Anchoring of CREB binding protein to the human T-cell leukemia virus type 1 promoter: a molecular mechanism of Tax transactivation.
The human T-cell leukemia virus type 1 (HTLV-1)-encoded Tax protein activates viral transcription through interaction with the cellular transcription factor CREB (cyclic AMP response element [CRE] binding protein). Although Tax stabilizes the binding of CREB to the Tax-responsive viral CREs in the HTLV-1 promoter, the precise molecular mechanism by which Tax mediates strong transcriptional activation through CREB remains unclear. In this report, we show that Tax promotes high-affinity binding of the KIX domain of CREB binding protein (CBP) to CREB-viral CRE complexes, increasing the stability of KIX in these nucleoprotein complexes by up to 4.4 kcal/mol. Comparable KIX binding affinities were measured for both phosphorylated and unphosphorylated forms of CREB, and in all cases high-affinity binding was dependent upon both Tax and the viral CRE. Tax also promoted association of KIX to a truncated form of CREB containing only the 73-amino-acid basic leucine zipper (bZIP) domain, indicating that the entire amino-terminal CBP-interacting domain of CREB is nonessential in the presence of Tax. Functional studies upheld the binding studies, as expression of the bZIP domain of CREB was sufficient to support Tax transactivation of HTLV-1 transcription in vivo. Finally, we show that transfection of a KIX expression plasmid, which lacks activation properties, inhibited Tax transactivation in vivo. This suggests that KIX occupies the CBP binding site on Tax, and therefore CBP is likely a cofactor in mediating Tax stimulation of HTLV-1 transcription. Together, these data support a model in which Tax anchors CBP to the HTLV-1 promoter, with strong transcriptional activation resulting from the CBP-associated activities of nucleosome remodeling and recruitment of the general transcription machinery.
Giebler
H A
HA
Department of Biochemistry and Molecular Biology, Colorado State University, Fort Collins 80523-1870, USA.
Loring
J E
JE
van Orden
K
K
Colgin
M A
MA
Garrus
J E
JE
Escudero
K W
KW
Brauweiler
A
A
Nyborg
J K
JK
D051697:Activating Transcription Factor 1; D002460:Cell Line; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism*; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins; D016356:Gene Products, tax / Q000235:genetics*; D006207:Half-Life; D015368:Human T-lymphotropic virus 1 / Q000235:genetics*; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D011487:Protein Conformation; D014157:Transcription Factors / Q000378:metabolism; D015533:Transcriptional Activation
D051697:Activating Transcription Factor 1; D017362:Cyclic AMP Response Element-Binding Protein; D004268:DNA-Binding Proteins; D016356:Gene Products, tax; D046911:Macromolecular Substances; D014157:Transcription Factors; D004247:DNA