THE COREPRESSOR N-COR AND ITS WARIANTS RIP13A AND RIP13-DELTA-1 DIRECTLY INTERACT WITH THE BASAL TRANSCRIPTION FACTORS TFIIB, TAF(II)32 ANDTAF(II)7O

Repression of transcription by the classical nuclear receptors (e.g. T R, RAR), the orphan nuclear receptors (e.g. Rev-erbA alpha/beta), Mxi- 1 and Mad bHLH-zip proteins and the oncoproteins PLZF and LAZ3/BCL6 is mediated by the corepressors N-CoR and SMRT. The interaction of the c orepressors with the components involved in chromatin remodelling, suc h as the recruiting proteins Sin3A/B and the histone deacteylases HDAc -1 and RPD3, has been analysed in detail. The N-CoR/ Sin3/HDAc complex es have a key role in the regulation of cellular proliferation and dif ferentiation. However, the interaction of these corepressors with the basal transcriptional machinery has remained obscure. In this study we demonstrated that the N-terminal repression domains and the receptor interaction domains (RID) of N-CoR and its splice variants, RIP13a and RIP13 Delta 1, directly interact with TAF(II)32 in vivo and in vitro. We show that interaction domain II within the N-CoR and RIP13a RID is required for the interaction with TAF(II)32. We also observed that N- CoR directly interacts with each of the basal factors, TFIIB and TAF(I I)70, and can simultaneously interact with all three basal factors in a non-competitive manner. Furthermore, we provide evidence that sugges ts the RVR/Rev-erb beta-corepressor complex also interacts with the ge neral transcriptional machinery, and that the physical association of TFIIB with N-CoR also occurs in the presence of Sin3B and HDAc-1. Inte restingly, we observed that N-CoR expression ablated the functional in teraction between TFIIB and TAF(II)32 that is critical to the initiati on of transcription. In conclusion, this study demonstrates that the N -terminal repressor region and the C-terminal RIDs are part of the cor epressor contact interface that mediates the interaction with the gene ral transcription factors, and demonstrates that TAFs can also directl y interact with corepressors to mediate signals from repressors to the basal machinery. We also suggest that N-CoR interacts with the centra l components of the transcriptional initiation process (TFIIB, TAFs) a nd locks them into a non-functional complex or conformation that is no t conducive to transcription.