title: Novel DNA binding by a basic helix-loop-helix protein. The role of the dioxin receptor PAS domain.
abstract: Central issues surrounding the basic helix-loop-helix (bHLH) superfamily of dimeric transcription factors concern how specificity of partner selection and DNA binding are achieved. bHLH proteins bind DNA through the basic sequence that is contiguous with a helix-loop-helix dimerization domain. For the two subgroups within the family, dimerization is further regulated by an adjacent Per-Arnt-Sim homology (PAS) or leucine zipper (LZ) domain. We provide evidence that for the bHLH.PAS transcription factors Dioxin Receptor (DR) and Arnt, the DR PAS A domain has a unique interaction with the bHLH region that underpins both dimerization strength and affinity for an atypical E-box DNA sequence. A PAS swap heterodimer, where the DR bHLH domain was fused to Arnt PAS A and the Arnt bHLH fused to DR PAS A, gave strong DNA binding, but dimerization was only effective with the native arrangement, suggesting the PAS A domain is critical for each process via distinct mechanisms. LZ domains, which regulate heterodimerization for the bHLH.LZ family members Myc and Max, could not replace the PAS domains for either dimerization or DNA binding in the DR/Arnt heterodimer. In vitro footprinting revealed that the PAS domains influence the conformation of target DNA in a manner consistent with DNA bending. These results provide the first insights for understanding mechanisms of selective dimerization and DNA interaction that distinguish bHLH.PAS proteins from the broader bHLH superfamily.
authors.affiliation: School of Molecular and Biomedical Science (Biochemistry), University of Adelaide, South Australia, Australia. anne.chapmansmith@adelaide.edu.au
authors.forename: Anne
authors.initials: A
authors.lastname: Chapman-Smith
authors.forename: Murray L
authors.initials: ML
authors.lastname: Whitelaw
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D051784:Aryl Hydrocarbon Receptor Nuclear Translocator / Q000737:chemistry*; D001483:Base Sequence; D050976:Basic-Leucine Zipper Transcription Factors / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D006801:Humans; D007930:Leucine / Q000737:chemistry; D051379:Mice; D008969:Molecular Sequence Data; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D018336:Receptors, Aryl Hydrocarbon / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid
chemical_list: C497396:ARNT protein, human; C497397:Arnt protein, mouse; D050976:Basic-Leucine Zipper Transcription Factors; C067850:Myc associated factor X; D016271:Proto-Oncogene Proteins c-myc; D018336:Receptors, Aryl Hydrocarbon; D051784:Aryl Hydrocarbon Receptor Nuclear Translocator; D004247:DNA; D007930:Leucine