Department of Biophysics and Biochemistry, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
EMBO J. 2012 May 30;31(11):2541-52. doi: 10.1038/emboj.2012.77. Epub 2012 Mar 27.
Helix-loop-helix (HLH) family transcription factors regulate numerous developmental and homeostatic processes. Dominant-negative HLH (dnHLH) proteins lack DNA-binding ability and capture basic HLH (bHLH) transcription factors to inhibit cellular differentiation and enhance cell proliferation and motility, thus participating in patho-physiological processes. We report the first structure of a free-standing human dnHLH protein, HHM (Human homologue of murine maternal Id-like molecule). HHM adopts a V-shaped conformation, with N-terminal and C-terminal five-helix bundles connected by the HLH region. In striking contrast to the common HLH, the HLH region in HHM is extended, with its hydrophobic dimerization interfaces embedded in the N- and C-terminal helix bundles. Biochemical and physicochemical analyses revealed that HHM exists in slow equilibrium between this V-shaped form and the partially unfolded, relaxed form. The latter form is readily available for interactions with its target bHLH transcription factors. Mutations disrupting the interactions in the V-shaped form compromised the target transcription factor specificity and accelerated myogenic cell differentiation. Therefore, the V-shaped form of HHM may represent an autoinhibited state, and the dynamic conformational equilibrium may control the target specificity.
螺旋-环-螺旋 (HLH) 家族转录因子调节许多发育和稳态过程。显性负性 HLH (dnHLH) 蛋白缺乏 DNA 结合能力，并捕获基本 HLH (bHLH) 转录因子，抑制细胞分化，增强细胞增殖和迁移，从而参与病理生理过程。我们报告了第一个游离态人源 dnHLH 蛋白 HHM（鼠源性母体 ID 样分子的人同源物）的结构。HHM 采用 V 形构象，由 N 端和 C 端五螺旋束通过 HLH 区连接。与常见的 HLH 形成鲜明对比的是，HHM 的 HLH 区被延伸，其疏水二聚化界面嵌入 N 端和 C 端螺旋束中。生化和物理化学分析表明，HHM 在这种 V 形构象和部分展开的松弛构象之间处于缓慢平衡。后一种形式很容易与目标 bHLH 转录因子相互作用。破坏 V 形构象相互作用的突变会损害目标转录因子的特异性并加速成肌细胞分化。因此，HHM 的 V 形构象可能代表一种自动抑制状态，而动态构象平衡可能控制靶特异性。
