KIX-Mediated Assembly of the CBP−CREB−HTLV-1 Tax Coactivator−Activator Complex†

Polypharmacology-based kinome screen identifies new regulators of KSHV reactivation

Kaposi's sarcoma-associated herpesvirus (KSHV) causes several human diseases including Kaposi's sarcoma (KS), a leading cause of cancer in Africa and in patients with AIDS. KS tumor cells harbor KSHV predominantly in a latent form, while typically <5% contain lytic replicating virus. Because both latent and lytic stages likely contribute to cancer initiation and progression, continued dissection of host regulators of this biological switch will provide insights into fundamental pathways controlling the KSHV life cycle and related disease pathogenesis. Several cellular protein kinases have been reported to promote or restrict KSHV reactivation, but our knowledge of these signaling mediators and pathways is incomplete. We employed a polypharmacology-based kinome screen to identify specific kinases that regulate KSHV reactivation. Those identified by the screen and validated by knockdown experiments included several kinases that enhance lytic reactivation: ERBB2 (HER2 or neu), ERBB3 (HER3), ERBB4 (HER4), MKNK2 (MNK2), ITK, TEC, and DSTYK (RIPK5). Conversely, ERBB1 (EGFR1 or HER1), MKNK1 (MNK1) and FRK (PTK5) were found to promote the maintenance of latency. Mechanistic characterization of ERBB2 pro-lytic functions revealed a signaling connection between ERBB2 and the activation of CREB1, a transcription factor that drives KSHV lytic gene expression. These studies provided a proof-of-principle application of a polypharmacology-based kinome screen for the study of KSHV reactivation and enabled the discovery of both kinase inhibitors and specific kinases that regulate the KSHV latent-to-lytic replication switch.

Rational peptide design for inhibition of the KIX-MLL interaction

The kinase-inducible domain interacting (KIX) domain is an integral part of the general transcriptional coactivator CREB-binding protein, and has been associated with leukemia, cancer, and various viral diseases. Hence, the KIX domain has attracted considerable attention in drug discovery and development. Here, we rationally designed a KIX inhibitor using a peptide fragment corresponding to the transactivation domain (TAD) of the transcriptional activator, mixed-lineage leukemia protein (MLL). We performed theoretical saturation mutagenesis using the Rosetta software to search for mutants expected to bind KIX more tightly than the wild-type MLL TAD. Mutant peptides with higher helical propensities were selected for experimental characterization. We found that the T2857W mutant of the MLL TAD peptide had the highest binding affinity for KIX compared to the other 12 peptides designed in this study. Moreover, the peptide had a high inhibitory effect on the KIX-MLL interaction with a half-maximal inhibitory concentration close to the dissociation constant for this interaction. To our knowledge, this peptide has the highest affinity for KIX among all previously reported inhibitors that target the MLL site of KIX. Thus, our approach may be useful for rationally developing helical peptides that inhibit protein-protein interactions implicated in the progression of various diseases.

Allostery in the dynamic coactivator domain KIX occurs through minor conformational micro-states

The coactivator KIX of CBP uses two binding surfaces to recognize multiple activators and exhibits allostery in ternary complex formation. Activator•coactivator interactions are central to transcriptional regulation, yet the microscopic origins of allostery in dynamic proteins like KIX are largely unknown. Here, we investigate the molecular recognition and allosteric manifestations involved in two KIX ternary systems c-Myb•KIX•MLL and pKID•KIX•MLL. Exploring the hypothesis that binary complex formation prepays an entropic cost for positive cooperativity, we utilize molecular dynamics simulations, side chain methyl order parameters, and differential scanning fluorimetry (DSF) to explore conformational entropy changes in KIX. The protein's configurational micro-states from structural clustering highlight the utility of protein plasticity in molecular recognition and allostery. We find that apo KIX occupies a wide distribution of lowly-populated configurational states. Each binding partner has its own suite of KIX states that it selects, building a model of molecular recognition fingerprints. Allostery is maximized with MLL pre-binding, which corresponds to the observation of a significant reduction in KIX micro-states observed when MLL binds. With all binding partners, the changes in KIX conformational entropy arise predominantly from changes in the most flexible loop. Likewise, we find that a small molecule and mutations allosterically inhibit/enhance activator binding by tuning loop dynamics, suggesting that loop-targeting chemical probes could be developed to alter KIX•activator interactions. Experimentally capturing KIX stabilization is challenging, particularly because of the disordered nature of particular activators. However, DSF melting curves allow for inference of relative entropic changes that occur across complexes, which we compare to our computed entropy changes using simulation methyl order parameters.

Deciphering the promiscuous interactions between intrinsically disordered transactivation domains and the KIX domain

The kinase-inducible domain interacting (KIX) domain of the transcriptional coactivator CBP protein carries 2 isolated binding sites (designated as the c-Myb site and the MLL site) and is capable of binding numerous intrinsically disordered transactivation domains (TADs), including c-Myb and pKID via the c-Myb site, and MLL, E2A and c-Jun via the MLL site. In this study we compared the kinetics for binding of various disordered TADs to the KIX domain via computational biophysical analyses. We found that the binding rates are heavily affected by long-range electrostatic interactions. The basal rate constants for forming the encounter complexes are similar for different KIX binding peptides, favorable electrostatic interactions between the MLL site and the peptides result in greater association rates when peptides bind to the MLL site. FOXO3a and p53 TAD each contains 2 copies of KIX binding motif and each motif interacts with both the MLL site and the c-Myb site. Our kinetics studies suggest that binding of FOXO3a or p53 TAD to the KIX domain is via a sequential mechanism, where one KIX binding motif binds to the MLL site first and then the other KIX binding motif binds to the c-Myb site. Considering the promiscuous interactions between FOXO3a and KIX, and p53 TAD and KIX, electrostatic steering simplifies the binding mechanism. This study highlights the importance of long-range electrostatic interactions in molecular recognition process involving multi-motif intrinsically disordered proteins and promiscuous interactions.

Experimental and molecular dynamics studies showed that CBP KIX mutation affects the stability of CBP:c-Myb complex

The coactivators CBP (CREBBP) and its paralog p300 (EP300), two conserved multi-domain proteins in eukaryotic organisms, regulate gene expression in part by binding DNA-binding transcription factors. It was previously reported that the CBP/p300 KIX domain mutant (Y650A, A654Q, and Y658A) altered both c-Myb-dependent gene activation and repression, and that mice with these three point mutations had reduced numbers of platelets, B cells, T cells, and red blood cells. Here, our transient transfection assays demonstrated that mouse embryonic fibroblast cells containing the same mutations in the KIX domain and without a wild-type allele of either CBP or p300, showed decreased c-Myb-mediated transcription. Dr. Wright's group solved a 3-D structure of the mouse CBP:c-Myb complex using NMR. To take advantage of the experimental structure and function data and improved theoretical calculation methods, we performed MD simulations of CBP KIX, CBP KIX with the mutations, and c-Myb, as well as binding energy analysis for both the wild-type and mutant complexes. The binding between CBP and c-Myb is mainly mediated by a shallow hydrophobic groove in the center where the side-chain of Leu302 of c-Myb plays an essential role and two salt bridges at the two ends. We found that the KIX mutations slightly decreased stability of the CBP:c-Myb complex as demonstrated by higher binding energy calculated using either MM/PBSA or MM/GBSA methods. More specifically, the KIX mutations affected the two salt bridges between CBP and c-Myb (CBP-R646 and c-Myb-E306; CBP-E665 and c-Myb-R294). Our studies also revealed differing dynamics of the hydrogen bonds between CBP-R646 and c-Myb-E306 and between CBP-E665 and c-Myb-R294 caused by the CBP KIX mutations. In the wild-type CBP:c-Myb complex, both of the hydrogen bonds stayed relatively stable. In contrast, in the mutant CBP:c-Myb complex, hydrogen bonds between R646 and E306 showed an increasing trend followed by a decreasing trend, and hydrogen bonds of the E665:R294 pair exhibited a fast decreasing trend over time during MD simulations. In addition, our data showed that the KIX mutations attenuate CBP's hydrophobic interaction with Leu302 of c-Myb. Furthermore, our 500-ns MD simulations showed that CBP KIX with the mutations has a slightly lower potential energy than wild-type CBP. The CBP KIX structures with or without its interacting protein c-Myb are different for both wild-type and mutant CBP KIX, and this is likewise the case for c-Myb with or without CBP, suggesting that the presence of an interacting protein influences the structure of a protein. Taken together, these analyses will improve our understanding of the exact functions of CBP and its interaction with c-Myb.

Role of Intrinsic Protein Disorder in the Function and Interactions of the Transcriptional Coactivators CREB-binding Protein (CBP) and p300

The transcriptional coactivators CREB-binding protein (CBP) and p300 undergo a particularly rich set of interactions with disordered and partly ordered partners, as a part of their ubiquitous role in facilitating transcription of genes. CBP and p300 contain a number of small structured domains that provide scaffolds for the interaction of disordered transactivation domains from a wide variety of partners, including p53, hypoxia-inducible factor 1α (HIF-1α), NF-κB, and STAT proteins, and are the targets for the interactions of disordered viral proteins that compete with cellular factors to disrupt signaling and subvert the cell cycle. The functional diversity of the CBP/p300 interactome provides an excellent example of the power of intrinsic disorder to facilitate the complexity of living systems.

Screening of the Human Kinome Identifies MSK1/2-CREB1 as an Essential Pathway Mediating Kaposi's Sarcoma-Associated Herpesvirus Lytic Replication during Primary Infection

Viruses often hijack cellular pathways to facilitate infection and replication. Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic gammaherpesvirus etiologically associated with Kaposi's sarcoma, a vascular tumor of endothelial cells. Despite intensive studies, cellular pathways mediating KSHV infection and replication are still not well defined. Using an antibody array approach, we examined cellular proteins phosphorylated during primary KSHV infection of primary human umbilical vein endothelial cells. Enrichment analysis identified integrin/mitogen-activated protein kinase (integrin/MAPK), insulin/epidermal growth factor receptor (insulin/EGFR), and JAK/STAT as the activated networks during primary KSHV infection. The transcriptional factor CREB1 (cyclic AMP [cAMP]-responsive element-binding protein 1) had the strongest increase in phosphorylation. While knockdown of CREB1 had no effect on KSHV entry and trafficking, it drastically reduced the expression of lytic transcripts and proteins and the production of infectious virions. Chemical activation of CREB1 significantly enhanced viral lytic replication. In contrast, CREB1 neither influenced the expression of the latent gene LANA nor affected KSHV infectivity. Mechanistically, CREB1 was not activated through the classic cAMP/protein kinase A (cAMP/PKA) pathway or via the AKT, MK2, and RSK pathways. Rather, CREB1 was activated by the mitogen- and stress-activated protein kinases 1 and 2 (MSK1/2). Consequently, chemical inhibition or knockdown of MSKs significantly inhibited the KSHV lytic replication program; however, it had a minimal effect on LANA expression and KSHV infectivity. Together, these results identify the MSK1/2-CREB1 proteins as novel essential effectors of KSHV lytic replication during primary infection. The differential effect of the MSK1/2-CREB1 pathway on the expression of viral latent and lytic genes might control the robustness of viral lytic replication, and therefore the KSHV replication program, during primary infection.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human tumor virus associated with several cancers. Through genome-wide kinase screening, we found that KSHV activates the MSK1/2-CREB1 pathway during primary infection and that it depends on this pathway for viral lytic replication. Inhibition of this pathway blocks KSHV lytic replication. These results illustrate a mechanism by which KSHV hijacks a cellular pathway for its replication, and they identify a potential therapeutic target.

Prepaying the entropic cost for allosteric regulation in KIX

The kinase-inducible domain interacting (KIX) domain of the CREB binding protein (CBP) is capable of simultaneously binding two intrinsically disordered transcription factors, such as the mixed-lineage leukemia (MLL) and c-Myb peptides, at isolated interaction sites. In vitro, the affinity for binding c-Myb is approximately doubled when KIX is in complex with MLL, which suggests a positive cooperative binding mechanism, and the affinity for MLL is also slightly increased when KIX is first bound by c-Myb. Expanding the scope of recent NMR and computational studies, we explore the allosteric mechanism at a detailed molecular level that directly connects the microscopic structural dynamics to the macroscopic shift in binding affinities. To this end, we have performed molecular dynamics simulations of free KIX, KIX-c-Myb, MLL-KIX, and MLL-KIX-c-Myb using a topology-based Gō-like model. Our results capture an increase in affinity for the peptide in the allosteric site when KIX is prebound by a complementary effector and both peptides follow an effector-independent folding-and-binding mechanism. More importantly, we discover that MLL binding lowers the entropic cost for c-Myb binding, and vice versa, by stabilizing the L12-G2 loop and the C-terminal region of the α3 helix on KIX. This work demonstrates the importance of entropy in allosteric signaling between promiscuous molecular recognition sites and can inform the rational design of small molecule stabilizers to target important regions of conformationally dynamic proteins.

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.

FP Tethering: a screening technique to rapidly identify compounds that disrupt protein-protein interactions

Tethering is a screening technique for discovering small-molecule fragments that bind to pre-determined sites via formation of a disulphide bond. Tethering screens traditionally rely upon mass spectrometry to detect disulphide bind formation, which requires a time-consuming liquid chromatography step. Here we show that Tethering can be performed rapidly and inexpensively using a homogenous fluorescence polarization (FP) assay that detects displacement of a peptide ligand from the protein target as an indirect readout of disulphide formation. We apply this method, termed FP Tethering, to identify fragments that disrupt the protein-protein interaction between the KIX domain of the transcriptional coactivator CBP and the transcriptional activator peptide pKID.

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.

Ordering a dynamic protein via a small-molecule stabilizer

Like many coactivators, the GACKIX domain of the master coactivator CBP/p300 recognizes transcriptional activators of diverse sequence composition via dynamic binding surfaces. The conformational dynamics of GACKIX that underlie its function also render it especially challenging for structural characterization. We have found that the ligand discovery strategy of Tethering is an effective method for identifying small-molecule fragments that stabilize the GACKIX domain, enabling for the first time the crystallographic characterization of this important motif. The 2.0 Å resolution structure of GACKIX complexed to a small molecule was further analyzed by molecular dynamics simulations, which revealed the importance of specific side-chain motions that remodel the activator binding site in order to accommodate binding partners of distinct sequence and size. More broadly, these results suggest that Tethering can be a powerful strategy for identifying small-molecule stabilizers of conformationally malleable proteins, thus facilitating their structural characterization and accelerating the discovery of small-molecule modulators.

Structural basis of CBP/p300 recruitment in leukemia induction by E2A-PBX1

E-proteins are critical transcription factors in B-cell lymphopoiesis. E2A, 1 of 3 E-protein-encoding genes, is implicated in the induction of acute lymphoblastic leukemia through its involvement in the chromosomal translocation 1;19 and consequent expression of the E2A-PBX1 oncoprotein. An interaction involving a region within the N-terminal transcriptional activation domain of E2A-PBX1, termed the PCET motif, which has previously been implicated in E-protein silencing, and the KIX domain of the transcriptional coactivator CBP/p300, critical for leukemogenesis. However, the structural details of this interaction remain unknown. Here we report the structure of a 1:1 complex between PCET motif peptide and the KIX domain. Residues throughout the helical PCET motif that contact the KIX domain are important for both binding KIX and bone marrow immortalization by E2A-PBX1. These results provide molecular insights into E-protein-driven differentiation of B-cells and the mechanism of E-protein silencing, and reveal the PCET/KIX interaction as a therapeutic target for E2A-PBX1-induced leukemia.

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.

The triplet puzzle of homologies in receptor heteromers exists also in other types of protein-protein interactions

Based on our theory, we point out main triplets of amino acid residues in the GABAB1 receptor, found in the central and peripheral nervous system, which seem to be critical for both receptor heterodimerization and chemokine binding. The obtained results suggest that these triplets may "guide-and-clasp" protein-protein interactions playing a role, e.g., in neuroinflammation disorders.

Transcriptional tools: Small molecules for modulating CBP KIX-dependent transcriptional activators

Previously it was demonstrated that amphipathic isoxazolidines are able to functionally replace the transcriptional activation domains of endogenous transcriptional activators. In addition, in vitro binding studies suggested that a key binding partner of these molecules is the CREB Binding Protein (CBP), more specifically the KIX domain within this protein. Here we show that CBP plays an essential role in the ability of isoxazolidine transcriptional activation domains to activate transcription in cells. Consistent with this model, isoxazolidines are able to function as competitive inhibitors of the activators MLL and Jun, both of which utilize a binding interaction with KIX to up-regulate transcription. Further, modification of the N2 side chain produced three analogs with enhanced potency against Jun-mediated transcription, although increased cytotoxicity was also observed. Collectively these small KIX-binding molecules will be useful tools for dissecting the role of the KIX domain in a variety of pathological processes.

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.

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.

Human T-cell leukemia virus I tax protein sensitizes p53-mutant cells to DNA damage

Mutations in p53 are a common cause of resistance of cancers to standard chemotherapy and, thus, treatment failure. Reports have shown that Tax, a human T-cell leukemia virus type I encoded protein that has been associated with genomic instability and perturbation of transcription and cell cycle, sensitizes HeLa cells to UV treatment. The extent to which Tax can sensitize cells and the mechanism by which it exerts its effect are unknown. In this study, we show that Tax sensitizes p53-mutant cells to a broad range of DNA-damaging agents, including mitomycin C, a bifunctional alkylator, etoposide, a topoisomerase II drug, and UV light, but not ionizing radiation, a double-strand break agent, or vinblastine, a tubulin poison. Tax caused hypersensitivity in all p53-deleted cell lines and several, but not all, mutant-expressed p53-containing cell lines, while unexpectedly being protective in p53 wild-type (wt) cells. The effect observed in p53-deleted lines could be reversed for this by transfection of wt p53. We also show that Tax activates a p53-independent proapoptotic program through decreased expression of the retinoblastoma protein and subsequent increased E2F1 expression. The expression of several proapoptotic proteins was also induced by Tax, including Puma and Noxa, culminating in a substantial increase in Bax dimerization. Our results show that Tax can sensitize p53-mutant cells to DNA damage while protecting p53 wt cells, a side benefit that might result in reduced toxicity in normal cells. Such studies hold the promise of a novel adjunctive therapy that could make cancer chemotherapy more effective.

Assessing the permissiveness of transcriptional activator binding sites

Both genetic and biochemical data suggest that transcriptional activators with little sequence homology nevertheless function through interaction with a shared group of coactivators. Here we show that a series of peptidomimetic transcriptional activation domains interact under cell-fiee and cellular conditions with the metazoan coactivator CBP despite differences in the positioning and identity of the constituent functional groups. Taken together, these results suggest that a key activator binding site within CBP is permissive, accepting multiple arrangements of hydrophobic functional groups. Further, this permissiveness is also observed with a coactivator from S. cerevisiae. Thus, the design of small molecule mimics of transcriptional activation domains with broad function may be more straightforward than previously envisioned.

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

The viral oncoprotein Tax mediates transcriptional activation of human T-cell leukemia virus type 1 (HTLV-1). Both Tax and the cellular transcription factor CREB bind to viral cyclic AMP response elements (vCREs) located in the viral promoter. Tax and serine 133 phosphorylated CREB (pCREB) bound to the HTLV-1 promoter facilitate viral transcription via the recruitment of the large cellular coactivators CBP/p300. While the interaction between the phosphorylated kinase inducible domain (pKID) of pCREB and the KIX domain of CBP/p300 has been well characterized, the molecular interactions between KIX, full-length Tax, and pCREB have not been examined. Here we biochemically characterized the interaction between Tax and KIX in a physiologically relevant complex containing pCREB and vCRE DNA. Our data show that Tax and pCREB simultaneously and independently bind two distinct surfaces on the KIX domain: Tax binds KIX at the previously characterized mixed-lineage leukemia (MLL) protein interaction surface while pCREB binds KIX at the pKID-KIX interface. These results provide evidence for a model in which Tax and pCREB bind distinct surfaces of KIX for effective CBP/p300 recruitment to the HTLV-1 promoter. We also show that MLL competes with Tax for KIX binding, suggesting a novel mechanism of Tax oncogenesis in which normal MLL function is disrupted by Tax.

Molecular characterization of the Tax-containing HTLV-1 enhancer complex reveals a prominent role for CREB phosphorylation in Tax transactivation

Transcriptional activation of human T-cell leukemia virus type 1 (HTLV-1) is mediated by the viral oncoprotein Tax, which utilizes cellular transcriptional machinery to perform this function. The viral promoter carries three cyclic AMP-response elements (CREs), which are recognized by the cellular transcription factor cAMP-response element-binding protein (CREB). Tax binds to GC-rich sequences that immediately flank the CREs. The coactivator CREB-binding protein (CBP)/p300 binds to this promoter-bound ternary complex, which promotes the initiation of HTLV-1 transcription. Protein kinase A phosphorylation of CREB at serine 133 facilitates transcription from cellular CREs by recruiting CBP/p300 via its KIX domain. However, it remains controversial whether CREB phosphorylation plays a role in Tax transactivation. In this study, we biochemically characterized the quaternary complex formed by Tax, CREB, KIX, and the viral CRE by examining the individual molecular interactions that contribute to Tax stabilization in the complex. Our data show KIX, Ser(133)-phosphorylated CREB, and vCRE DNA are all required for stable Tax incorporation into the complex in vitro. Consonant with a fundamental role for CREB phosphorylation in Tax recruitment to the complex, we found that CREB is highly phosphorylated in a panel of HTLV-1-infected human T-cell lines. Significantly, we show that Tax is directly responsible for promoting elevated levels of CREB phosphorylation. Together, these data support a model in which Tax promotes CREB phosphorylation in vivo to ensure availability for Tax transactivation. Because pCREB has been implicated in leukemogenesis, enhancement of CREB phosphorylation by the virus may play a role in the etiology of adult T-cell leukemia.

Functional inactivation of p53 by human T-cell leukemia virus type 1 Tax protein: mechanisms and clinical implications

Human T-cell leukemia virus type 1 (HTLV-I) has been implicated with the etiology of adult T-cell leukemia (ATL) and certain other clinical disorders. Although the leukemogenic mechanism of HTLV-1 is not fully understood yet, the viral Tax protein is widely regarded as a key factor in this mechanism. Tax can modulate the synthesis or function of many regulatory factors which control a wide range of normal and oncogenic cellular processes and therefore, it acts as a potent oncoprotein. In the last few years, special attention has been attracted to Tax interference with the transactivation function of p53, a tumor-suppressor protein that is involved in regulation of the cell-cycle and apoptosis and in maintaining the cellular genome integrity. p53 is mutated in approximately 60% of all human tumors. In contrast, mutant p53 is found in only small percentage of ATL patients. Nevertheless, p53 is inactive in the leukemic cells of most ATL patients and in most HTLV-1 transformed cells. By inactivating p53, Tax can immortalize the HTLV-1-infected cells and destabilize their genome. Consequently, such cells can progress toward the ultimate leukemic state by a stepwise accumulation of oncogenic mutations and other types of chromosomal aberrations. Furthermore, since p53 exists in most ATL patients in its wild-type form, its reactivation by therapeutic drugs might be an effective approach for ATL therapy. Several mechanisms have been proposed so far for Tax-induced p53 inactivation. Understanding the exact mechanism of this Tax effect is essential for designing effective means for this therapeutic approach. In this review article, we discuss the various mechanisms proposed for Tax interference with p53 functions and their clinical and therapeutic implications.

The human T-cell leukemia virus type 1 (HTLV-1): New insights into the clinical aspects and molecular pathogenesis of adult t-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-associated myelopathy (TSP/HAM)

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus to be identified in the early 1980s. The isolation and identification of a related virus, HTLV-2, and the distantly related human immunodeficiency virus (HIV) immediately followed. Of the three retroviruses, two are associated definitively with specific diseases, HIV, with acquired immune deficiency syndrome (AIDS) and HTLV-1, with adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). While an estimated 10-20 million people worldwide are infected with HTLV-I, infection is endemic in the Caribbean, parts of Africa, southwestern Japan, and Italy. Approximately 4% of HTLV-I infected individuals develop ATLL, a disease with a poor prognosis. The clinical manifestations of infection and the current biology of HTLV viruses with emphasis on HTLV-1 are discussed in detail. The implications for improvements in diagnosis, treatment, intervention, and vaccination are included, as well as a discussion of the emergence of HTLV-1 and -2 as copathogens among HIV-1-infected individuals.

Structural basis for cooperative transcription factor binding to the CBP coactivator

Regulation of transcription requires interactions between transcriptional activators and transcriptional co-activator CREB binding protein (CBP). The KIX domain of CBP can bind simultaneously to two different proteins, providing an additional mechanism for transcriptional regulation. Here we describe the solution structure of the ternary complex formed by cooperative binding of activation domains from the c-Myb and mixed lineage leukemia (MLL) transcription factors to the KIX domain. The MLL and c-Myb domains form helices that bind to two distinct hydrophobic grooves on opposite faces of KIX. Compared to the binary KIX:c-Myb complex, significant changes are observed in the structure of KIX at the MLL binding interface in the ternary complex. Two regions of KIX that are disordered in the binary complex become structured in the ternary complex: a flexible loop forms intimate contacts with bound MLL, and the C-terminal helix is extended and stabilized by MLL binding. This structural change results in the formation of additional electrostatic/polar interactions between KIX and the bound c-Myb, providing a structural basis for the cooperativity observed for the ternary complex.

Intrinsically unstructured proteins and their functions

Many gene sequences in eukaryotic genomes encode entire proteins or large segments of proteins that lack a well-structured three-dimensional fold. Disordered regions can be highly conserved between species in both composition and sequence and, contrary to the traditional view that protein function equates with a stable three-dimensional structure, disordered regions are often functional, in ways that we are only beginning to discover. Many disordered segments fold on binding to their biological targets (coupled folding and binding), whereas others constitute flexible linkers that have a role in the assembly of macromolecular arrays.

Relevance of CREB phosphorylation in the anti-apoptotic function of human T-lymphotropic virus type 1 tax protein in serum-deprived murine fibroblasts

The human T-cell leukemia virus type 1 (HTLV-1) Tax transactivator is thought to play a primary role in the development of HTLV-1-mediated diseases. Using a murine fibroblast model, we previously showed that Tax reduces apoptosis induced by serum starvation by preventing cytochrome c release from the mitochondria. As Tax can enhance the transcriptional activity of nuclear factor NF-kB and cAMP-responsive element binding protein/activating transcription factor-1 (CREB/ATF-1), we investigated the relevance of these routes in the anti-apoptotic effects of Tax. Results showed that a Tax mutant retaining CREB/ATF-1 transactivating activity protects murine fibroblasts from serum-depletion-induced apoptosis, while two CREB/ATF-1-defective mutants did not. Treatment with forskolin, an activator of CREB, significantly attenuated cytochrome c release and Bax translocation in response of serum deprivation. In analogy to forskolin treatment, Tax expression results in sustained phosphorylation of CREB at Ser(133) during serum starvation. Considered together, these results underscore a primary role of CREB transcriptional activation in preventing apoptosis triggered by growth factor withdrawal, and suggest that Tax might in part function by affecting the phosphorylation state of CREB.

Role of Tax protein in human T-cell leukemia virus type-I leukemogenicity

HTLV-1 is the etiological agent of adult T-cell leukemia (ATL), the neurological syndrome TSP/HAM and certain other clinical disorders. The viral Tax protein is considered to play a central role in the process leading to ATL. Tax modulates the expression of many viral and cellular genes through the CREB/ATF-, SRF- and NF-κB-associated pathways. In addition, Tax employs the CBP/p300 and p/CAF co-activators for implementing the full transcriptional activation competence of each of these pathways. Tax also affects the function of various other regulatory proteins by direct protein-protein interaction. Through these activities Tax sets the infected T-cells into continuous uncontrolled replication and destabilizes their genome by interfering with the function of telomerase and topoisomerase-I and by inhibiting DNA repair. Furthermore, Tax prevents cell cycle arrest and apoptosis that would otherwise be induced by the unrepaired DNA damage and enables, thereby, accumulation of mutations that can contribute to the leukemogenic process. Together, these capacities render Tax highly oncogenic as reflected by its ability to transform rodent fibroblasts and primary human T-cells and to induce tumors in transgenic mice. In this article we discuss these effects of Tax and their apparent contribution to the HTLV-1 associated leukemogenic process. Notably, however, shortly after infection the virus enters into a latent state, in which viral gene expression is low in most of the HTLV-1 carriers' infected T-cells and so is the level of Tax protein, although rare infected cells may still display high viral RNA. This low Tax level is evidently insufficient for exerting its multiple oncogenic effects. Therefore, we propose that the latent virus must be activated, at least temporarily, in order to elevate Tax to its effective level and that during this transient activation state the infected cells may acquire some oncogenic mutations which can enable them to further progress towards ATL even if the activated virus is re-suppressed after a while. We conclude this review by outlining an hypothetical flow of events from the initial virus infection up to the ultimate ATL development and comment on the risk factors leading to ATL development in some people and to TSP/HAM in others.