AN EXTENDED ALPHA-HELIX AND SPECIFIC AMINO-ACID-RESIDUES OPPOSITE THEDNA-BINDING SURFACE OF THE CAMP RESPONSE ELEMENT-BINDING PROTEIN BASIC DOMAIN ARE IMPORTANT FOR HUMAN T-CELL LYMPHOTROPIC RETROVIRUS TYPE-ITAX BINDING

The human T cell lymphotropic retrovirus type I (HTLV-I) trans-activat or, Tax, interacts specifically with the basic-domain/leucine-zipper ( bZip) protein, cAMP response element binding protein (CREB), bound to the viral Tax-responsive element consisting of three imperfect 21-base pair repeats, each with a cAMP response element core flanked by G/C-r ich sequences. Here, the minimal CREB-bZip necessary for Tax binding i s shown to be composed of amino acid residues 280 341, The Tax-CREB in teraction involves an uninterrupted and extended a-helix in CREB that spans most of its basic domain to include amino acid residues localize d to the NH, terminus of the DNA binding region. Mutational analyses i ndicate that three residues, Arg(284), Met(291), and Glu(299) unique t o this region of the CREB/activating transcription factor-1 subfamily of bZip proteins, constitute the contact surface for Tax, Amino acid s ubstitutions in these positions had little impact on CREB-bZip binding to DNA but abrogated its binding to Tax. Each of the contact residues for Tax are spaced approximately two helical turns apart on the side of the bZip helix directly opposite to that of the invariant DNA-bindi ng residues. Molecular modeling reveals the Tax-contact residues to be near the minor groove of the G/C-rich DNA in the al-base pair repeat. They most likely position Tax for minor groove contact with the G/C-r ich sequences.