AN EXPOSED KID-LIKE DOMAIN IN HUMAN T-CELL LYMPHOTROPIC VIRUS TYPE-1 TAX IS RESPONSIBLE FOR THE RECRUITMENT OF COACTIVATORS CBP P300/

Human T-cell lymphotropic virus type 1 (HTLV-1) transcriptional activa tion is mediated by the viral transactivator, Tax, and three 21-bp rep eats (Tax response element [TxRE]) located in the U3 region of the vir al long terminal repeat (LTR). Each TxRE contains a core cyclic AMP re sponse element (CRE) flanked by 5' G-rich and 3' C-rich sequences. The TxRE binds CREB (CRE-binding protein) and Tax to form a ternary compl ex and confers Tax-dependent transactivation. Recent data indicate tha t Tax functions as a specific link to connect CREB-binding protein (CB P)/p300 in a phosphorylation-independent manner to CREB/ATF-1 assemble d on the viral 21-bp repeats. Glutathione S-transferase pull-down perf ormed with Tax deletion mutants and peptide competition have localized the site in Tax critical for binding CBP/p300 to a highly protease-se nsitive region around amino acid residues 81 to 95 ((81)QRTSKTLKTVLTPP IT(95)) which lies between the domains previously proposed to be impor tant for CREB binding and Tax subunit dimerization, Amino acid residue s around the trypsin- and chymotrypsin-sensitive sites (88KVL90) of Ta x bear resemblance to those in the kinase-inducible domain of CREB (12 9SRRPSYRKILNE140) surrounding Ser-133, which undergoes signal-induced phosphorylation to recruit CBP/p300. Site-directed mutagenesis of resi dues in this domain (R82A, K85A, K88A, and V89A) resulted in proteins which failed to transactivate from the HTLV-1 LTR in vivo. These mutan ts (K85A, K88A, and V89A) bind CREB with similar affinities as wild-ty pe Tax, vet interaction with CBP/p300 is abrogated in various biochemi cal assays, indicating that the recruitment of CBP/p300 is crucial for Tax transactivation. A Tax mutant, M47, defective in the COOH-termina l transactivation domain, continued to interact with CBP/p300, suggest ing that interactions with additional cellular factors are required fo r proper Tax function.