GCN5 and ADA Adaptor Proteins Regulate Triiodothyronine/GRIP1 and SRC-1 Coactivator-Dependent Gene Activation by the Human Thyroid Hormone Receptor

We have used yeast genetics and in vitro protein-protein interaction experiments to explore the possibility that GCN5 (general control nonrepressed protein 5) and several other ADA (alteration/deficiency in activation) adaptor proteins of the multimeric SAGA complex can regulate T3/GRIP1 (glucocorticoid receptor interacting protein 1) and SRC-1 (steroid receptor coactivator-1) coactivator-dependent activation of transcription by the human T3 receptor β1 (hTRβ1). Here, we show that in vivo activation of a T3/GRIP1 or SRC-1 coactivator-dependent T3 hormone response element by hTRβ1 is dependent upon the presence of yeast GCN5, ADA2, ADA1, or ADA3 adaptor proteins and that the histone acetyltransferase (HAT) domains and bromodomain (BrD) of yGCN5 must be intact for maximal activation of transcription. We also observed that hTRβ1 can bind directly to yeast or human GCN5 as well as hADA2, and that the hGCN5387−837 sequence could bind directly to either GRIP1 or SRC-1 coactivator. Importantly, the T3-dependent binding of hTRβ1to hGCN5387−837 could be markedly increased by the presence of GRIP1 or SRC1. Mutagenesis of GRIP1 nuclear receptor (NR) Box II and III LXXLL motifs also substantially decreased both in vivo activation of transcription and in vitro T3-dependent binding of hTRβ1 to hGCN5. Taken together, these experiments support a multistep model of transcriptional initiation wherein the binding of T3 to hTRβ1 initiates the recruitment of p160 coactivators and GCN5 to form a trimeric transcriptional complex that activates target genes through interactions with ADA/SAGA adaptor proteins and nucleosomal histones.