Coactivators for a proline-rich activator purified from the multisubunit human TFIID complex.

The mechanisms of transcriptional activation directed by sequence-specific regulators is central to understanding gene regulation. Here, we report the isolation of coactivators responsible for mediating transcriptional activation by Gal4-Pro, a hybrid regulator containing the proline-rich activation domain of human CTF/NFI. Chromatographic studies indicate that endogenous human TFIID consists of a multisubunit complex containing the TATA-binding protein (TBP), coactivators, and other associated factors. A fraction containing the coactivator activity was separated from the endogenous TBP after disrupting the tightly associated complex with urea. The urea-purified TBP was active for basal level transcription but no longer could support activation by Gal4-Pro. However, when the two separated components were added together, activated levels of transcription were restored in the presence of Gal4-Pro. Immunoaffinity purification of the TFIID complex identifies several polypeptides specifically associated with the endogenous TBP, some or all of which function as coactivators when reconstituted with Gal4-Pro. The isolated coactivators also mediate activation by a chimeric glutamine-rich activator derived from Sp1 but not the Gal4-VP16 activator, suggesting distinct factor requirements for different types of transcriptional regulators.