The human T-cell leukemia virus type 1 (HTLV-1)-encoded Tax protein activates viral transcription through interaction with the cellular transcription factor CREB (cyclic AMP response element [CRE] binding protein).Although Tax stabilizes the binding of CREB to the Tax-responsive viral CREs in the HTLV-1 promoter, the precise molecular mechanism by which Tax mediates strong transcriptional activation through CREB remains unclear. In this report, we show that Tax Human T-cell leukemia virus type 1 (HTLV-1) is a complex retrovirus responsible for an aggressive and fatal malignancy called adult T-cell leukemia (for a review, see reference 16). The viral genome encodes a unique oncoprotein, called Tax, which is a key regulatory protein that appears to facilitate the transition from viral latency to high levels of virion production in the infected T cell. Tax mediates the emergence from latency via strong transcriptional activation of the HTLV-1 genome. The precise molecular mechanism by which Tax activates viral transcription has been widely studied but is not fully understood. Tax does not bind DNA directly (3,8,19,32) but interacts with host cell proteins to stimulate viral transcription through three 21-bp repeat sequences in the transcriptional control region of the virus (10,17,23,31,36,38,39). The three 21-bp repeats each contain an off-consensus core octanucleotide sequence with similarity to the cyclic AMP (cAMP) response element (CRE). A short run of GC-rich nucleotides immediately flanks the core CRE sequences within each of the 21-bp repeats. Together, the CRE and GC-rich flanks form a critical DNA element (called the viral CRE) that is obligatory for Tax transactivation in vivo (11,18,23,29,30). These viral CREs have been shown to serve as binding sites for several members of the basic leucine zipper (bZIP) family of cellular transcription factors. Specifically, the CRE binding protein (CREB) appears to have the most prominent role in mediating Tax transcriptional activation through the viral CREs in the HTLV-1 promoter (1,2,9,11,15,27,(45)(46)(47)(48)(49). Recent studies have shown that in the absence of Tax, the interaction between CREB and the viral CRE is highly unstable, resulting in rapid dissociation of CREB from the viral promoter. In the presence of Tax, the dissociation rate of CREB from the viral CRE is decreased and the equilibrium binding affinity is increased (7,11,44,45). Several lines of evidence indicate that Tax interacts primarily, although not exclusively, with the bZIP region of CREB to stabilize CREB binding to the weak viral CRE sequences (2,7,11,15,46). This binding stabilization by Tax appears to be accomplished through both an increase in CREB dimerization and stabilization of the helical structure of CREB's bZIP domain (7,37,42). Together, these studies support a model in which Tax transactivates HTLV-1 gene expression by increasing the number of CREB molecules bound to the viral promoter, leading to transcriptional activation of the virus and enhanced virion production. Unfort...

The complex human T-cell leukemia virus type 1 (HTLV-1) retrovirus encodes several proteins that are unique to the virus within its 3-end region. Among them, the viral transactivator Tax and posttranscriptional regulator Rex are well characterized, and both positively regulate HTLV-1 viral expression. Less is known about the other regulatory proteins encoded in this region of the provirus, including the recently discovered HBZ protein. HBZ has been shown to negatively regulate basal and Tax-dependent HTLV-1 transcription through its ability to interact with specific basic-leucine zipper (bZIP) proteins. In the present study, we found that HBZ reduces HTLV-1 transcription and virion production. We then characterized the interaction between HBZ and the cellular transcription factor CREB. CREB plays a critical role in Tax-mediated HTLV-1 transcription by forming a complex with Tax that binds to viral cyclic AMP-response elements (CREs) located within the viral promoter. We found that HBZ and CREB interact in vivo and directly in vitro, and this interaction occurs through the bZIP domain of each protein. We also found that CREM-Ia and ATF-1, which share significant homology in their bZIP domains with the bZIP domain of CREB, interact with HBZ-bZIP. The interaction between CREB and HBZ prevents CREB binding to the viral CRE elements in vitro and in vivo, suggesting that the reduction in HTLV-1 transcription by HBZ is partly due to the loss of CREB at the promoter. We also found that HBZ displaces CREB from a cellular CRE, suggesting that HBZ may deregulate CREB-dependent cellular gene expression.Human T-cell leukemia virus type 1 (HTLV-1) is a human retrovirus that is associated with two distinct diseases: adult T-cell leukemia (ATL), an abnormal proliferation of infected CD4 ϩ T lymphocytes, and HTLV-1-associated myelopathy and/or tropical spastic paraparesis, a neurodegenerative disorder (19,48,49). The molecular mechanisms leading to the development of both diseases are unclear, although the viral protein Tax is postulated to play an important role in these processes. Tax functions as a transcription factor and is essential for strong HTLV-1 transcription. Tax activates transcription through three 21-bp repeats that contain imperfect cyclic AMP responsive elements (called viral CREs) situated within the long terminal repeat of the HTLV-1 genome (1,6,14,15,22,27). Tax does not bind DNA alone but interacts with cellular transcription factors from the ATF/CREB family to form complexes that associate with the DNA. Within these complexes, Tax contacts the GC-rich sequences flanking the CRE core (31,37,38,41). The formation of Tax/CREB/DNA complexes is critical for the recruitment of the cellular coactivators CBP/p300 and subsequent high transcriptional activation of the virus (18,21,32,39,58).A number of cellular factors containing basic leucine zipper (bZIP) motifs have been shown to bind the viral CREs in HTLV-1-infected T cells. These factors included the ATF/ CREB family members (ATF-1, ATF-2, CREB, CREB-2, and CRE...

Tax protein activates transcription of the human T-cell leukaemia virus type I (HTLV-I) genome through three imperfect cyclic AMP-responsive element (CRE) target sites located within the viral promoter. Previous work has shown that Tax interacts with the bZIP element of proteins that bind the CRE target site to promote peptide dimerization, suggesting an association between Tax and bZIP coiled coil. Here we show that the site of interaction with Tax is not the coiled coil, but the basic segment. This interaction increases the stability of the GCN4 bZIP dimer by 1.7 kcal mol-1 and the DNA affinity of the dimer by 1.9 kcal mol-1. The differential effect of Tax on several bZip-DNA complexes that differ in peptide sequence or DNA conformation suggests a model for Tax action based on stabilization of a distinct DNA-bound protein structure. This model may explain how Tax interacts with transcription factors of considerable sequence diversity to alter patterns of gene expression.

The yeast nucleosome assembly protein 1 (yNAP1) participates in many diverse activities, such as the assembly of newly synthesized DNA into chromatin and the rearrangement of nucleosomes during transcriptional activation. yNAP1 does not require ATP hydrolysis to perform these functions and is a valuable tool for in vitro chromatin assembly. Using recombinant histone complexes, we show that yNAP1 has a preference for binding the (H3-H4) 2 tetramer over the (H2A-H2B) dimer. We find that the loss of the histone tails abrogates this preference for H3 and H4, and we demonstrate a direct interaction between yNAP1 and the amino-terminal tails of H3 and H4. yNAP1 binds to one histone fold domain, thus specifying the stoichiometry of the complexes formed with the histone dimer and tetramer. Finally, we provide evidence that the acidic carboxyl-terminal region of yNAP1, although dispensable for nucleosome assembly in vitro, contributes to binding via structure-independent electrostatic interactions. Our results are consistent with recent mechanistic investigations of NAP1 and expand our understanding of the histone chaperone family of assembly factors.

The human T-cell leukemia virus type I (HTLV-I) is the causative agent of an aggressive T-cell malignancy in humans. While the virus appears to maintain a state of latency in most infected cells, high level virion production is an essential step in the HTLV-I life cycle. The virally-encoded Tax protein, a potent activator of gene expression, is believed to control the switch from latency to replication. Tax stimulation of HTLV-I transcription is mediated through cellular activating transcription factor/cAMP response element binding proteins, which bind the three 21-base pair (bp) repeat viral enhancer elements. In this report, we show that viral latency may result from a highly unstable interaction between CREB and the HTLV-I 21-bp repeats, resulting in rapid dissociation of CREB from the viral promoter. In the presence Tax, the dissociation rate of CREB from a 21-bp repeat element is decreased. This stabilization is highly specific, requiring the amino-terminal region of CREB and appropriate 21-bp repeat sequences. We suggest that Tax stabilization of CREB binding to the viral promoter leads to an increase in gene expression, possibly providing the switch from latency to high level replication of the virus.

The virally encoded Tax protein is implicated in the various clinical manifestations associated with infection by human Tcell leukemia virus type 1 (HTLV-1), including an aggressive and fatal T-cell malignancy (for a review, see reference 18). The mechanism of lymphocyte transformation by Tax is not known, although it appears to be linked to the pleiotropic transcriptional deregulation properties associated with the Tax oncoprotein. Following T-cell infection, the retrovirus appears to establish and maintain a state of latency, with very low levels of viral gene expression (31, 34). The transition from latency to high-level viral RNA synthesis is initiated by an ill-defined signal which triggers Tax expression. Tax protein is transported into the nucleus, where it interacts with the three conserved 21-bp repeat elements located in the transcriptional control region of the virus, strongly activating viral gene expression (11,19,32,48,50,51).The interaction between Tax and the 21-bp repeats is facilitated by cellular transcription factors, which provide the primary recognition of these Tax-responsive elements (3,9,23,43,44). Centered within each 21-bp repeat is an off-consensus cyclic AMP (cAMP) response element, termed a viral CRE, which serves as the recognition element for members of the activating transcription factor/CRE binding protein (ATF/ CREB) family of cellular transcription factors. The CRE binding activity of these proteins is derived from their basic amino acid DNA binding domain and adjacent leucine zipper dimerization domain (bZIP domain). While several members of the ATF/CREB family bind to the CREs in the viral promoter, CREB appears to play the most prominent role in mediating Tax transactivation (1,2,4,9,12,17,24,36,56,(59)(60)(61)(62)(63).Although the precise molecular events which lead to Tax transactivation mediated through CREB are not well understood, several studies have established that Tax enters into a stable ternary complex with CREB and the viral CRE (12,26,46,54,61,63). In the absence of Tax, CREB forms an unstable complex with the viral CRE; however, in the presence of Tax, the dissociation rate of CREB is decreased (12). Studies with a variety of bZIP family members suggest that Tax interacts primarily, although not exclusively, with the bZIP region of CREB (1,7,12,17,49,56,60). This interaction may stabilize the ␣-helical structure of the parallel bZIP dimers, resulting in both enhanced DNA binding and dimerization of CREB (7,49,56). Together, these observations led to the suggestion that transactivation may occur as a consequence of Tax stabilization of CREB bound to the off-consensus viral CREs (12), leading to increased viral mRNA synthesis through the activation properties associated with CREB.This model for Tax transactivation was incomplete, because it did not address the role of CREB phosphorylation, which is necessary for CREB transcriptional activity (25). Recently, we and others (24, 36) have shown Tax directly interacts with the cellular coactivator CREB binding protei...

The Tax protein, encoded by the human

The human T-cell leukemia virus-encoded oncoprotein Tax is a potent deregulator of cellular gene expression. Here we report that Tax represses transcription of the human bax gene, a gene whose protein product accelerates apoptosis. This repression is mediated through a 27-bp sequence in the bax promoter that contains a putative basic helix-loop-helix binding site. Deletion of this sequence abolishes Tax-mediated repression of bax. Repression of the bax gene may be biologically significant, as we also show that HTLV-I-infected cell lines are resistant to a variety of physical, chemical, and biological stimuli which induce apoptosis in uninfected T-cells. The repression of genes involved in promoting apoptosis, including the bax gene, may contribute to retroviral survival, and initiate a pathway toward malignant transformation.