Thyroid hormone receptor-associated proteins and general positive cofactors mediate thyroid hormone receptor function in the absence of the TATA box-binding protein-associated factors of TFIID

Coactivators previously implicated in ligand-dependent activation functions by thyroid hormone receptor (TR) include p300 and CREB-binding protein (CB P), the steroid receptor coactivator-1 (SRC-1)-related family of proteins, and the multicomponent TR-associated protein (TRAP) complex. Here we show t hat two positive cofactors (PC2 and PC4) derived from the upstream stimulat ory activity (USA) cofactor fraction act synergistically to mediate thyroid hormone (T-3)-dependent activation either by TR or by a TR-TRAP complex in an in vitro system reconstituted with purified factors and DNA templates. Significantly, the TRAP-mediated enhancement of activation by TR does not r equire the TATA box-binding protein-associated factors of TFIID. Furthermor e, neither the pleiotropic coactivators CBP and p300 nor members of the SRC -1 family were detected in either the TR-TRAP complex or the other componen ts of the in vitro assay system. These results show that activation by TR a t the level of naked DNA templates is enhanced by cooperative functions of the TRAP coactivators and the general coactivators PC2 and PC4, and they fu rther indicate a potential functional redundancy between TRAPs and TATA box -binding protein-associated factors in TFIID. In conjunction with earlier s tudies on other nuclear receptor interacting cofactors, the present study a lso suggests a multistep pathway, involving distinct sets of cofactors, for activation of hormone responsive genes.