INTERACTIONS OF THYROID-HORMONE RECEPTOR WITH THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 (HIV-1) LONG TERMINAL REPEAT AND THE HIV-1 TAT TRANSACTIVATOR

Thyroid hormone (T3) receptor (T3R) regulates the human immunodeficien cy virus type 1 (HIV-1) long terminal repeat (LTR) by binding to and a ctivating thyroid hormone response elements (TREs) embedded within the viral NF-kappa B and Sp1 motifs. The TREs within the NF-kappa B sites are necessary for activation by T3 in the absence of Tat, while those in the Sp1 motifs function as TREs only when Tat is expressed, sugges ting that Tat and T3R interact in the cell. Transactivation of the HIV -1 LTR by T3R alpha and several receptor mutants revealed that the 50- amino-acid N-terminal A/B region of T3R alpha, known to interact with the basal transcription factor TFIIB, is critical for activation of bo th Tat-dependent and Tat-independent responsive sequences of the LTR. A single amino acid change in the highly conserved tau(i) region in th e ligand-binding domain of T3R alpha eliminates Tat-independent but no t Tat-dependent activation of the HIV-1 LTR by T3. Ro 5-3335 hloro-5-( 2-pyrryl)-3H-1,4-benzodiazepin-2(H)-one], which inhibits Tat-mediated transactivation of HIV-1, also inhibits the functional interaction bet ween Tat and T3R alpha. Binding studies,vith glutathione-S-transferase fusion proteins and Western (immunoblot) analysis indicate that T3R a lpha interacts with Tat through amino acids within the DNA-binding dom ain of T3R alpha. Mutational analysis revealed that amino acid residue s in the basic and C-terminal regions of Tat are required for the bind ing of Tat to T3R alpha, while the N terminus of Tat is not required. These studies provide functional and physical evidence that stimulatio n of the HIV-1 LTR by T3 involves an interaction between T3R alpha and Tat. Our results also suggest a model in which multiple domains of T3 R alpha interact with Tat and other factors to form transcriptionally important complexes.