CBP-dependent and independent enhancing activity of steroid receptor coactivator-1 in thyroid hormone receptor-mediated transactivation.

Full-length of steroid receptor coactivator-1 (F-SRC-1) has been shown to interact with thyroid hormone receptors (TRs) in a ligand-dependent manner and to stimulate receptor-dependent transcription. To identify functional domains of F-SRC-1, several internal deletion mutants of F-SRC-1 were constructed. Although in vitro pull down assay with TR showed interaction of all of these mutants with TR, lack of mid legion (amino acids 398-1172) lost enhancing activity of TR-mediated transcription in a transient transfection assay. However, F-SRC-1 mutant lacking CBP-interacting domain still preserved enhancing activity. Surprisingly, F-SRC-1 mutants also increased basal level of viral promoter activity depending upon their deleted region. Yeast activation function assay revealed that these F-SRC-1 mutants had intrinsic activation function when bound to DNA. Analyses of small fragments of F-SRC-1 identified three separable activation domains. In vitro binding assay showed that TBP and TFIIB bound to C-terminal half of F-SRC-1. These results suggest that F-SRC-1 can function via both CBP-dependent and independent manners using various sets of activation domains and that direct interactions between F-SRC-1 and TBP or TFIIB may not be important for CBP-independent transcription.