DHR51, the Drosophila melanogaster Homologue of the Human Photoreceptor Cell-Specific Nuclear Receptor, Is a Thiolate Heme-Binding Protein
Résumé
Heme has been recently described as a regulating ligand for the activity of the human nuclear receptors (NR) REV-ERB alpha and REV-ERB beta and their Drosophila homologue E75. Here, we report the cloning, expression in Escherichia coli, purification, and screening for the heme-binding ability of 11 NR ligand-binding domains of Drosophila melanogaster (DHR3, DHR4, DHR39, DHR51, DHR78, DHR83, HNF4, TLL, ERR, FTZ-F1, and E78), of unknown structure. One of these NRs, DHR51, homologous to the human photoreceptor cell-specific nuclear receptor (PNR), specifically binds heme and exhibits a UV-visible spectrum identical to that of heme-bound E75-LBD. EPR and UV-visible absorption spectroscopy indicates that, like in E75, the heme contains a hexa-coordinated low spin ferric iron. One of its axial ligands is a tightly bound cysteine, while the other one is a histidine. A dissociation constant of 0.5 mu M for the heme was measured by isothermal titration calorimetry. We show that DHR51 binds NO and CO and discuss the possibility that DHR51 may be either a gas or a heme sensor.