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...

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 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 Tax protein, encoded by the human

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...

No abstract

The human T-cell leukemia virus type I oncoprotein Tax transcriptionally deregulates a wide variety of viral and cellular genes. Tax deregulation of gene expression is mediated through interaction with a variety of structurally unrelated cellular transcription factors, as Tax does not bind DNA in a sequence-specific manner. Although most of these cellular transcription factors have been shown to mediate activation by Tax, we have recently demonstrated that members of the basic helixloop-helix (bHLH) family of transcription factors, which play a critical role in progression through the cell cycle, mediate repression by Tax. In this report, we examined whether Tax might repress transcription of the tumor suppressor p53, as the p53 gene has recently been demonstrated to be regulated by the bHLH protein c-Myc. Furthermore, loss or inactivation of the p53 gene has been shown to be causally associated with oncogenic transformation. We show that Tax represses transcription of the p53 gene and that this repression is dependent upon the bHLH recognition element in the p53 promoter. Together, these results suggest that Tax may promote malignant transformation through repression of p53 transcription.

Robust transcription of human T-cell leukemia virus type 1 (HTLV-1) genome requires the viral transactivator Tax. Although Tax has been previously shown to interact with the KIX domain of CBP/p300 in vitro, the precise functional relevance of this interaction remains unclear. Using two distinct approaches to interrupt the physical interaction between Tax and KIX, we find that Tax transactivation from chromatin templates is strongly dependent on CBP/p300 recruitment via the KIX domain. Additionally, we find that the primary functional contribution of CBP/p300 to Tax transactivation resides in the intrinsic acetyltransferase activity of the coactivators. These studies unexpectedly uncover a specific requirement for CBP/p300 acetyltransferase activity on chromatin templates assembled with nucleosomes lacking their amino-terminal tails. Together, these data indicate that the KIX domain of CBP/p300 is essential for targeting the acetyltransferase activity of the coactivator to the Tax-CREB (Tax/CREB) complex. Significantly, these observations reveal the presence of one or more CBP/p300 acetyltransferase targets that function specifically on chromatin templates, are independent of the histone tails, and are critical to Tax transactivation.The human T-cell leukemia virus type 1 (HTLV-1)-encoded Tax protein is required for high-level transcription of the viral genome (for review, see reference 66). Tax stimulates HTLV-1 transcription through interaction with three conserved 21-bp repeat DNA elements, called viral CREs, located in the transcriptional control region of the virus. These sequences serve as binding sites for Tax in complex with the cellular transcription factor CREB (or other members of the ATF/CREB family of transcription factors) (1,16,22,67). Tax binds to the viral CRE sequences through protein-DNA interactions with GCrich minor groove sequences (34,43,44,48) and proteinprotein interactions with CRE-bound CREB (1, 22). The formation of this promoter-bound Tax-CREB complex (Tax/ CREB) is critical for the recruitment of the multifunctional cellular coactivators CBP and p300 (19,26,32,39,65).CBP and p300 are very large paralogous proteins that coordinate highly regulated gene expression in metazoans. Several conserved domains in CBP and p300 serve as binding sites for a wide variety of structurally unrelated cellular and viral transcription factors. These domains in CBP/p300 include two cysteine-histidine-rich domains (CH1 and CH3), the SRC-interacting domain, and the KIX domain (for recent reviews, see references 11 and 21). The transcription factor-coactivator interactions mediated by these domains facilitate recruitment of the coactivators to target promoters. Subsequent to recruitment by the transcription factors, CBP and p300 appear to stimulate transcription through multiple, distinct mechanisms.