HTLV-1 Tax protein recruitment into IKKε and TBK1 kinase complexes enhances IFN-I expression.

The Tax protein expressed by human T-cell leukemia virus type 1 (HTLV-1) plays a pivotal role in the deregulation of cellular pathways involved in the immune response, inflammation, cell survival, and cancer. Many of these effects derive from Tax multiple interactions with host factors, including the subunits of the IKK-complex that are required for NF-κB activation. IKKɛ and TBK1 are two IKK-related kinases that allow the phosphorylation of interferon regulatory factors that trigger IFN type I gene expression. We observed that IKKɛ and TBK1 recruit Tax into cellular immunocomplexes. We also found that TRAF3, which regulates cell receptor signaling effectors, forms complexes with Tax. Transactivation analyses revealed that expression of Tax, in presence of IKKɛ and TBK1, enhances IFN-ß promoter activity, whereas the activation of NF-κB promoter is not modified. We propose that Tax may be recruited into the TBK1/IKKɛ complexes as a scaffolding-adaptor protein that enhances IFN-I gene expression.

Transcriptional regulation of the human Raver2 ribonucleoprotein gene.

Raver2 is a putative modulator of the activity of the polypyrimidine-tract binding protein (PTB), one of the most intensively studied splicing repressors. Little is known about Raver2 expression, and all current data is from mice where it shows tissue specificity. In the present study, by comparing Raver2 transcript expression in human and mouse tissues, we found that human Raver2 is ubiquitously expressed in adult tissues. In order to investigate human Raver2 transcription regulation, we identified and characterized a putative promoter region in a 1000bp region upstream of the transcription starting site of the gene. Dual luciferase reporter assays demonstrated that this region had promoter activity conferred by the first 160bp. By mutagenic analyses of putative cis-acting regulatory sequences, we identified an individual site that decreased the promoter activity by up to 40% when mutated. Together, our results suggest that regulation of human Raver2 expression involves TATA-less transcriptional activity.

Intracellular localization and cellular factors interaction of HTLV-1 and HTLV-2 Tax proteins: similarities and functional differences.

Human T-lymphotropic viruses type 1 (HTLV-1) and type 2 (HTLV-2) present very similar genomic structures but HTLV-1 is more pathogenic than HTLV-2. Is this difference due to their transactivating Tax proteins, Tax-1 and Tax-2, which are responsible for viral and cellular gene activation? Do Tax-1 and Tax-2 differ in their cellular localization and in their interaction pattern with cellular factors? In this review, we summarize Tax-1 and Tax-2 structural and phenotypic properties, their interaction with factors involved in signal transduction and their localization-related behavior within the cell. Special attention will be given to the distinctions between Tax-1 and Tax-2 that likely play an important role in their transactivation activity.

Association of HTLV Tax proteins with TAK1-binding protein 2 and RelA in calreticulin-containing cytoplasmic structures participates in Tax-mediated NF-κB activation.

HTLV-1 is more pathogenic than HTLV-2 despite having a similar genome and closely related transactivating oncoproteins. Both Tax-1 protein from HTLV-1 and Tax-2 from HTLV-2 activate the NF-κB pathway. The mechanisms involved in Tax-1 deregulation of this signalling pathway have been thoroughly investigated, but little is known about regulation by Tax-2. We have compared the interaction of Tax-1 and Tax-2 with two key NF-κB signalling factors: TAK1-binding protein 2 (TAB2), an adaptor involved in the activation of TAK1 kinase, and RelA, the active subunit of the canonical RelA/p50 NF-κB transcription factor. Tax-2 formed stable complexes with both RelA and TAB2. These two NF-κB factors colocalized with Tax proteins in dotted cytoplasmic structures targeted by calreticulin, a multi-process calcium-buffering chaperone. Co-expression of RelA and/or TAB2 markedly increased Tax-mediated NF-κB activation. These findings provide new insights into the role of RelA, TAB2 and Tax in the deregulation of the NF-κB pathway.

Functional characterization of the ribonucleoprotein, PTB-binding 1/Raver1 promoter region.

Raver1 is a ribonucleoprotein, evolutionarily conserved in mammals, which acts as a polypyrimidine tract-binding protein (PTB/hnRNPI) co-repressor in regulating alternative splicing events. The mouse homologue has been identified as a dual compartment protein that interacts with PTB within perinucleolar structures, and localizes at microfilament plasma membrane attachment sites in fibroblasts, epithelial and muscle cells. Human Raver1 gene is localized on chromosome 19p13.2 and encodes for an inferred 756 amino acid protein sharing 87% similarity with the mouse orthologue. The human Raver1 gene expression has not been previously investigated. Here we report the mRNA expression profile of human Raver1 gene and the molecular characterization of its promoter region. From the in silico analysis of 1542 bp of the Raver1 5'-flanking region (GC content=61%), no canonical TATA or CAAT boxes can be highlighted, whereas several consensus Sp1 putative binding sequences can be predicted within 1 kb from the transcription start site (TSS) that we determined by 5'-RACE. Functional analyses established a minimal region involved in the regulation of the human Raver1 promoter activity. Mutational analyses and transfection studies indicated that a GGGAGCTCCC sequence at -531 represents a putative cis signal acting as a negative regulator element of the promoter function. Altogether, our results indicate that human Raver1 gene promoter region shares common features of ubiquitously expressed gene with the interacting splicing regulator PTB.