain or the KRAB.KAP-1 complex. Using purified components, a combination of biochemical and biophysical analyses has revealed that the KRAB domain from the KOX1 protein is predominantly a monomer and that the KAP-1 protein is predominantly a trimer in solution. The analyses of electrophoretic mobility shift assays, GST association assays, and plasmon resonance interaction data have indicated that the KRAB binding to KAP-1 is direct, highly specific, and high affinity. The optical biosensor data for the complex was fitted to a model of a one-binding step interaction with fast association and slow dissociation rates, with a calculated K(d) of 142 nm. The fitted R(max) indicated three molecules of KAP-1 binding to one molecule of the KRAB domain, a stoichiometry that is consistent with quantitative SDS-polyacrylamide gel electrophoresis analysis of the complex. These structural and dynamic parameters of the KRAB/KAP-1 interaction have implications for identifying downstream effectors of KAP-1 silencing and the de novo design of new repression domains.

Reconstitution of the KRAB-KAP-1 repressor complex: a model system for defining the molecular anatomy of RING-B box-coiled-coil domain-mediated protein-protein interactions.

The KRAB domain is a 75 amino acid residue transcriptional repression module commonly found in eukaryotic zinc-finger proteins. KRAB-mediated gene silencing requires binding to the corepressor KAP-1. The KRAB:KAP-1 interaction requires the RING-B box-coiled coil (RBCC) domain of KAP-1, which is a widely distributed motif, hypothesized to be a protein-protein interface. Little is known about RBCC-mediated ligand binding and the role of the individual sub-domains in recognition and specificity. We have addressed these issues by reconstituting and characterizing the KRAB:KAP-1-RBCC interaction using purified components. Our results show that KRAB binding to KAP-1 is direct and specific, as the related RBCC domains from TIF1alpha and MID1 do not bind the KRAB domain. A combination of gel filtration, analytical ultracentrifugation, chemical cross-linking, non-denaturing gel electrophoresis, and site-directed mutagenesis techniques has revealed that the KAP-1-RBCC must oligomerize likely as a homo-trimer in order to bind the KRAB domain. The RING finger, B2 box, and coiled-coil region are required for oligomerization of KAP-1-RBCC and KRAB binding, as mutations in these domains concomitantly abolished these functions. KRAB domain binding stabilized the homo-oligomeric state of the KAP-1-RBCC as detected by chemical cross-linking and velocity sedimentation studies. Mutant KAP-1-RBCC molecules hetero-oligomerize with the wild-type KAP-1, but these complexes were inactive for KRAB binding, suggesting a potential dominant negative activity. Substitution of the coiled-coil region with heterologous dimerization, trimerization, or tetramerization domains failed to recapitulate KRAB domain binding. Chimeric KAP-1-RBCC proteins containing either the RING, RING-B box, or coiled-coil regions from MID1 also failed to bind the KRAB domain. The KAP-1-RBCC mediates a highly specific, direct interaction with the KRAB domain, and it appears to function as an integrated, possibly cooperative structural unit wherein each sub-domain contributes to oligomerization and/or ligand recognition. These observations provide the first principles for RBCC domain-mediated protein-protein interaction and have implications for identifying new ligands for RBCC domain proteins.

Structure-function studies of the BTB/POZ transcriptional repression domain from the promyelocytic leukemia zinc finger oncoprotein.

The evolutionarily conserved BTB/POZ domain from the promyelocytic leukemia zinc finger (PLZF) oncoprotein mediates transcriptional repression through the recruitment of corepressor proteins containing histone deacetylases in acute promyelocytic leukemia. We have determined the 2.0 A crystal structure of the BTB/POZ domain from PLZF (PLZF-BTB/POZ), and have carried out biochemical analysis of PLZF-BTB/POZ harboring site-directed mutations to probe structure-function relationships. The structure reveals a novel alpha/beta homodimeric fold in which dimer interactions occur along two surfaces of the protein subunits. The conservation of BTB/POZ domain residues at the core of the protomers and at the dimer interface implies an analogous fold and dimerization mode for BTB/POZ domains from otherwise functionally unrelated proteins. Unexpectedly, the BTB/POZ domain forms dimer-dimer interactions in the crystals, suggesting a mode for higher-order protein oligomerization for BTB/POZ-mediated transcriptional repression. Biochemical characterization of PLZF-BTB/POZ harboring mutations in conserved residues involved in protein dimerization reveals that the integrity of the dimer interface is exquisitely sensitive to mutation and that dimer formation is required for wild-type levels of transcriptional repression. Interestingly, similar mutational analysis of residues within a pronounced protein cleft along the dimer interface, which had been implicated previously for interaction with corepressors, has negligible effects on dimerization or transcriptional repression. Together, these studies form a structure-function framework for understanding BTB/POZ-mediated oligomerization and transcriptional repression properties.

KAP-1 corepressor protein interacts and colocalizes with heterochromatic and euchromatic HP1 proteins: a potential role for Krüppel-associated box-zinc finger proteins in heterochromatin-mediated gene silencing.

Krüppel-associated box (KRAB) domains are present in approximately one-third of all human zinc finger proteins (ZFPs) and are potent transcriptional repression modules. We have previously cloned a corepressor for the KRAB domain, KAP-1, which is required for KRAB-mediated repression in vivo. To characterize the repression mechanism utilized by KAP-1, we have analyzed the ability of KAP-1 to interact with murine (M31 and M32) and human (HP1alpha and HP1gamma) homologues of the HP1 protein family, a class of nonhistone heterochromatin-associated proteins with a well-established epigenetic gene silencing function in Drosophila. In vitro studies confirmed that KAP-1 is capable of directly interacting with M31 and hHP1alpha, which are normally found in centromeric heterochromatin, as well as M32 and hHP1gamma, both of which are found in euchromatin. Mapping of the region in KAP-1 required for HP1 interaction showed that amino acid substitutions which abolish HP1 binding in vitro reduce KAP-1 mediated repression in vivo. We observed colocalization of KAP-1 with M31 and M32 in interphase nuclei, lending support to the biochemical evidence that M31 and M32 directly interact with KAP-1. The colocalization of KAP-1 with M31 is sometimes found in subnuclear territories of potential pericentromeric heterochromatin, whereas colocalization of KAP-1 and M32 occurs in punctate euchromatic domains throughout the nucleus. This work suggests a mechanism for the recruitment of HP1-like gene products by the KRAB-ZFP-KAP-1 complex to specific loci within the genome through formation of heterochromatin-like complexes that silence gene activity. We speculate that gene-specific repression may be a consequence of the formation of such complexes, ultimately leading to silenced genes in newly formed heterochromatic chromosomal environments.

Induction of antisense Pax-3 expression leads to the rapid morphological differentiation of neuronal cells and an altered response to the mitogenic growth factor bFGF.

Mutations within the Pax-3 gene lead to a range of developmental abnormalities in both humans and mice. In this report, we have investigated the role that Pax-3 plays in neuronal cell development by specifically downregulating Pax-3 expression within a neuronal cell line. This was achieved by stably transfecting the neuronal cell line ND7 with an expression vector in which antisense Pax-3 RNA was produced under the control of the inducible MMTV promoter. In the stable transfectants, we found that the addition of dexamethasone led to the induction of antisense Pax-3 RNA and a rapid downregulation in endogenous Pax-3 protein expression. The decrease in endogenous Pax-3 protein expression corresponded with a dramatic change in the morphology of the cell: the normally rounded ND7 cells exhibited increased cell to substrate adhesion, extended long neurite processes and expressed genes such as snap-25 that are characteristic of a mature neuron. The morphological differentiation induced by a reduction in Pax-3 expression was followed 24-48 hours later by a cessation in cell proliferation. Interestingly the morphological differentiation and cessation in cell proliferation inducted in the cell lines lacking Pax-3 could be reversed by the addition of the mitogenic growth factor EGF but not by bFGF, whose receptor was downregulated in these cells. These results suggest that the expression of Pax-3 is essential to maintain the undifferentiated phenotype of these immature neuronal cells, and in its absence the cells acquire many of the characteristics of a mature neuronal cell. The slow onset of cell cycle arrest in the cells lacking Pax-3 argues against this transcription factor playing a direct role in the regulation of neuronal cell proliferation.

Nuclear redistribution of BRCA1 during viral infection.

The functions and the intracellular localization of the breast/ovarian susceptibility gene product, BRCA1, has been controversial. To arrive at a clear understanding of its localization and relative position to other nuclear structures, a new monoclonal antibody was produced and characterized by immunohistochemical techniques with other BRCA1 antibodies. Each of the antibodies specifically detected BRCA1 as localized to specific nuclear domains and did so in a variety of cells and in a cell cycle-dependent manner. However, all antibodies also cross-reacted with the centrosomal domain, suggesting that BRCA1 is also localized to this important mitotic component. We found that the BRCA1-containing nuclear domains are different than any of the well-defined nuclear domains. However, a cell cycle-related partial overlap was found for HP1alpha, a chromo-domain-containing protein involved in heterochromatin maintenance. Cellular stimuli, such as heat shock and herpes virus infection, dispersed BRCA1 from its domains. In contrast, infection with adenovirus 5 recruited BRCA1 to regions of viral transcription and replication. These disparate distributions of BRCA1 may provide clues to its function.

Characterization of monoclonal antibodies directed to the amino-terminus of the WT1, Wilms' tumor suppressor protein.

We have produced and characterized three monoclonal antibodies (MAbs) directed to the amino terminus of the WT1 Wilms' tumor suppressor transcription factor and compared their properties to rabbit polyclonal sera raised to the same immunogen. A recombinant protein consisting of amino acids 1-181 of human WT1 was overexpressed in E. coli, purified, and used as the immunogen. Three MAbs designated 6F-H2, 6F-H7, and 6F-HC-17--all of the IgG1 subclass--were selected and further characterized. Each recognized all isoforms of the full-length WT1 protein in Western blot assays and immunoprecipitated WT1 in both physiologic buffers and under high detergent/high salt (RIPA) conditions. Preliminary epitope mapping suggests that all three MAbs recognize a region in the amino terminal 84 amino acids of WT1 and that the MAbs do not recognize the polyglycine or polyproline regions of the protein. The WT1 antibodies do not recognize the structurally and functionally related early growth response (EGR)1, EGR2, EGR3, or EGR4 proteins. All WT1 MAbs recognize the murine WT1 protein and immunohistochemical staining of murine embryonic and newborn kidney sections show strong staining of condensing metanephric mesenchyme and primitive podocytes in developing glomeruli. These WT1-specific MAbs should be useful in characterizing the biochemical and developmental roles of WT1 and in defining the emerging role of WT1 as a diagnostic and/or prognostic marker in mesothelioma, leukemias, and breast cancer.

BAP1: a novel ubiquitin hydrolase which binds to the BRCA1 RING finger and enhances BRCA1-mediated cell growth suppression.

We have identified a novel protein, BAP1, which binds to the RING finger domain of the Breast/Ovarian Cancer Susceptibility Gene product, BRCA1. BAP1 is a nuclear-localized, ubiquitin carboxy-terminal hydrolase, suggesting that deubiquitinating enzymes may play a role in BRCA1 function. BAP1 binds to the wild-type BRCA1-RING finger, but not to germline mutants of the BRCA1-RING finger found in breast cancer kindreds. BAP1 and BRCA1 are temporally and spatially co-expressed during murine breast development and remodeling, and show overlapping patterns of subnuclear distribution. BAP1 resides on human chromosome 3p21.3; intragenic homozygous rearrangements and deletions of BAP1 have been found in lung carcinoma cell lines. BAP1 enhances BRCA1-mediated inhibition of breast cancer cell growth and is the first nuclear-localized ubiquitin carboxy-terminal hydrolase to be identified. BAP1 may be a new tumor suppressor gene which functions in the BRCA1 growth control pathway.

Constitutive expression of the Wilms' tumor gene (WT1) in the leukemic cell line U937 blocks parts of the differentiation program.

The Wilms tumor gene, WT1, encodes a zinc-finger DNA binding protein which is thought to function as a tissue specific transcription factor, regulating cell growth and differentiation. High expression of WT1 has been detected in a range of acute leukemias. To elucidate a role for WT1 in leukemogenesis, we transfected the monoblastic cell line U937, which lacks detectable levels of endogenous WT1, with two isoforms of WT1. We showed that, in contrast to U937 control cells, cells constitutively expressing either of the isoforms, WT1(-KTS) or WT1(+KTS), did not respond to differentiation induction by retinoic acid or vitamin D3, as judged by the capacity to reduce nitro blue tetrazolium and morphology. Although U937 cells expressing WT1 were hampered in their ability to differentiate on incubation with retinoic acid and vitamin D3, the induced G1/G0-accumulation was similar to differentiating control cells treated with inducers. Furthermore, distinct effects on the maturation process were indicated by downregulation of the myeloid cell surface makers CD13 and CD15, while the upregulation of CD14 and CD11c on WT1 transfected cells was similar to control cells upon incubation with retinoic acid and vitamin D3. Taken together our results demonstrate that a constitutive expression of WT1 in the leukemic cell line U937 leads to impairment of differentiation responses, indicating that a high expression of WT1 can contribute to the differentiation block of acute leukemia.

The DNA binding activity of the paired box transcription factor Pax-3 is rapidly downregulated during neuronal cell differentiation.

Mutations in the murine Pax-3 gene lead to a range of developmental abnormalities including deficiencies in sensory and sympathetic neurones. We have investigated Pax-3 expression during neuronal differentiation and show levels of Pax-3 DNA binding decrease upon cell cycle arrest and morphological differentiation. The fall in Pax-3 DNA binding occurs within 1 h of the induction of differentiation and is mediated in part by a decrease in Pax-3 mRNA. This decrease in Pax-3 binding activity precedes any changes in cell proliferation or morphology, suggesting that the downregulation of this transcription factor may be an important prerequisite for the differentiation of neuronal progenitor cells.

Overexpression, purification, characterization, and crystallization of the BTB/POZ domain from the PLZF oncoprotein.

The BTB/POZ domain defines a conserved region of about 120 residues and has been found in over 40 proteins to date. It is located predominantly at the N terminus of Zn-finger DNA-binding proteins, where it may function as a repression domain, and less frequently in actin-binding and poxvirus-encoded proteins, where it may function as a protein-protein interaction interface. A prototypic human BTB/POZ protein, PLZF (promyelocytic leukemia zinc finger) is fused to RARalpha (retinoic acid receptor alpha) in a subset of acute promyelocytic leukemias (APLs), where it acts as a potent oncogene. The exact role of the BTB/POZ domain in protein-protein interactions and/or transcriptional regulation is unknown. We have overexpressed, purified, characterized, and crystallized the BTB/POZ domain from PLZF (PLZF-BTB/POZ). Gel filtration, dynamic light scattering, and equilibrium sedimentation experiments show that PLZF-BTB/POZ forms a homodimer with a Kd below 200 nM. Differential scanning calorimetry and equilibrium denaturation experiments are consistent with the PLZF-BTB/POZ dimer undergoing a two-state unfolding transition with a Tm of 70.4 degrees C, and a DeltaG of 12.8 +/- 0.4 kcal/mol. Circular dichroism shows that the PLZF-BTB/POZ dimer has significant secondary structure including about 45% helix and 20% beta-sheet. We have prepared crystals of the PLZF-BTB/POZ that are suitable for a high resolution structure determination using x-ray crystallography. The crystals form in the space group I222 or I212121 with a = 38.8, b = 77.7, and c = 85.3 A and contain 1 protein subunit per asymmetric unit with approximately 40% solvent. Our data support the hypothesis that the BTB/POZ domain mediates a functionally relevant dimerization function in vivo. The crystal structure of the PLZF-BTB/POZ domain will provide a paradigm for understanding the structural basis underlying BTB/POZ domain function.

Repression of the HSV-1 latency-associated transcript (LAT) promoter by the early growth response (EGR) proteins: involvement of a binding site immediately downstream of the TATA box.

During herpes simplex virus (HSV) latency, in neurons of the nervous system, a single family of viral transcripts (the Latency-Associated Transcripts or LATs) are synthesized. Within the LAT promoter region, we have identified a consensus sequence for the EGR proteins in an unusual position immediately downstream of the TATA box. The early growth response (EGR) proteins are rapidly induced in cells by stimuli which also induce HSV to reactivate from latency. In order to determine if EGR proteins play any role in control of LAT transcription, we have analyzed the interactions between EGR proteins and the LAT promoter. Gel retardation and DNase I protection assays demonstrated that EGR1 zinc finger protein bound specifically to the LAT promoter region EGR consensus sequence. To determine if EGR proteins could modulate transcription through the LAT promoter, cotransfection assays were performed using chloramphenicol acetyltransferase (CAT) reporter constructs driven by either the wild-type LAT promoter or a LAT promoter with a mutated EGR binding site. Contransfection of the wild-type LAT promoter construct with EGR expression plasmids resulted in inhibition of the basal level of CAT activity with EGR-2 but not EGR-1 or 3. However, normal levels of CAT activity were observed in cotransfections using the mutant LAT promoter CAT construct suggesting that repression was mediated by the binding of EGR-2 proteins to the LAT promoter. Furthermore, data from combination binding assays using EGR1 and TATA binding protein (TBP) in vitro support the hypothesis that binding of EGR proteins to the LAT promoter prevents binding of TBP and thus suppresses transcription. These results may provide a link between stress responses in neurons of the CNS which activate the EGR family of proteins and HSV reactivation from latency due to the same stress response.