Binding of the human T-cell leukemia virus Tax protein to the coactivator CBP interferes with CBP-mediated transcriptional control.

The HTLV-I oncoprotein Tax is required for high level viral transcription and is strongly linked to HTLV-I-associated malignant transformation. Tax stimulates HTLV-I transcription through high affinity binding to the KIX domain of CBP, a pleiotropic coactivator. Several cellular proteins, including c-jun, also bind to KIX and utilize CBP as a coactivator. To test whether Tax binding to KIX may disable cellular CBP function, we examined the potential interplay between Tax and c-jun for binding to KIX. We show that Tax represses the transcription function of c-jun in vivo and demonstrate that both transcription factors bind to an overlapping minimal region of KIX in vitro. c-jun binding to KIX is displaced by Tax, indicating that their binding is mutually exclusive and providing a molecular basis for the observed repression. The competition between Tax and cellular transcription factors for CBP represents a novel pathway for HTLV-I dependent deregulation of gene expression, and may have significant implications for cellular homeostasis and transformation in the HTLV-I infected T-cell.