Yahi Hakima, Fritsch Lauriane, Philipot Ophelie, Guasconi Valentina, Souidi Mouloud, Robin Philippe, Polesskaya Anna, Losson Regine, Harel-Bellan Annick, Ait-Si-Ali Slimane
Institut André Lwoff, CNRS, FRE 2944, 7 rue Guy Moquet, Villejuif, France.
J Biol Chem. 2008 Aug 29;283(35):23692-700. doi: 10.1074/jbc.M802647200. Epub 2008 Jul 2.
Mechanisms of transcriptional repression are important during cell differentiation. Mammalian heterochromatin protein 1 isoforms HP1alpha, HP1beta, and HP1gamma play important roles in the regulation of chromatin structure and function. We explored the possibility of different roles for the three HP1 isoforms in an integrated system, skeletal muscle terminal differentiation. In this system, terminal differentiation is initiated by the transcription factor MyoD, whose target genes remain mainly silent until myoblasts are induced to differentiate. Here we show that HP1alpha and HP1beta isoforms, but not HP1gamma, interact with MyoD in myoblasts. This interaction is direct, as shown using recombinant proteins in vitro. A gene reporter assay revealed that HP1alpha and HP1beta, but not HP1gamma, inhibit MyoD transcriptional activity, suggesting a model in which MyoD could serve as a bridge between nucleosomes and chromatin-binding proteins such as HDACs and HP1. Chromatin immunoprecipitation assays show a preferential recruitment of HP1 proteins on MyoD target genes in proliferating myoblasts. Finally, modulation of HP1 protein level impairs MyoD target gene expression and muscle terminal differentiation. Together, our data show a nonconventional interaction between HP1 and a tissue-specific transcription factor, MyoD. In addition, they strongly suggest that HP1 isoforms play important roles during muscle terminal differentiation in an isoform-dependent manner.
转录抑制机制在细胞分化过程中至关重要。哺乳动物异染色质蛋白1亚型HP1α、HP1β和HP1γ在染色质结构和功能的调节中发挥重要作用。我们探讨了这三种HP1亚型在骨骼肌终末分化这一整合系统中发挥不同作用的可能性。在这个系统中，终末分化由转录因子MyoD启动，其靶基因在成肌细胞被诱导分化之前主要保持沉默。在此我们表明，HP1α和HP1β亚型而非HP1γ在成肌细胞中与MyoD相互作用。如体外使用重组蛋白所显示，这种相互作用是直接的。基因报告分析表明，HP1α和HP1β而非HP1γ抑制MyoD的转录活性，提示一种模型，即MyoD可作为核小体与诸如HDACs和HP1等染色质结合蛋白之间的桥梁。染色质免疫沉淀分析显示，在增殖的成肌细胞中，HP1蛋白优先募集到MyoD靶基因上。最后，HP1蛋白水平的调节损害MyoD靶基因表达和肌肉终末分化。总之，我们的数据显示了HP1与组织特异性转录因子MyoD之间的一种非常规相互作用。此外，它们强烈提示HP1亚型在肌肉终末分化过程中以亚型依赖的方式发挥重要作用。
