Nuclear receptor corepressors partner with class II histone deacetylases in a Sin3-independent repression pathway

Transcriptional repression mediated by corepressors N-CoR and SMRT is a critical function of nuclear hormone receptors, and is dysregulated in human myeloid leukemias. At the present time, these corepressors are thought to act exclusively through an mSin3/HDAC1 complex. Surprisingly, however, numerous biochemical studies have not detected N-CoR or SMRT in mSin3- and HDAC1-containing complexes. Each corepressor contains multiple repression domains (RDs), the significance of which is unknown. Here we show that these RDs are nonredundant, and that one RD, which is conserved in N-CoR and SMRT, represses transcription by interacting directly with class II HDAC4 and HDAC5. Endogenous N-CoR and SMRT each associate with HDAC4 in a complex that does not contain mSin3A or HDAC1. This is the first example of a single corepressor utilizing distinct domains to engage multiple HDAC complexes. The alternative HDAC complexes may mediate specific repression pathways in normal as well as leukemic cells.