Two receptor interaction domains in the corepressor, N-CoR/RIP13, are required for an efficient interaction with Rev-erbA alpha and RVR: physical association is dependent on the E region of the orphan receptors.
Rev-erbA alpha and RVR/Rev-erb beta/BD73 are orphan steroid receptors that have no known ligands in the 'classical sense'. These 'orphans' do not activate transcription, but function as dominant transcriptional silencers. The thyroid hormone receptor (TR) and the retinoic acid receptor (RAR) act as transcriptional silencers by binding corepressors (e.g. N-CoR/RIP13 and SMRT/TRAC-2) in the absence of ligands. The molecular basis of repression by orphan receptors, however, remains obscure, and it is unclear whether these corepressors mediate transcriptional silencing by Rev-erbA alpha and RVR. Recently, two new variants of N-CoR have been described, RIP13a and RIP13delta1. The characterisation of these splice variants has identified a second receptor interaction domain (ID-II), in addition to the previously characterised interaction domain (ID-I). This investigation utilised the mammalian two hybrid system and transfection analysis to demonstrate that Rev-erbA alpha and RVR will not efficiently interact with either ID-I or ID-II separately from RIP13a or RIP13delta1. However, they interact efficiently with a domain composed of ID-I and ID-II from RIP13a. Interestingly, the interaction of Rev-erbA alpha and RVR is strongest with ID-I and ID-II from RIP13delta1. Detailed deletion analysis of the orphan receptor interaction with RIP13/N-CoR rigorously demonstrated that the physical association was critically dependent on an intact E region of Rev-erbA alpha and RVR. Over-expression of the corepressor interaction domains (i.e. dominant negative forms of N-CoR/RIP13) could alleviate orphan receptor-mediated repression of transactivation by GALVP16. This demonstrated that these regions could function as anti-repressors. In conclusion, these data from two independent approaches demonstrate that repression by Rev-erbA alpha and RVR is mediated by an interaction of ID-I and ID-II of N-CoR, RIP13a and delta1 with the putative ligand binding domain of the orphan receptors.