Heme mediates derepression of Maf recognition element through direct binding to transcription repressor Bach1

Abstract

Heme controls expression of genes involved in the synthesis of globins and heme. The mammalian transcription factor Bach1 functions as a repressor of the Maf recognition element (MARE) by forming antagonizing hetero-oligomers with the small Maf family proteins. We show here that heme binds specifically to Bach1 and regulates its DNA-binding activity. Deletion studies demonstrated that a heme-binding region of Bach1 is confined within its C-terminal region that possesses four dipeptide cysteine-proline (CP) motifs. Mutations in all of the CP motifs of Bach1 abolished its interaction with heme. The DNA-binding activity of Bach1 as a MafK hetero-oligomer was markedly inhibited by heme in gel mobility shift assays. The repressor activity of Bach1 was lost upon addition of hemin in transfected cells. These results suggest that increased levels of heme inactivate the repressor Bach1, resulting in induction of a host of genes with MARES:

Heme: a versatile signaling molecule controlling the activities of diverse regulators ranging from transcription factors to MAP kinases

“…MAREs are found in the regulatory regions of genes involved in heme metabolism, such as oxidative stress response genes, globin genes, heme oxygenase-1 and erythroid-specific ALAS [72,[78][79][80][81]. It has been suggested that the transcription of genes possessing MAREs is regulated via a balance between transcriptional activation and repression [22,80]. Under normal conditions, when expressed with small Maf proteins, Bach1 is located in the nucleus [77].…”

“…Bach1 has two functional domains: BTB/POZ and bZip, which regulate protein interaction and DNA binding, respectively ( Figure 2) [22,73]. Studies have revealed that Bach1 possesses six HRM (CP) motifs (see Figure 2A and 2B) that are essential for heme-mediated regulation [22,82].…”

“…Studies have revealed that Bach1 possesses six HRM (CP) motifs (see Figure 2A and 2B) that are essential for heme-mediated regulation [22,82]. Two HRM (CP) motifs are downstream of the BTB/ POZ domain, three are upstream of the bZip domain and one is downstream of the bZip domain [22,77]. Heme binding by Bach1 is dependent on the HRM (CP) motifs, and there is evidence suggesting that heme binding to the HRM (CP) motifs of Bach1 is cooperative [22].…”

“…Two HRM (CP) motifs are downstream of the BTB/ POZ domain, three are upstream of the bZip domain and one is downstream of the bZip domain [22,77]. Heme binding by Bach1 is dependent on the HRM (CP) motifs, and there is evidence suggesting that heme binding to the HRM (CP) motifs of Bach1 is cooperative [22]. No single HRM (CP) motif is indispensable for heme regulation of Bach1, indicating that there is functional redundancy among the HRM (CP) motifs [22].…”

“…As such, heme has been shown to control the activities of a variety of signal transducers and transcriptional regulators [17][18][19][20]. These include the iron regulatory protein (IRR) in the bacterium Bradyrhizobium japonicum [21], the yeast Saccharomyces cerevisiae transcriptional regulator, the heme activator protein Hap1 [14], the mammalian transcriptional repressor Bach1 [22], the heme-regulated inhibitor (HRI) kinase [23] and components of the Ras-MAPK signaling pathway ( Figure 1, Table 1) [17]. In this review, we aim to summarize the current knowledge of the molecular mechanisms by which heme controls the activities of these diverse signal transducers and regulators.…”