Chapman-Smith Anne, Whitelaw Murray L
School of Molecular and Biomedical Science (Biochemistry), University of Adelaide, South Australia, Australia.
J Biol Chem. 2006 May 5;281(18):12535-45. doi: 10.1074/jbc.M512145200. Epub 2006 Mar 6.
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.
围绕二聚体转录因子基本螺旋-环-螺旋（bHLH）超家族的核心问题涉及如何实现伴侣选择特异性和DNA结合特异性。bHLH蛋白通过与螺旋-环-螺旋二聚化结构域相邻的碱性序列结合DNA。对于该家族中的两个亚组，二聚化进一步由相邻的Per-Arnt-Sim同源结构域（PAS）或亮氨酸拉链（LZ）结构域调控。我们提供的证据表明，对于bHLH.PAS转录因子二噁英受体（DR）和芳香烃受体核转运蛋白（Arnt），DR的PAS A结构域与bHLH区域存在独特的相互作用，这种相互作用支撑着二聚化强度以及对非典型E盒DNA序列的亲和力。一种PAS交换异二聚体，其中DR的bHLH结构域与Arnt的PAS A结构域融合，而Arnt的bHLH结构域与DR的PAS A结构域融合，表现出很强的DNA结合能力，但二聚化仅在天然组合时有效，这表明PAS A结构域通过不同机制对每个过程都至关重要。调节bHLH.LZ家族成员Myc和Max异二聚化的LZ结构域，在DR/Arnt异二聚体中，无论是二聚化还是DNA结合都无法取代PAS结构域。体外足迹分析表明，PAS结构域以与DNA弯曲一致的方式影响靶DNA的构象。这些结果为理解区分bHLH.PAS蛋白与更广泛的bHLH超家族的选择性二聚化和DNA相互作用机制提供了初步见解。
