Specific interaction of HTLV tax protein and a human type IV neuronal intermediate filament protein

A globally applicable PCR-based detection and discrimination of BK and JC polyomaviruses

BKV and JCV belong to the Polyomaviridae family and are opportunistic agents associated with complications in immunocompromised individuals. Although a single screening assay for both viruses would be convenient, the diversity of BKV and JCV serotypes and genotypes is a methodological challenge. In this paper, we developed a PCR method able to detect and segregate BKV and JCV, despite these genetic discrepancies. A duplex semi-nested PCR (duplex snPCR) was designed to target a conserved region (639nt-1516nt) within the VP2 gene. In the first PCR, a primer set common to all BKV and JCV serotypes/ genotypes was used, followed by a semi-nested PCR with internal primers for BKV and JCV segregation. The limit of detection of the duplex snPCR was as low as 10 copies of BKV or JCV plasmids/μL. Specific products were observed when JCV and BKV plasmids were mixed in the same reaction. In field sample testing, the duplex snPCR detected and distinguished both viruses in different biological samples. Results were confirmed by Sanger's sequencing. The geographical complexity of BKV and JCV serotypes and genotypes imposes limits to a simple and universal method that could detect each virus. However, we describe here a sensitive and reliable PCR technique for BKV and JCV diagnosis that overcomes these limitations and could be universally applied.

α-Internexin and Peripherin: Expression, Assembly, Functions, and Roles in Disease

α-Internexin and peripherin are neuronal-specific intermediate filament (IF) proteins. α-Internexin is a type IV IF protein like the neurofilament triplet proteins (NFTPs, which include neurofilament light chain, neurofilament medium chain, and neurofilament high chain) that are generally considered to be the primary components of the neuronal IFs. However, α-internexin is often expressed together with the NFTPs and has been proposed as the fourth subunit of the neurofilaments in the central nervous system. α-Internexin is also expressed earlier in the development than the NFTPs and is a maker for neuronal IF inclusion disease. α-Internexin can self-polymerize in vitro and in transfected cells and it is present in the absence of the NFTP in development and in granule cells in the cerebellum. In contrast, peripherin is a type III IF protein. Like α-internexin, peripherin is specific to the nervous system, but it is expressed predominantly in the peripheral nervous system (PNS). Peripherin can also self-assemble both in vitro and in transfected cells. It is as abundant as the NFTPs in the sciatic nerve and can be considered a fourth subunit of the neurofilaments in the PNS. Peripherin has multiple isoforms that arise from intron retention, cryptic intron receptor site or alternative translation initiation. The functional significance of these isoforms is not clear. Peripherin is a major component found in inclusions of patients with amyotrophic lateral sclerosis (ALS) and peripherin expression is upregulated in ALS patients.

Expression pattern of neuronal intermediate filament α-internexin in anterior pituitary gland and related tumors

PURPOSE

α-Internexin (INA) is a class IV neuronal intermediate filament protein that maintains the morphogenesis of neurons. It is expressed in developing neuroblasts and represents the major component of the cytoskeleton in cerebellar granule cells of adult central nervous system tissue. Data concerning INA expression in the human frontal pituitary lobe and related adenomas (PA) is missing.

METHODS

Using immunohistochemistry we examined the distribution pattern of INA in a large cohort of 152 PA, 11 atypical PA, 4 pituitary carcinomas and 20 normal pituitaries (overall n = 187). Quantity of INA protein expression was semi-quantitatively evaluated and grouped into five categories (0 = 0%; 1 = >0-5%; 2 = >5-35%; 3 = >35-80%; 4 = >80% of cells).

RESULTS

Cellular staining intensity of INA appeared significantly higher in gonadotropinomas (Go, n = 62), null cell adenomas (NC, n = 7) and thyrotropinomas (TSHomas, n = 7) compared to the other tumor subtypes (p ≤ 0.001). Furthermore, Go and NC showed a peculiar pseudorosette-like staining pattern surrounding blood vessels in 85.5% (59/69) of cases. Interestingly, areas exhibiting homogenous INA staining were often associated with oncocytic cell changes and decreased immunohistochemically detectable hormone expression. Only 8.5% (8/94) of other PA showed a comparable INA distribution (p ≤ 0.001).

CONCLUSION

Go, NC as well as TSHomas exhibit high levels of intracellular INA protein indicating neuronal transdifferentiation. A possible impact on pathogenesis and endocrine activity needs further investigation.

Tax-1 and Tax-2 similarities and differences: focus on post-translational modifications and NF-κB activation

Although human T cell leukemia virus type 1 and 2 (HTLV-1 and HTLV-2) share similar genetic organization, they have major differences in their pathogenesis and disease manifestation. HTLV-1 is capable of transforming T lymphocytes in infected patients resulting in adult T cell leukemia/lymphoma whereas HTLV-2 is not clearly associated with lymphoproliferative diseases. Numerous studies have provided accumulating evidence on the involvement of the viral transactivators Tax-1 versus Tax-2 in T cell transformation. Tax-1 is a potent transcriptional activator of both viral and cellular genes. Tax-1 post-translational modifications and specifically ubiquitylation and SUMOylation have been implicated in nuclear factor-kappaB (NF-κB) activation and may contribute to its transformation capacity. Although Tax-2 has similar protein structure compared to Tax-1, the two proteins display differences both in their protein–protein interaction and activation of signal transduction pathways. Recent studies on Tax-2 have suggested ubiquitylation and SUMOylation independent mechanisms of NF-κB activation. In this present review, structural and functional differences between Tax-1 and Tax-2 will be summarized. Specifically, we will address their subcellular localization, nuclear trafficking and their effect on cellular regulatory proteins. A special attention will be given to Tax-1/Tax-2 post-translational modification such as ubiquitylation, SUMOylation, phosphorylation, acetylation, NF-κB activation, and protein–protein interactions involved in oncogenecity both in vivo and in vitro.

Murine neurofibroma reversion by antisense RNA for HTLV-I tax

Neurofibroma cell lines derived from mice transgenic for HTLV-I LTR tax express high levels of HTLV-I tax mRNA and protein and exhibit a transformed phenotype. A retrovirus vector carrying HTLV-I tax cDNA in reversed transcriptional orientation was stably transfected into the neurofibroma cells. Antisense RNA inhibited expression of the tax gene with a decrease of more than 40% in both tax mRNA and protein. Tax antisense RNA reversed the transformed phenotype as exhibited by dramatic changes in cell morphology and growth characteristics. Expression of several cellular genes which are activated by Tax protein including GM-CSF, IL-6, LT/TNF, c-myc and LIF was down-regulated, while M-CSF and c-src proto-oncogene expressions were up-regulated. Accumulation of beta-actin mRNA was not affected. The changes that occurred in the tax antisense expressing neurofibroma cells could be the consequence of the decreased concentration of Tax protein. These results also indicate that HTLV-I Tax protein is crucial for maintaining the transformed cell morphology, growth and proliferation of murine neurofibroma cells and suggest that deregulation of endogenous cellular genes by Tax protein is the mechanism through which neurofibromas occur in tax mice.

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.

Alpha-internexin is structurally and functionally associated with the neurofilament triplet proteins in the mature CNS

Alpha-internexin, a neuronal intermediate filament protein implicated in neurodegenerative disease, coexists with the neurofilament (NF) triplet proteins (NF-L, NF-M, and NF-H) but has an unknown function. The earlier peak expression of alpha-internexin than the triplet during brain development and its ability to form homopolymers, unlike the triplet, which are obligate heteropolymers, have supported a widely held view that alpha-internexin and neurofilament triplet form separate filament systems. Here, we demonstrate, however, that despite a postnatal decline in expression, alpha-internexin is as abundant as the triplet in the adult CNS and exists in a relatively fixed stoichiometry with these subunits. Alpha-internexin exhibits transport and turnover rates identical to those of triplet proteins in optic axons and colocalizes with NF-M on single neurofilaments by immunogold electron microscopy. Alpha-internexin also coassembles with all three neurofilament proteins into a single network of filaments in quadruple-transfected SW13vim(-) cells. Genetically deleting NF-M alone or together with NF-H in mice dramatically reduces alpha-internexin transport and content in axons throughout the CNS. Moreover, deleting alpha-internexin potentiates the effects of NF-M deletion on NF-H and NF-L transport. Finally, overexpressing a NF-H-LacZ fusion protein in mice induces alpha-internexin and neurofilament triplet to aggregate in neuronal perikarya and greatly reduces their transport and content selectively in axons. Our data show that alpha-internexin and the neurofilament proteins are functionally interdependent. The results strongly support the view that alpha-internexin is a fourth subunit of neurofilaments in the adult CNS, providing a basis for its close relationship with neurofilaments in CNS diseases associated with neurofilament accumulation.

Identification of Japanese encephalitis virus-inducible genes in mouse brain and characterization of GARG39/IFIT2 as a microtubule-associated protein

Several mouse central nervous system genes have been identified that are differentially regulated during Japanese encephalitis virus (JEV) infection, including those which have not been reported to be induced by any other neurotropic virus. Interestingly, approximately 80 % of JEV-inducible genes identified in this study are also induced by Sindbis virus, indicating activation of common host signalling pathways by these two viruses, despite their diverse life cycles. One of these, the glucocorticoid attenuated response gene 39 (GARG39, also known as IFIT2, ISG54 and MuP54) was characterized further. It was demonstrated that GARG39 protein interacts with microtubules in vitro, co-localizes with beta-tubulin in vivo and is enriched in the mitotic spindle of non-neuronal cells undergoing mitosis. While GARG39 was known for a long time as an inflammation-inducible glucocorticoid attenuated protein, its identification as a microtubule-associated protein in this study suggests a possible role for this protein in cell proliferation, virion assembly/transport and microtubule dynamics.

Quantitative proteomics analysis of differential protein expression and oxidative modification of specific proteins in the brains of old mice

The brain is susceptible to oxidative stress, which is associated with age-related brain dysfunction, because of its high content of peroxidizable unsaturated fatty acids, high oxygen consumption per unit weight, high content of key components for oxidative damage, and the relative scarcity of antioxidant defense systems. Protein oxidation, which results in functional disruption, is not random but appears to be associated with increased oxidation in specific proteins. By using a proteomics approach, we have compared the protein levels and specific protein carbonyl levels, an index of oxidative damage in the brains of old mice, to these parameters in the brains of young mice and have identified specific proteins that are altered as a function of aging. We show here that the expression levels of dihydropyrimidinase-like 2 (DRP2), alpha-enolase (ENO1), dynamin-1 (DNM1), and lactate dehydrogenase 2 (LDH2) were significantly increased in the brains of old versus young mice; the expression levels of three unidentified proteins were significantly decreased. The specific carbonyl levels of beta-actin (ACTB), glutamine synthase (GS), and neurofilament 66 (NF-66) as well as a novel protein were significantly increased, indicating protein oxidation, in the brains of old versus young mice. These results were validated by immunochemistry. In addition, enzyme activity assays demonstrated that oxidation was associated with decreased GS activity, while the activity of lactate dehydrogenase was unchanged in spite of an up-regulation of LDH2 levels. Several of the up-regulated and oxidized proteins in the brains of old mice identified in this report are known to be oxidized in neurodegenerative diseases as well, suggesting that these proteins may be particularly susceptible to processes associated with neurodegeneration. Our results establish an initial basis for understanding protein alterations that may lead to age-related cellular dysfunction in the brain.

Muscle wasting induced by HTLV-1 tax-1 protein: an in vitro and in vivo study

Besides tropical spastic paraparesis/human T-cell leukemia virus type-1 (HTLV-1)-associated myelopathy, the human retrovirus HTLV-1 causes inflammatory disorders such as myositis. Although the pathogenesis of HTLV-1-associated myositis is primarily unknown, a direct effect of cytokines or viral proteins in myocytotoxicity is suspected. We have developed an in vitro cell culture model to study the interactions between primary human muscle cells and HTLV-1 chronically infected cells. When HTLV-1-infected cell lines were added to differentiated muscle cultures, cytopathic changes such as fiber shrinking were observed as early as 1 day after contact. This was accompanied by alterations in desmin and vimentin organization, occurring in the absence of muscle cell infection but with Tax-1 present in myotubes. Cytopathic changes were also observed when infected culture supernatants were added to the muscle cells. Fiber atrophy and cytoskeletal disorganization were confirmed in muscle biopsies from two HTLV-1-infected patients with myositis. Transduction of cultured muscle cells with a lentiviral vector containing the HTLV-1 Tax gene reproduced such effects in vitro. The present data indicate that the myocytotoxicity that is observed in HTLV-1-associated myopathies can be due to a direct effect of the Tax-1 protein expressed in infected inflammatory cells, in the absence of muscle cell infection.

Deregulation of cell-signaling pathways in HTLV-1 infection

Human T-lymphotropic virus type 1 (HTLV-1) infection is associated with the clonal expansion and transformation of mature T lymphocytes. While the mechanisms involved are incompletely understood the viral regulatory protein Tax plays a central role in these processes. Recent studies employing genomic and proteomic approaches have demonstrated the marked complexity of gene deregulation associated with Tax expression and confirmed the remarkable pleiotropism of this protein as evidenced by the numerous Tax-cellular protein interactions in infected cells. In this review, we summarize the role of Tax in the deregulation of selected cellular-signaling pathways. Specifically, this has focused on the influence and interaction of Tax with the AP-1 and NF-AT transcription factors, PDZ domain-containing proteins, Rho-GTPases, and the Janus kinase/signal transducer and activator of transcription and transforming growth factor-beta-signaling pathways. In addition to identifying the deregulation of events within these pathways, attempts have been made to highlight differences between HTLV-1 and -2, which may relate to differences in their pathogenic properties.

Human T-lymphotropic virus, type 1, tax protein triggers microtubule reorientation in the virological synapse

We showed recently that the human T-lymphotropic virus, type 1 (HTLV-1), spreads directly from cell to cell via a virological synapse. The HTLV-1 virological synapse resembles the immunological synapse; each is a specialized contact between a lymphocyte and another cell that contains organized protein microdomains, and each involves repolarization of the T-cell microtubule cytoskeleton. However, formation of the virological synapse is not triggered by T-cell receptor-mediated antigen recognition. On the basis of our previous data, we postulated that formation of the viral synapse was triggered by a conjunction of two signals, one from HTLV-1 infection of the T-cell and one from cell-cell contact. We have recently identified ICAM-1 engagement as a cell-contact signal that causes the microtubule polarization associated with the virological synapse. Here we used confocal microscopy of T-lymphocytes naturally infected with HTLV-1 or transfected with individual HTLV-1 genes to investigate the role of the viral transcriptional transactivator protein Tax. Polarization of the microtubules was induced by cell-cell contact or by cross-linking T-cell surface molecules with monoclonal antibodies adsorbed to latex beads. We show that Tax, which is mainly found in the nucleus, is also present at two specific extranuclear sites as follows: around the microtubule organizing center in association with the cis-Golgi and in the cell-cell contact region. We show that expression of Tax provides an intracellular signal that synergizes with ICAM-1 engagement to cause the T-cell microtubule polarization observed at the virological synapse.

Protein profile of tax-associated complexes

Infection with human T-cell leukemia virus type 1 (HTLV-1) results in adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. Tax, a 40-kDa protein, regulates viral and cellular transcription, host signal transduction, the cell cycle, and apoptosis. Tax has been shown to modulate cellular CREB and NFkappaB pathways; however, to date, its role in binding to various host cellular proteins involved in tumorigenesis has not been fully described. In this study, we describe the Tax-associated proteins and their functions in cells using several approaches. Tax eluted from a sizing column mostly at an apparent molecular mass of 1800 kDa. Following Tax immunoprecipitation, washes with high salt buffer, two-dimensional gel separation, and mass spectrometric analysis, a total of 32 proteins was identified. Many of these proteins belong to the signal transduction and cytoskeleton pathways and transcription/chromatin remodeling. A few of these proteins, including TXBP151, have been shown previously to bind to Tax. The interaction of Tax with small GTPase-cytoskeleton proteins, such as ras GAP1m, Rac1, Cdc42, RhoA, and gelsolin, indicates how Tax may regulate migration, invasion, and adhesion in T-cell cancers. Finally, the physical and functional association of Tax with the chromatin remodeling SWI/SNF complex was assessed using in vitro chromatin remodeling assays, chromatin remodeling factor BRG1 mutant cells, and RNA interference experiments. Collectively, Tax is able to bind and regulate many cellular proteins that regulate transcription and cytoskeletal related pathways, which might explain the pleiotropic effects of Tax leading to T-cell transformation and leukemia in HTLV-1-infected patients.

SR-related protein TAXREB803/SRL300 is an important cellular factor for the transactivational function of human T-cell lymphotropic virus type 1 Tax

Expression of the human T-cell lymphotropic virus type 1 (HTLV-1) genes is transcriptionally activated by the cognate oncoprotein Tax which enhances the binding of the cyclin AMP-responsive element binding protein (CREB) to the Tax responsive element (TxRE) located in its long terminal repeat (LTR). TxRE is highly homologous to the cyclic AMP-responsive element (CRE) except for the GC-rich sequence flanking the CRE. We cloned the cDNA for a cellular factor, TAXREB803, of which the DNA-binding domain bound to TxRE and the binding was dependent on the 3' GC-rich sequence in TxRE. TAXREB803 is an SR-related protein composed of 2,752 amino acids including numerous arginine/serine (RS) motifs. TAXREB803 enhanced both the Tax dependent transcription and the CREB binding to TxRE in cooperation with Tax. The interaction of TAXREB803 and Tax was detected by coimmunoprecipitation assays as well as by indirect immunofluorescence assays. Significantly, Tax transactivation for the HTLV-1 LTR decreased dramatically when the expression level of the endogenous TAXREB803 was suppressed by the small interfering RNA. These results suggest that TAXREB803 functions as a transcriptional coactivator for Tax and plays a critical role in the expression of HTLV-1 genes.

Congruence of tissue expression profiles from Gene Expression Atlas, SAGEmap and TissueInfo databases

BACKGROUND

Extracting biological knowledge from large amounts of gene expression information deposited in public databases is a major challenge of the postgenomic era. Additional insights may be derived by data integration and cross-platform comparisons of expression profiles. However, database meta-analysis is complicated by differences in experimental technologies, data post-processing, database formats, and inconsistent gene and sample annotation.

RESULTS

We have analysed expression profiles from three public databases: Gene Expression Atlas, SAGEmap and TissueInfo. These are repositories of oligonucleotide microarray, Serial Analysis of Gene Expression and Expressed Sequence Tag human gene expression data respectively. We devised a method, Preferential Expression Measure, to identify genes that are significantly over- or under-expressed in any given tissue. We examined intra- and inter-database consistency of Preferential Expression Measures. There was good correlation between replicate experiments of oligonucleotide microarray data, but there was less coherence in expression profiles as measured by Serial Analysis of Gene Expression and Expressed Sequence Tag counts. We investigated inter-database correlations for six tissue categories, for which data were present in the three databases. Significant positive correlations were found for brain, prostate and vascular endothelium but not for ovary, kidney, and pancreas.

CONCLUSION

We show that data from Gene Expression Atlas, SAGEmap and TissueInfo can be integrated using the UniGene gene index, and that expression profiles correlate relatively well when large numbers of tags are available or when tissue cellular composition is simple. Finally, in the case of brain, we demonstrate that when PEM values show good correlation, predictions of tissue-specific expression based on integrated data are very accurate.

Coiled-coil motif as a structural basis for the interaction of HTLV type 1 Tax with cellular cofactors

Human T lymphotropic virus type 1 (HTLV-1) Tax is a multifunctional protein centrally involved in transcriptional regulation, cell cycle control, and viral transformation. The regulatory functions of Tax are thought to be mediated through protein-protein interaction with cellular cofactors. Previously we have identified several novel binding partners for Tax, including human mitotic checkpoint protein MAD1 (TXBP181), G-protein pathway suppressor GPS2 (TXBP31), and IkappaB kinase regulatory subunit IKK-gamma. Here we described two additional Tax partners, TXBP151 and TXBP121. A closer examination of the sequences of eight independent cellular Tax-binding proteins identified by us and others revealed that all of them share a single characteristic, a highly structured coiled-coil domain. We also noted that Tax and the Tax-binding coiled-coil proteins can homodimerize. Additionally, the same domain in Tax is responsible for interaction with different coiled-coil proteins. Taken together, our findings point to a particular coiled-coil structure as one of the Tax-recognition motifs. The interaction of Tax with a particular subgroup of cellular coiled-coil proteins represents one mechanism by which Tax dysregulates cell growth and proliferation.

The role of human T-lymphotropic virus type 1 (HTLV-1)-infected dendritic cells in the development of HTLV-1-associated myelopathy/tropical spastic paraparesis

The development of human T-lymphotropic virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is closely associated with the activation of T cells which are HTLV-1 specific but may cross-react with neural antigens (Ags). Immature dendritic cells (DCs), differentiated from normal donor monocytes by using recombinant granulocyte-macrophage colony-stimulating factor and recombinant interleukin-4, were pulsed with HTLV-1 in vitro. The pulsed DCs contained HTLV-1 proviral DNA and expressed HTLV-1 Gag Ag on their surface 6 days after infection. The DCs matured by lipopolysaccharides stimulated autologous CD4(+) T cells and CD8(+) T cells in a viral dose-dependent manner. However, the proliferation level of CD4(+) T cells was five- to sixfold higher than that of CD8(+) T cells. In contrast to virus-infected DCs, DCs pulsed with heat-inactivated virions activated only CD4(+) T cells. To clarify the role of DCs in HAM/TSP development, monocytes from patients were cultured for 4 days in the presence of the cytokines. The expression of CD86 Ag on DCs was higher and that of CD1a Ag was more down-regulated than in DCs generated from normal monocytes. DCs from two of five patients expressed HTLV-1 Gag Ag. Furthermore, both CD4(+) and CD8(+) T cells from the patients were greatly stimulated by contact with autologous DCs pulsed with inactivated viral Ag as well as HTLV-1-infected DCs. These results suggest that DCs are susceptible to HTLV-1 infection and that their cognate interaction with T cells may contribute to the development of HAM/TSP.

No requirement of alpha-internexin for nervous system development and for radial growth of axons

Alpha-Internexin is a type IV intermediate filament protein that is expressed abundantly in neurons during development of the peripheral and central nervous systems as well as in few neurons of the adult central nervous system. It has been suggested that alpha-internexin may act as a scaffold for the formation of neuronal intermediate filaments during early development. In addition, recent reports suggest that alpha-internexin could play a major role in two degenerative neurological disorders. We report here an analysis of mice with a targeted disruption of alpha-internexin gene. Unexpectedly, alpha-internexin -/- mice developed normally and did not exhibit overt phenotypes. Moreover, the absence of alpha-internexin did not interfere with neurite extension of cultured DRG neurons. The number and caliber of L4 ventral root axons remained unchanged in alpha-internexin -/- mice. In the retina, alpha-internexin begins to be expressed in retinal ganglion cells when their first axons reach the optic chiasma. Using HRP tracer, we show that the projection pattern of the RGC axons is not modified by the absence of alpha-internexin. Electron microscopy did not reveal significant differences in axonal calibers, in myelination of axons and in neurofilament structures between alpha-internexin -/- and control mice during development and at adult stage. These data indicate that alpha-internexin is not required for the polymerization of neurofilament in vivo. Mice deficient for both alpha-internexin and neurofilament light chain (NF-L) exhibited no over phenotypes as well. No intermediate filament structures were detectable in optic nerve of alpha-internexin -/-; NF-L -/- mice. Ours results do not support the hypothesis of a role for type IV intermediate filaments in axonal outgrowth during development of nervous system.

CREB-2, a cellular CRE-dependent transcription repressor, functions in association with Tax as an activator of the human T-cell leukemia virus type 1 promoter

The Tax protein of the human T-cell leukemia virus type 1 (HTLV-1) has been implicated in human T-cell immortalization. The primary function of Tax is to transcriptionally activate the HTLV-1 promoter, but Tax is also known to stimulate expression of cellular genes. It has been reported to associate with several transcription factors, as well as proteins not involved in transcription. To better characterize potential cellular targets of Tax present in infected cells, a Saccharomyces cerevisiae two-hybrid screening was performed with a cDNA library constructed from the HTLV-1-infected MT2 cell line. From this study, we found 158 positive clones representing seven different cDNAs. We focused our attention on the cDNA encoding the transcription factor CREB-2. CREB-2 is an unconventional member of the ATF/CREB family in that it lacks a protein kinase A (PKA) phosphorylation site and has been reported to negatively regulate transcription from the cyclic AMP response element of the human enkephalin promoter. In this study, we demonstrate that CREB-2 cooperates with Tax to enhance viral transcription and that its basic-leucine zipper C-terminal domain is required for both in vitro and in vivo interactions with Tax. Our results confirm that the activation of the HTLV-1 promoter through Tax and factors of the ATF/CREB family is PKA independent.