Structure of a dominant-negative helix-loop-helix transcriptional regulator suggests mechanisms of autoinhibition

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 freestanding 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. The EMBO Journal (2012) 31, 2541-2552. doi: 10.1038/emboj.2012.77; Published online 27 March 2012

螺旋-环-螺旋（HLH）家族转录因子调控众多发育和内稳态过程。显性负性HLH（dnHLH）蛋白缺乏DNA结合能力，并且会捕获碱性HLH（bHLH）转录因子以抑制细胞分化并增强细胞增殖和运动性，从而参与病理生理过程。我们报道了一种独立的人类dnHLH蛋白HHM（小鼠母体Id样分子的人类同源物）的首个结构。HHM呈V形构象，其N端和C端的五螺旋束通过HLH区域相连。与常见的HLH形成鲜明对比的是，HHM中的HLH区域是延伸的，其疏水二聚化界面嵌入在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形形式和部分展开的松弛形式之间存在缓慢的平衡。后一种形式易于与其靶标bHLH转录因子相互作用。破坏V形形式中相互作用的突变会损害靶标转录因子特异性并加速肌原细胞分化。因此，HHM的V形形式可能代表一种自身抑制状态，并且动态构象平衡可能控制靶标特异性。《欧洲分子生物学组织杂志》（2012年）31卷，2541 - 2552页。doi:10.1038/emboj.2012.77；2012年3月27日在线发表