ANCHORING OF CREB BINDING-PROTEIN TO THE HUMAN T-CELL LEUKEMIA-VIRUS TYPE-1 PROMOTER - A MOLECULAR MECHANISM OF TAX TRANSACTIVATION

The human T-cell leukemia virus type 1 (HTLV-1)-encoded Tax protein ac tivates viral transcription through interaction with the cellular tran scription factor CREB (cyclic AMP response element [CRE] binding prote in), 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 rem ains unclear. In this report, we show that: Tax promotes high-affinity binding of the KIX domain of CREB binding protein (CBP) to CRER-viral CRE complexes, increasing the stability of KIX in these nucleoprotein complexes by up to 4.4 kcal/mol. Comparable KIX binding affinities we re measured for both phosphorylated and unphosphorylated forms of CREB , and in all cases high-affinity binding was dependent upon both Tax a nd the viral CRE. Tax also promoted association of KIX to a truncated form of CREB containing only the 73-amino-acid basic leucine zipper (b ZIP) domain, indicating that the entire amino-terminal CBP-interacting domain of CREB is nonessential in the presence of Tax, Functional stu dies upheld the binding studies, as expression of the bZIP domain of C REB was sufficient to support Tax transactivation of HTLV-1 transcript ion in vivo, Finally, we show that transfection of a KIX expression pl asmid, which lacks activation properties, inhibited Tax transactivatio n in vivo, This suggests that KIX occupies the CBP binding site on Tax , and therefore: CBP is likely a cofactor in mediating Tax stimulation of HTLV-1 transcription, Together, these data support a model in whic h Tax anchors CBP to the HTLV-I promoter, with strong transcriptional activation resulting from the CBP-associatcd activities of nucleosome remodeling and recruitment of the general transcription machinery.