Human T-lymphotropic virus type 1 p30^II functions as a transcription factor and differentially modulates CREB-responsive promoters

Human T-lymphotropic virus type 1 (HTLV-1), a complex retrovirus, causes adult T-cell lymphoma/leukemia and is linked to a variety of immune-mediated disorders. The roles of proteins encoded in the pX open reading frame (ORF) II gene region in HTLV-1 replication or in mediating virus-associated diseases remain to be defined. A nucleus-localizing 30-kDa protein, p30^II, encoded within pX ORF II has limited homology with the POU family of transcription factors. Recently, we reported that selected mutations in pX ORF II diminish the ability of HTLV-1 to maintain high viral loads in infected rabbits. Herein we have tested the transcriptional ability of p30^II in mammalian cells by using yeast Gal4 fusion protein vectors and transfection of luciferase reporter genes driven by CREB-responsive promoters, p30^II as a Gal4 DNA-binding domain (DBD) fusion protein transactivates Gal4-driven luciferase reporter gene activity up to 25-fold in 293 and HeLa-tat cells. We confirmed nuclear localization of p30^II and demonstrate dose-dependent binding of p30^II-Gal4(DBD) to Gal4 DNA-binding sites. The transcriptional activity of p30^II-Gal4(DBD) was independent of TATA box flanking sequences, as shown by using two different Gal4 reporter systems. Studies of selected p30^II mutants indicated that domains that mediate transcription are restricted to a central core region of the protein between amino acids 62 and 220. Transfection of a p30^II-expressing plasmid repressed cellular CRE-driven reporter gene activity, with or without Tax expression. In contrast, p30^II at lower concentrations enhanced HTLV-1 long terminal repeat-driven reporter gene activity independent of Tax expression. These data are the first to demonstrate a transcriptional function for p30^II and suggest a mechanism by which this nuclear protein may influence HTLV-1 replication or cellular gene expression in vivo.