Molecular interactions involved in the transactivation of the human T-cellleukemia virus type 1 promoter mediated by Tax and CREB-2 (ATF-4)

The human T-cell leukemia virus type 1 (HTLV-1) Tax protein activates viral transcription through three 21-bp repeats located in the U3 region of the HTLV-1 long terminal repeat and called Tax-responsive! elements (TxREs). Ea ch TYRE contains nucleotide sequences corresponding to imperfect cyclic AMP response elements (CRE). In this study, we demonstrate that the bZIP trans criptional factor CREB-2 is able to bind in vitro to the TxREs and that CRE B-2 binding to each of the 21-bp motifs is enhanced by Tax. We also demonst rate that Tax can weakly interact with CREB-2 bound to a cellular palindrom ic CRE motif such as that found in the somatostatin promoter. Mutagenesis o f Tax and CREB-2 demonstrates that both N- and C-terminal domains of Tax an d the C-terminal region of CREB-2 are required for direct interaction betwe en the two proteins. In addition, the Tax mutant M47, defective for HTLV-1 activation, is unable to form in vitro a ternary complex with CREB-2 and Tx RE, In agreement with recent results suggesting that Tax can recruit the co activator CREB-binding protein (CBP) on the HTLV-1 promoter, we provide evi dence that Tax, CREB-2 and CBP are capable of cooperating to stimulate vira l transcription. Taken together, our data highlight the major role played b y CREB-2 in Tax-mediated transactivation.