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Corepressors N-CoR and SMRT participate in diverse repression pathways and exist in large protein complexes including HDAC3, TBL1 and TBLR1. However, the roles of these proteins in SMRT-N-CoR complex function are largely unknown. Here we report the purification and functional characterization of the human N-CoR complex. The purified N-CoR complex contains 10-12 associated proteins, including previously identified components and a novel actin- binding protein IR1O. We show that TBL1/TBLR1 associates with N-CoR through two independent inter- actions: the N-terminal region and the C-terminal WD-40 repeats interact with the N-CoR RIM and RIM region, respectively. In vitro, TBL1/TBLR1 bind his- tones H2B and H4, and, importantly, repression by TBL1/TBLR1 correlates with their interaction with histones. By using specific small interference RNAs (siRNAs), we demonstrate that HDAC3 is essential, whereas TBL1 and TBLR1 are functionally redundant but essential for repression by untiganded thyroid hormone receptor. Together, our data reveal the roles of HDAC3 and TBL/TBLR1 and provide evidence for the functional importance of histone interaction in repression mediated by SMRT-N-CoR complexes. [ABSTRACT FROM AUTHOR]