High Affinity Heme Binding to a Heme Regulatory Motif on the Nuclear Receptor Rev-erb{beta} Leads to its Degradation and Indirectly Regulates its Interaction with Nuclear Receptor Corepressor [Metabolism]

December 15th, 2015 by Carter, E. L., Gupta, N., Ragsdale, S. W.

Rev-erbα and Rev-erbβ are heme-binding nuclear receptors (NRs) that repress the transcription of genes involved in regulating metabolism, inflammation and the circadian clock. Previous gene expression and co-immunoprecipitation studies led to a model in which heme binding to Rev-erbα recruits nuclear receptor corepressor 1 (NCoR1) into an active repressor complex. However, in contradiction, biochemical and crystallographic studies have shown that heme decreases affinity of the ligand-binding domain (LBD) of Rev-erbs for NCoR1 peptides. One explanation for this discrepancy is that the LBD and NCoR1 peptides used for in vitro studies cannot replicate key features of the full-length proteins used in cellular studies. However, combined in vitro and cellular results described here demonstrate that heme does not directly promote interactions between full-length Rev-erbβ (FLRev-erbβ) and an NCoR1 construct encompassing all three NR interaction domains. NCoR1 tightly binds both apo- and heme- replete FLRev-erbβ:DNA complexes; furthermore, heme, at high concentrations, destabilizes the FLRev-erbβ-NCoR1 complex. The interaction between FLRev-erbβ and NCoR1 as well as Rev-erbβ repression at the Bmal1 promoter appear to be modulated by another cellular factor(s), at least one of which is related to the ubiquitin-proteasome pathway. Our studies suggest that heme is involved in regulating degradation of Rev-erbβ in a manner consistent with its role in circadian rhythm maintenance. Finally, the very slow rate constant (10-6 s-1) for heme dissociation from Rev-erbβ, rules out a prior proposal that Rev-erbβ acts as an intracellular heme sensor.
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