Structural basis for heme-dependent NCoR binding to the transcriptional repressor REV-ERBβ

Heme is the endogenous ligand for the constitutively repressive REV-ERB nuclear receptors, REV-ERBα (NR1D1) and REV-ERBβ (NR1D2), but how heme regulates REV-ERB activity remains unclear. While cellular studies indicate heme is required for the REV-ERBs to bind the corepressor NCoR and repress transcription, fluorescence-based biochemical assays and crystal structures suggest that heme displaces NCoR. Here, we found that heme artifactually influences detection of NCoR interaction in fluorescence-based assays. Using fluorescence-independent methods, including isothermal titration calorimetry, NMR spectroscopy, and XL-MS, we determined that heme remodels the thermodynamic profile of NCoR binding to REV-ERBβ ligand-binding domain (LBD) and directly increases LBD binding affinity for an NCoR interaction motif. We further report two crystal structures of REV-ERBβ LBD cobound to heme and NCoR peptides, which reveal the heme-dependent NCoR binding mode. By resolving previous contradictory biochemical, structural, and cellular studies, our findings should facilitate renewed progress toward understanding heme-dependent REV-ERB activity.