Both corepressor proteins SMRT and N-CoR exist in large protein complexes containing HDAC3

Area of Science:

• Molecular Biology
• Epigenetics
• Protein Biochemistry

Background:

• Nuclear receptors, such as thyroid hormone receptors (TRs), regulate gene expression.
• Corepressors like SMRT (Silencing Mediator for Retinoid and Thyroid hormone Receptors) and N-CoR (Nuclear Receptor Corepressor) are crucial for transcriptional repression.
• The precise composition and function of these corepressor complexes are not fully understood.

Purpose of the Study:

• To investigate the protein composition of SMRT and N-CoR complexes.
• To determine the role of histone deacetylase 3 (HDAC3) in these complexes.
• To elucidate the mechanism by which SMRT and N-CoR mediate transcriptional repression by unliganded TRs.

Main Methods:

• Conventional and immunoaffinity chromatography were used to isolate SMRT complexes from HeLa nuclear extracts.
• Purified complexes were analyzed by SDS-PAGE and mass spectrometry.
• In vitro binding assays were performed to assess the interaction of complexes with unliganded TRs.
• Experiments in Xenopus oocytes involved antibody injection to disrupt complex function.

Main Results:

• SMRT and N-CoR exist in large protein complexes (1.5-2 MDa).
• Histone deacetylase 3 (HDAC3) and transducin (beta)-like 1 (TBL1) were identified as subunits of the SMRT complex.
• HDAC3-containing SMRT and N-CoR complexes bind to unliganded TRs in vitro.
• Antibodies against HDAC3 or SMRT/N-CoR partially relieved repression by unliganded TR/RXR in Xenopus oocytes.

Conclusions:

• SMRT and N-CoR form large, bona fide HDAC-containing complexes.
• HDAC3 is a key component of these corepressor complexes.
• These findings provide new molecular insights into the function of SMRT and N-CoR in transcriptional repression mediated by nuclear receptors.