EXPANSION OF CREBS DNA RECOGNITION SPECIFICITY BY TAX RESULTS FROM INTERACTION WITH ALA-ALA-ARG AT POSITIONS-282-284 NEAR THE CONSERVED DNA-BINDING DOMAIN OF CREB

The transactivator protein of human T-lymphotropic virus type I (HTLV- I), Tax, forms multiprotein complexes with the ubiquitous transcriptio n factor CREB and the CREB/ATF-1 heterodimer. The interaction between Tax and CREB is highly specific and results in increased binding of th e Tax/CREB complexes to the HTLV-I 21-bp repeats. Despite the extensiv e sequence similarities between CREB and ATF-1, Tax interacts with ATF -1 only marginally. Compared with CREB, Tax/CREB exhibits greatly incr eased DNA recognition specificity and preferentially assembles on a co nsensus binding site, GGGGG(T/A)TGACG(T/C)(A/C)TA(T/C)C-CCCC, homologo us to the HTLV-I 21-bp repeats. Here we report that Tax affects CREB b inding to the Tax-inducible DNA elements by interacting with the basic -leucine zipper (bZip) domain of CREB. We show by domain switching tha t the basic region in CREB bZip can confer on c-Jun and ATF-1 leucine zippers the ability to interact with Tax in vitro. Mutational analyses further demonstrate that the amino acid residues of CREB critical for Tax/CREB interaction are Ala-Ala-Arg at positions 282-284 (AAR(284)), immediately upstream of the highly conserved DNA-binding domain (R/K) XX(R/K) N(R/K)XAAXX(S/C)RX(R/K)(K/R) characteristic of all bZip protei ns. Specific amino acid substitutions in AAR(284) of CREB weakened or abolished Tax/CREB interaction, whereas reciprocal changes in ATF-1 al lowed it to interact with Tax. These results support a model in which the specific interaction between Tax and the AAR(284) residues near th e DNA-binding domain of CREB results in a multiprotein complex with al tered DNA recognition property. This protein complex assembles selecti vely on the viral Tax-responsive 21-bp repeats to augment transcriptio n.