Determinants of CoRNR-dependent repression complex assembly on nuclear hormone receptors

Ligand-dependent exchange of coactivators and corepressors is the fundamental regulator of nuclear hormone receptor (NHR) function. The interaction surfaces of coactivators and corepressors are similar but distinct enough to allow the ligand to function as a switch. Multiple NHRs share features that allow corepressor binding, and each of two distinct corepressors (N-CoR and SMRT) contains two similar CoRNR motifs that interact with NHRs. Here we report that the specificity of corepressor-NHR interaction is determined by the individual NHR interacting with specific CoRNR boxes within a preferred corepressor. First, receptors have distinct preferences for CoRNR1 versus CoRNR2. For example, the retinoic acid receptor binds CoRNR1, while RXR interacts almost exclusively with CoRNR2. Second, the NHR preference for N-CoR or SMRT is due to differences in CoRNR1 but not CoRNR2. Moreover, within a single corepressor, affinity for different NHRs is determined by distinct regions flanking CoRNR1. The highly specific determinants of NHR-corepressor interaction and preference suggest that repression is regulated by the permissibility of selected receptor-CoRNR-corepressor combinations. Interestingly, different NHR surfaces contribute to binding of CoRNR1 and CoRNR2, suggesting a model to explain corepressor binding to NHR heterodimers.     

The N-CoR-HDAC3 nuclear receptor corepressor complex inhibits the JNK pathway through the integral subunit GPS2

The corepressors N-CoR and SMRT partner with histone deacetylases (HDACs) in diverse repression pathways. We report here that GPS2, a protein involved in intracellular signaling, is an integral subunit of the N-CoR-HDAC3 complex. We have determined structural motifs that direct the formation of a highly stable and active deacetylase complex. GPS2 and TBL1, another component of the N-CoR-HDAC3 complex, interact cooperatively with repression domain 1 of N-CoR to form a heterotrimeric structure and are indirectly linked to HDAC3 via an extended N-CoR SANT domain that also activates latent HDAC3 activity. More importantly, we show here that the N-CoR-HDAC3 complex inhibits JNK activation through the associated GPS2 subunit and thus could potentially provide an alternative mechanism for hormone-mediated antagonism of AP-1 function.     

Purification and functional characterization of the human N-CoR complex: the roles of HDAC3, TBL1 and TBLR1

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 IR10. We show that TBL1/TBLR1 associates with N-CoR through two independent interactions: the N-terminal region and the C-terminal WD-40 repeats interact with the N-CoR RD1 and RD4 region, respectively. In vitro, TBL1/TBLR1 bind histones 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 unliganded 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.