The basic-helix-loop-helix-PAS orphan MOP3 forms transcriptionally active complexes with circadian and hypoxia factors

We report that MOP3 is a general dimerization partner for a subset of the basic-helix-loop-helix (bHLH)-PER-ARNT-SIM (PAS) superfamily of transcriptional regulators. We demonstrated that MOP3 interacts with MOP4, CLOCK, hypoxia inducible factor 1 alpha (HIF1 alpha), and HIF2 alpha. A DNA selection protocol revealed that the MOP3 MOP4 heterodimer bound a CACGTGA-containing DNA element, Transient transfection experiments demonstrated that the MOP3-MOP4 and MOP3-CLOCK complexes bound this element in COS-1 cells and drove transcription from a linked luciferase reporter gene, We also deduced the high-affinity DNA binding sites for MOP3-HIF1 alpha complex (TACGTGA) and used transient transfection experiments to demonstrate that the MOP3-HIF1 alpha and MOP3-HIF2 alpha heterodimers bound this element, drove transcription, and responded to cellular hypoxia. Finally. we found that MOP3 mRNA expression overlaps in a number of tissues with each of its four potential partner molecules in vivo.