Functional interaction of adenovirus E1A with holo-TFIID.

The activation domains of several regulatory transcription factors have been shown to bind directly in vitro to the TATA box-binding protein (TBP). Yet TBP must also interact with multiple associated polypeptides, called TAFs, for these same activators to stimulate transcription. These findings raise the question of how TBP can interact with so many proteins, both activators and TAFs, simultaneously. Here, we show that the activation domain of the adenovirus large E1A protein can bind specifically and stably to isolated holo-TFIID, the multisubunit protein complex consisting of TBP plus TAFs. Consequently, the surface of TBP that interacts with E1A must be exposed in the holo-TFIID complex. To assess the functional significance of this interaction, we established an in vitro transcription system responsive to the E1A activation domain. The addition of excess E1A to this system inhibits (squelches) both E1A-dependent and E1A-independent transcription by sequestering a target factor required for E1A activation. From among the component activities that collectively reconstitute E1A-responsive transcription in this system, holo-TFIID alone is singularly capable of reversing the inhibition of transcription mediated by excess E1A, indicating that holo-TFIID is the direct functional target of the E1A activation domain.