Molecular characterization of HTLV-1 Tax interaction with the KIX domain of CBP/p300

Molecular basis for SOX2-dependent regulation of super-enhancer activity

Pioneer transcription factors (TFs) like SOX2 are vital for stemness and cancer through enhancing gene expression within transcriptional condensates formed with coactivators, RNAs and mediators on super-enhancers (SEs). Despite their importance, how these factors work together for transcriptional condensation and activation remains unclear. SOX2, a pioneer TF found in SEs of pluripotent and cancer stem cells, initiates SE-mediated transcription by binding to nucleosomes, though the mechanism isn't fully understood. To address SOX2's role in SEs, we identified mSE078 as a model SOX2-enriched SE and p300 as a coactivator through bioinformatic analysis. In vitro and cell assays showed SOX2 forms condensates with p300 and SOX2-binding motifs in mSE078. We further proved that SOX2 condensation is highly correlated with mSE078's enhancer activity in cells. Moreover, we successfully demonstrated that p300 not only elevated transcriptional activity but also triggered chromatin acetylation via its direct interaction with SOX2 within these transcriptional condensates. Finally, our validation of SOX2-enriched SEs showcased their contribution to target gene expression in both stem cells and cancer cells. In its entirety, this study imparts valuable mechanistic insights into the collaborative interplay of SOX2 and its coactivator p300, shedding light on the regulation of transcriptional condensation and activation within SOX2-enriched SEs.

Vital for Viruses: Intrinsically Disordered Proteins

Viruses infect all kingdoms of life; their genomes vary from DNA to RNA and in size from 2kB to 1 MB or more. Viruses frequently employ disordered proteins, that is, protein products of virus genes that do not themselves fold into independent three-dimensional structures, but rather, constitute a versatile molecular toolkit to accomplish a range of functions necessary for viral infection, assembly, and proliferation. Interestingly, disordered proteins have been discovered in almost all viruses so far studied, whether the viral genome consists of DNA or RNA, and whatever the configuration of the viral capsid or other outer covering. In this review, I present a wide-ranging set of stories illustrating the range of functions of IDPs in viruses. The field is rapidly expanding, and I have not tried to include everything. What is included is meant to be a survey of the variety of tasks that viruses accomplish using disordered proteins.

The effect of alcoholic extract from Eucalyptus camaldulensis leaves on HTLV-1 Tax activities

HTLV-1 is a human retrovirus responsible for adult T-cell leukemia (ATL) and certain other clinical disorders. The viral Tax oncoprotein plays a central role in HTLV-1 pathogenicity, mainly due to its capacity of inducing the transcriptional activity of various transcriptional factors like NFқB. Eucalyptus camaldulensis (Ec) is considered as a traditional medicinal plant with valuable therapeutic effects. Here we evaluated the activity of its ethanolic leave extract on different Tax activities by testing its influence on Tax-induced activity of NFқB and HTLV-1 LTR in Jurkat cells. Our results showed that Ec inhibited Tax induced activation of NFқB -, SRF- dependent promoters and HTLV-1 LTR. Ec extract has no effect on the binding of Tax to NFқB while it strongly prevented the degradation of IҝBα induced by Tax probably as a result of preventing the link between Tax and IKKγ. In addition, increasing the cellular level of P-TEFb-cyclinT1 significantly reduced the inhibitory effect of Ec on Tax activities, probably by preventing the interaction between Tax and P-TEFb-cyclin T1. The 40%-MeOH fraction of this extract, which is rich with polyphenols, offered the highest inhibitory effect against Tax activities. Further studies are required for the isolation and identification of active component/s in this extract which may be developed in the future as preventive/curing drugs for HTLV-1 related diseases.

Charge Interactions Modulate the Encounter Complex Ensemble of Two Differently Charged Disordered Protein Partners of KIX

Disordered proteins play important roles in cell signaling and are frequently involved in protein-protein interactions. They also have a larger proportion of charged and polar residues than their folded counterparts. Here, we developed a structure-based model and applied molecular dynamics simulations to examine the presence and importance of electrostatic interactions in the binding processes of two differently charged intrinsically disordered ligands of the KIX domain of CBP. We observed non-native opposite-charged contacts in the encounter complexes for both ligands with KIX, and this may be a general feature of coupled folding and binding reactions. The ensemble of successful encounter complexes is a diverse set of structures, and in the case of the highly charged ligand, this ensemble was found to be malleable with respect to ionic strength. There are only minor differences between encounter complex ensembles for successful and unsuccessful collisions with no key interactions that appear to make the process far more productive. The energy landscape at this early stage in the process does not appear highly funneled. Strikingly we observed many native interactions that appear to reduce chances of an encounter complex being productive. Instead it appears that collectively non-native electrostatic interactions in the encounter complex increase the likelihood of productivity by holding the proteins together long enough for folding to take place. This mechanism is more effective for the more highly charged ligand.

Structural basis for cooperative regulation of KIX-mediated transcription pathways by the HTLV-1 HBZ activation domain

The human T cell leukemia virus I basic leucine zipper protein (HTLV-1 HBZ) maintains chronic viral infection and promotes leukemogenesis through poorly understood mechanisms involving interactions with the KIX domain of the transcriptional coactivator CBP and its paralog p300. The KIX domain binds regulatory proteins at the distinct MLL and c-Myb/pKID sites to form binary or ternary complexes. The intrinsically disordered N-terminal activation domain of HBZ (HBZ AD) deregulates cellular signaling pathways by competing directly with cellular and viral transcription factors for binding to the MLL site and by allosterically perturbing binding of the transactivation domain of the hematopoietic transcription factor c-Myb. Crystal structures of the ternary KIX:c-Myb:HBZ complex show that the HBZ AD recruits two KIX:c-Myb entities through tandem amphipathic motifs (L/V)(V/L)DGLL and folds into a long α-helix upon binding. Isothermal titration calorimetry reveals strong cooperativity in binding of the c-Myb activation domain to the KIX:HBZ complex and in binding of HBZ to the KIX:c-Myb complex. In addition, binding of KIX to the two HBZ (V/L)DGLL motifs is cooperative; the structures suggest that this cooperativity is achieved through propagation of the HBZ α-helix beyond the first binding motif. Our study suggests that the unique structural flexibility and the multiple interaction motifs of the intrinsically disordered HBZ AD are responsible for its potency in hijacking KIX-mediated transcription pathways. The KIX:c-Myb:HBZ complex provides an example of cooperative stabilization in a transcription factor:coactivator network and gives insights into potential mechanisms through which HBZ dysregulates hematopoietic transcriptional programs and promotes T cell proliferation.

Differential effects of HTLV-1 Tax oncoprotein on the different estrogen-induced-ER α-mediated transcriptional activities

The activated estrogen (E2) receptor α (ERα) is a potent transcription factor that is involved in the activation of various genes by 2 different pathways; a classical and non-classical. In classical pathway, ERα binds directly to E2-responsive elements (EREs) located in the appropriate genes promoters and stimulates their transcription. However, in non-classical pathway, the ERα can indirectly bind with promoters and enhance their activity. For instance, ERα activates BRCA1 expression by interacting with jun/fos complex bound to the AP-1 site in BRCA1 promoter. Interference with the expression and/or functions of BRCA1, leads to high risk of breast or/and ovarian cancer. HTLV-1Tax was found to strongly inhibit BRCA1 expression by preventing the binding of E2-ERα complex to BRCA1 promoter. Here we examined Tax effect on ERα induced activation of genes by the classical pathway by testing its influence on E2-induced expression of ERE promoter-driven luciferase reporter (ERE-Luc). Our findings showed that E2 profoundly stimulated this reporter expression and that HTLV-1Tax significantly induced this stimulation. This result is highly interesting because in our previous study Tax was found to strongly block the E2-ERα-mediated activation of BRCA1 expression. ERα was found to produce a big complex by recruiting various cofactors in the nucleus before binding to the ERE region. We also found that only part of the reqruited cofactors are required for the transcriptional activity of ERα complex. Chip assay revealed that the binding of Tax to the ERα complex, did not interfere with its link to ERE region.

Allostery within a transcription coactivator is predominantly mediated through dissociation rate constants

The kinase-inducible domain interacting (KIX) domain of CREB binding protein binds to multiple intrinsically disordered transcription factors in vivo at two distinct sites on its surface. Several reports have been made of allosteric communication between these two sites in this well-characterized model system. In this work, we have performed fluorescence stopped-flow measurements to investigate the kinetics of binding of five KIX binding proteins. We find that they all have similar association and dissociation rate constants for complex formation, despite their wide range of intrinsic helical propensities. Furthermore, by careful arrangement of pseudofirst-order conditions, we have been able to show that both association and dissociation rate constants are decreased when a partner is bound at the alternative site. These decreases suggest that positive allosteric effects are not mediated by structural changes in binding sites but rather, through a more general mechanism, largely mediated through dissociation, which we propose is largely related to changes in the flexibility of the KIX domain itself.

Functional redundancy between the transcriptional activation domains of E2A is mediated by binding to the KIX domain of CBP/p300

The E-protein transcription factors play essential roles in lymphopoiesis, with E12 and E47 (hereafter called E2A) being particularly important in B cell specification and maturation. The E2A gene is also involved in a chromosomal translocation that results in the leukemogenic oncoprotein E2A-PBX1. The two activation domains of E2A, AD1 and AD2, display redundant, independent, and cooperative functions in a cell-dependent manner. AD1 of E2A functions by binding the transcriptional co-activator CBP/p300; this interaction is required in oncogenesis and occurs between the conserved ϕ-x-x-ϕ-ϕ motif in AD1 and the KIX domain of CBP/p300. However, co-activator recruitment by AD2 has not been characterized. Here, we demonstrate that the first of two conserved ϕ-x-x-ϕ-ϕ motifs within AD2 of E2A interacts at the same binding site on KIX as AD1. Mutagenesis uncovered a correspondence between the KIX-binding affinity of AD2 and transcriptional activation. Although AD2 is dispensable for oncogenesis, experimentally increasing the affinity of AD2 for KIX uncovered a latent potential to mediate immortalization of primary hematopoietic progenitors by E2A-PBX1. Our findings suggest that redundancy between the two E2A activation domains with respect to transcriptional activation and oncogenic function is mediated by binding to the same surface of the KIX domain of CBP/p300.

HTLV-1 Tax oncoprotein inhibits the estrogen-induced-ER α-Mediated BRCA1 expression by interaction with CBP/p300 cofactors

BRCA1 is a multifunctional tumor suppressor, whose expression is activated by the estrogen (E2)-liganded ERα receptor and regulated by certain recruited transcriptional co-activators. Interference with BRCA1 expression and/or functions leads to high risk of breast or/and ovarian cancer. Another multifunctional protein, HTLV-1Tax oncoprotein, is widely regarded as crucial for developing adult T-cell leukemia and other clinical disorders. Tax profile reveals that it can antagonize BRCA1 expression and/or functionality. Therefore, we hypothesize that Tax expression in breast cells can sensitize them to malignant transformation by environmental carcinogens. Here we examined Tax effect on BRCA1 expression by testing its influence on E2-induced expression of BRCA1 promoter-driven luciferase reporter (BRCA1-Luc). We found that E2 strongly stimulated this reporter expression by liganding to ERα, which consequently associated with BRCA1 promoter, while ERα concomitantly recruited CBP/p300 to this complex for co-operative enhancement of BRCA1 expression. Introducing Tax into these cells strongly blocked this E2-ERα-mediated activation of BRCA1 expression. We noted, also, that Tax exerted this inhibition by binding to CBP/p300 without releasing them from their complex with ERα. Chip assay revealed that the binding of Tax to the CBP/p300-ERα complex, prevented its link to AP1 site. Interestingly, we noted that elevating the intracellular pool of CBP or p300 to excessive levels dramatically reduced the Tax-mediated inhibition of BRCA1 expression. Exploring the mechanism of this reduction revealed that the excessive co-factors were sufficient to bind separately the free Tax molecules, thus lowering their amount in the CBP/p300-ERα complex and relieving, thereby, the inhibition of BRCA1 expression.

Molecular recognition by the KIX domain and its role in gene regulation

The kinase-inducible domain interacting (KIX) domain is a highly conserved independently folding three-helix bundle that serves as a docking site for transcription factors, whereupon promoter activation and target specificity are achieved during gene regulation. This docking event is a harbinger of an intricate multi-protein assembly at the transcriptional apparatus and is regulated in a highly precise manner in view of the critical role it plays in multiple cellular processes. KIX domains have been characterized in transcriptional coactivators such as p300/CREB-binding protein and mediator of RNA polymerase II transcription subunit 15, and even recQ protein-like 5 helicases in various organisms. Their targets are often intrinsically disordered regions within the transactivation domains of transcription factors that attain stable secondary structure only upon complexation with KIX. In this article, we review the KIX domain in terms of its sequence and structure and present the various implications of its ability to act as a transcriptional switch, the mechanistic basis of molecular recognition by KIX, its binding specificity, target promiscuity, combinatorial potential and unique mode of regulation via allostery. We also discuss the possible roles of KIX domains in plants and hope that this review will accelerate scientific interest in KIX and pave the way for novel avenues of research on this critical domain.

Transcriptional/epigenetic regulator CBP/p300 in tumorigenesis: structural and functional versatility in target recognition

In eukaryotic cells, gene transcription is regulated by sequence-specific DNA-binding transcription factors that recognize promoter and enhancer elements near the transcriptional start site. Some coactivators promote transcription by connecting transcription factors to the basal transcriptional machinery. The highly conserved coactivators CREB-binding protein (CBP) and its paralog, E1A-binding protein (p300), each have four separate transactivation domains (TADs) that interact with the TADs of a number of DNA-binding transcription activators as well as general transcription factors (GTFs), thus mediating recruitment of basal transcription machinery to the promoter. Most promoters comprise multiple activator-binding sites, and many activators contain tandem TADs, thus multivalent interactions may stabilize CBP/p300 at the promoter, and intrinsically disordered regions in CBP/p300 and many activators may confer adaptability to these multivalent complexes. CBP/p300 contains a catalytic histone acetyltransferase (HAT) domain, which remodels chromatin to 'relax' its superstructure and enables transcription of proximal genes. The HAT activity of CBP/p300 also acetylates some transcription factors (e.g., p53), hence modulating the function of key transcriptional regulators. Through these numerous interactions, CBP/p300 has been implicated in complex physiological and pathological processes, and, in response to different signals, can drive cells towards proliferation or apoptosis. Dysregulation of the transcriptional and epigenetic functions of CBP/p300 is associated with leukemia and other types of cancer, thus it has been recognized as a potential anti-cancer drug target. In this review, we focus on recent exciting findings in the structural mechanisms of CBP/p300 involving multivalent and dynamic interactions with binding partners, which may pave new avenues for anti-cancer drug development.

Profiling the dynamic interfaces of fluorinated transcription complexes for ligand discovery and characterization

The conformationally dynamic binding surfaces of transcription complexes present a particular challenge for ligand discovery and characterization. In the case of the KIX domain of the master coactivator CBP/p300, few small molecules have been reported that target its two allosterically regulated binding sites despite the important roles that KIX plays in processes ranging from memory formation to hematopoiesis. Taking advantage of the enrichment of aromatic amino acids at protein interfaces, here we show that the incorporation of six (19)F-labeled aromatic side chains within the KIX domain enables recapitulation of the differential binding footprints of three natural activator peptides (MLL, c-Myb, and pKID) in complex with KIX and effectively reports on allosteric changes upon binding using 1D NMR spectroscopy. Additionally, the examination of both the previously described KIX protein-protein interaction inhibitor Napthol-ASE-phosphate and newly discovered ligand 1-10 rapidly revealed both the binding sites and the affinities of these small molecules. Significantly, the utility of using fluorinated transcription factors for ligand discovery was demonstrated through a fragment screen leading to a new low molecular weight fragment ligand for CBP/p300, 1G7. Aromatic amino acids are enriched at protein-biomolecule interfaces; therefore, this quantitative and facile approach will be broadly useful for studying dynamic transcription complexes and screening campaigns complementing existing biophysical methods for studying these dynamic interfaces.

HTLV-1 HBZ protein deregulates interactions between cellular factors and the KIX domain of p300/CBP

The complex retrovirus human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia. Deregulation of cellular transcription is thought to be an important step for T-cell transformation caused by viral infection. HTLV-1 basic leucine zipper factor (HBZ) is one of the viral proteins believed to be involved in this process, as it deregulates the expression of numerous cellular genes. In the context of the provirus, HBZ represses HTLV-1 transcription, in part, by binding to the homologous cellular coactivators p300 and CBP. These coactivators play a central role in transcriptional regulation. In this study, we determined that HBZ binds with high affinity to the KIX domain of p300/CBP. This domain contains two binding surfaces that are differentially targeted by multiple cellular factors. We show that two φXXφφ motifs in the activation domain of HBZ mediate binding to a single surface of the KIX domain, the mixed-lineage leukemia (MLL) binding surface. Formation of this interaction inhibits binding of MLL to the KIX domain while enhancing the binding of the transcription factor c-Myb to the opposite surface of KIX. Consequently, HBZ inhibits transcriptional activation mediated by MLL and enhances activation mediated by c-Myb. CREB, which binds the same surface of KIX as c-Myb, also exhibited an increase in activity through HBZ. These results indicate that HBZ is able to alter gene expression by competing with transcription factors for the occupancy of one surface of KIX while enhancing the binding of factors to the other surface.

Human T-lymphotropic virus type 1 Tax protein complexes with P-TEFb and competes for Brd4 and 7SK snRNP/HEXIM1 binding

Positive transcription elongation factor b (P-TEFb) plays an important role in stimulating RNA polymerase II elongation for viral and cellular gene expression. P-TEFb is found in cells in either an active, low-molecular-weight (LMW) form or an inactive, high-molecular-weight (HMW) form. We report here that human T-lymphotropic virus type 1 (HTLV-1) Tax interacts with the cyclin T1 subunit of P-TEFb, forming a distinct Tax/P-TEFb LMW complex. We demonstrate that Tax can play a role in regulating the amount of HMW complex present in the cell by decreasing the binding of 7SK snRNP/HEXIM1 to P-TEFb. This is seen both in vitro using purified Tax protein and in vivo in cells transduced with Tax expression constructs. Further, we find that a peptide of cyclin T1 spanning the Tax binding domain inhibits the ability of Tax to disrupt HMW P-TEFb complexes. These results suggest that the direct interaction of Tax with cyclin T1 can dissociate P-TEFb from the P-TEFb/7SK snRNP/HEXIM1 complex for activation of the viral long terminal repeat (LTR). We also show that Tax competes with Brd4 for P-TEFb binding. Chromatin immunoprecipitation (ChIP) assays demonstrated that Brd4 and P-TEFb are associated with the basal HTLV-1 LTR, while Tax and P-TEFb are associated with the activated template. Furthermore, the knockdown of Brd4 by small interfering RNA (siRNA) activates the HTLV-1 LTR promoter, which results in an increase in viral expression and production. Our studies have identified Tax as a regulator of P-TEFb that is capable of affecting the balance between its association with the large inactive complex and the small active complex.

The HTLV-1 tax protein cooperates with phosphorylated CREB, TORC2 and p300 to activate CRE-dependent cyclin D1 transcription

Adult T-cell leukemia/lymphoma is a fatal malignancy etiologically linked to infection with the human T-cell leukemia virus (HTLV-1). The virally-encoded oncoprotein Tax activates transcription of HTLV-1 and cellular genes by cooperating with cellular transcription factors. Cyclin D1 is a pivotal regulator of cell cycle progression, and increased expression strongly correlates with malignant transformation. Here, we characterize the mechanism of Tax transactivation of cyclin D1. We find that cyclin D1 transcript levels are elevated in HTLV-1 infected cells and that Tax physically associates with the cyclin D1 gene in vivo. Tax binds the cyclin D1 promoter-proximal cyclic AMP response element (CRE) in the presence of phosphorylated CREB (pCREB) in vitro, and together the Tax/pCREB complex recruits the cellular coactivator p300 to the promoter via this unconventional Tax-responsive element. We further show that Transducer of Regulated CREB 2 (TORC2) cooperates with Tax to further enhance p300 recruitment to the cyclin D1 promoter in vitro, consistent with enhanced cyclin D1 expression in the presence of Tax and TORC2. Together, our findings support a model in which Tax-induced accumulation of cyclin D1 shortens the G1 phase of the cell cycle, promotes mitotic replication of the virus, and drives selection and expansion of malignant T-cells.

Mapping the interactions of the p53 transactivation domain with the KIX domain of CBP

Molecular interactions between the tumor suppressor p53 and the transcriptional coactivators CBP/p300 are critical for the regulation of p53 transactivation and stability. The transactivation domain (TAD) of p53 binds directly to several CBP/p300 domains (TAZ1, TAZ2, NCBD, and KIX). Here we map the interaction between the p53 TAD and the CBP KIX domain using isothermal titration calorimetry and NMR spectroscopy. KIX is a structural domain in CBP/p300 that can simultaneously bind two polypeptide ligands, such as the activation domain of MLL and the kinase-inducible activation domain (pKID) of CREB, using distinct interaction surfaces. The p53 TAD consists of two subdomains (AD1 and AD2); peptides corresponding to the isolated AD1 and AD2 subdomains interact with KIX with relatively low affinity, but a longer peptide containing both subdomains binds KIX tightly. In the context of the full-length p53 TAD, AD1 and AD2 bind synergistically to KIX. Mapping of the chemical shift perturbations onto the structure of KIX shows that isolated AD1 and AD2 peptides bind to both the MLL and pKID sites. Spin-labeling experiments show that the complex of the full-length p53 TAD with KIX is disordered, with the AD1 and AD2 subdomains each interacting with both the MLL and pKID binding surfaces. Phosphorylation of the p53 TAD at Thr18 or Ser20 increases the KIX binding affinity. The affinity is further enhanced by simultaneous phosphorylation of Thr18 and Ser20, and the specificity of the interaction is increased. The p53 TAD simultaneously occupies the two distinct sites that have been identified on the CBP KIX domain and efficiently competes for these sites with other known KIX-binding transcription factors.

Amphipathic small molecules mimic the binding mode and function of endogenous transcription factors

Small molecules that reconstitute the binding mode(s) of a protein and in doing so elicit a programmed functional response offer considerable advantages in the control of complex biological processes. The development challenges of such molecules are significant, however. Many protein-protein interactions require multiple points of contact over relatively large surface areas. More significantly, several binding modes can be superimposed upon a single sequence within a protein, and a true small molecule replacement must be preprogrammed for such multimodal binding. This is the case for the transcriptional activation domain or TAD of transcriptional activators as these motifs utilize a poorly characterized multipartner binding profile in order to stimulate gene expression. Here we describe a unique class of small molecules that exhibit both function and a binding profile analogous to natural transcriptional activation domains. Of particular note, the small molecules are the first reported to bind to the KIX domain within the CREB binding protein (CBP) at a site that is utilized by natural activators. Further, a comparison of functional and nonfunctional small molecules indicates that an interaction with CBP is a key contributor to transcriptional activity. Taken together, the evidence suggests that the small molecule TADs mimic both the function and mechanism of their natural counterparts and thus present a framework for the broader development of small molecule transcriptional switches.

Dimerization and a novel Tax speckled structure localization signal are required for Tax nuclear localization

The human T-cell leukemia virus type 1 oncoprotein Tax has pleiotropic activities, a subset of which likely leads to immortalization of T cells. Tax is expressed and known to function in both the cell nucleus and the cytoplasm. Tax has defined nuclear localization (NLS) and nuclear export signals that enable shuttling between the two compartments. In this study, we identified a novel region in Tax that targets the protein to discrete nuclear foci that we have previously termed Tax speckled structures (TSS). We demonstrated that the identified region is both necessary and sufficient for directing proteins to TSS. This novel TSS localization signal (TSLS), spanning amino acids 50 to 75, is separable from and adjacent to the NLS of Tax. Coexpression of a Tax NLS mutant and a Tax TSLS mutant rescued the nuclear entry and subnuclear TSS targeting of both proteins, demonstrating that these signals are independent domains. Our analysis also revealed that Tax proteins deficient for dimerization fail to localize to the nucleus. Consequently, when we restored dimerization via induction of a heterologous "dimerizer" domain, nuclear localization was rescued. Thus, we defined additional domains in Tax specific for nuclear localization and subnuclear targeting. Our results reveal a more complex network for regulation of Tax subcellular localization and subsequent function.

The proto-oncogene Bcl3, induced by Tax, represses Tax-mediated transcription via p300 displacement from the human T-cell leukemia virus type 1 promoter

The etiology of human T-cell leukemia virus type 1 (HTLV-1)-induced adult T-cell leukemia is linked to the expression of the viral oncoprotein Tax. Although the mechanism of retroviral transformation is unknown, Tax interferes with fundamental cellular processes, including proliferation and apoptosis, and these events may directly link Tax to early steps in malignant progression. In this study, we examined the interplay between Tax and the potent proto-oncogene B-cell chronic leukemia protein 3 (Bcl3). Bcl3 is a critical regulator of cell survival and proliferation and is overexpressed in HTLV-1-infected cells. We found that Tax induced Bcl3 expression through stimulation of the NF-kappaB pathway. An intronic NF-kappaB binding site within the Bcl3 gene served as the primary target of Tax-induced NF-kappaB activation. We next considered the consequence of Bcl3 overexpression on Tax function. Interestingly, we found that Bcl3 formed a stable complex with Tax and that this complex potently inhibited Tax-dependent HTLV-1 transcription. Importantly, Bcl3 associated with the HTLV-1 promoter in a Tax-dependent manner and inhibited the binding of the critical cellular coactivator p300. The conserved ankyrin repeat domain of Bcl3 mediated both Tax binding and inhibition of p300 recruitment to the HTLV-1 promoter. Together, these data suggest that Tax-induced Bcl3 overexpression benefits the virus in two important ways. First, Bcl3 may promote cell division and thus clonal proliferation of the virus. Second, Bcl3 may attenuate virion production, facilitating immune evasion. One consequence of this regulatory loop may be Bcl3-induced malignant transformation of the host cell.

The HTLV-1 Tax interactome

The Tax1 oncoprotein encoded by Human T-lymphotropic virus type I is a major determinant of viral persistence and pathogenesis. Tax1 affects a wide variety of cellular signalling pathways leading to transcriptional activation, proliferation and ultimately transformation. To carry out these functions, Tax1 interacts with and modulates activity of a number of cellular proteins. In this review, we summarize the present knowledge of the Tax1 interactome and propose a rationale for the broad range of cellular proteins identified so far.

The human T-cell leukemia virus type 1 tax protein confers CBP/p300 recruitment and transcriptional activation properties to phosphorylated CREB

The human T-cell leukemia virus-encoded oncoprotein Tax is a potent activator of viral transcription. Tax function is strictly dependent upon the cellular transcription factor CREB, and together they bind cAMP response elements within the viral promoter and mediate high-level viral transcription. Signal-dependent CREB phosphorylation at Ser(133) (pCREB) correlates with the activation of transcription. This activation has been attributed to recruitment of the coactivators CBP/p300 via physical interaction with the KIX domain. Here we show that the promoter-bound Tax/pCREB complex strongly recruits the recombinant, purified full-length coactivators CBP and p300. Additionally, the promoter-bound Tax/pCREB (but not Tax/CREB) complex recruits native p300 and potently activates transcription from chromatin templates. Unexpectedly, pCREB alone failed to detectably recruit the full-length coactivators, despite strong binding to KIX. These observations are in marked contrast to those in published studies that have characterized the physical interaction between KIX and pCREB and extrapolated these results to the full-length proteins. Consistent with our observation that pCREB is deficient for binding of CBP/p300, pCREB alone failed to support transcriptional activation. These data reveal that phosphorylation of CREB is not sufficient for CBP/p300 recruitment and transcriptional activation. The regulation of transcription by pCREB is therefore more complex than is generally recognized, and coregulators, such as Tax, likely play a critical role in the modulation of pCREB function.