Characterization of receptor interaction and transcriptional repression by the corepressor SMRT

SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) and N-CoR (nuclear receptor corepressor) are two related transcriptional corepressors that contain separable domains capable of interacting with unliganded nuclear receptors and repressing basal transcription. To decipher the mechanisms of receptor interaction and transcriptional repression by SMRT/N-CoR, we have characterized protein-protein interacting surfaces between SMRT and nuclear receptors and defined transcriptional repression domains of both SMRT and N-CoR. Deletional analysis reveals two individual nuclear receptor domains necessary for stable association with SMRT and a C-terminal helix essential for corepressor dissociation. Coordinately, two SMRT domains are found to interact independently with the receptors. Functional analysis reveals that SMRT contains two distinct repression domains, and the corresponding regions in N-CoR also repress basal transcription. Both repression domains in SMRT and N-CoR interact weakly with mSin3A, which in turn associates with a histone deacetylase HDAC1 in a mammalian two-hybrid assay. Far-Western analysis demonstrates a direct protein-protein interaction between two N-CoR repression domains with mSin3A. Finally we demonstrate that overexpression of full-length SMRT further represses basal transcription from natural promoters. Together, these results support a role of SMRT/N-CoR in corepression through the utilization of multiple mechanisms for receptor interactions and transcriptional repression.