C. elegans mixed stage embryo total RNA levels by microarray: L4440 RNAi, smo-1 RNAi and sdc-2 (y93)+RNAi

The essential process of dosage compensation equalizes X-chromosome gene expression between C. elegans XO males and XX hermaphrodites through a dosage compensation complex (DCC) that resembles condensin. The DCC binds to both X chromosomes of hermaphrodites to repress transcription by half. Here we show that post-translational modification by the SUMO conjugation pathway is essential for sex-specific assembly of the DCC onto X. Depletion of the SUMO peptide in vivo severely disrupts binding of particular DCC subunits and causes changes in X-linked gene expression similar to those caused by disrupting genes encoding DCC subunits. Three DCC subunits are themselves SUMOylated, and depletion of SUMO preferentially reduces their binding to X, suggesting that SUMOylation of DCC subunits is essential for robust association with X. DCC SUMOylation is triggered by the signal that initiates DCC assembly onto X. The initial step of assembly--binding of X-targeting factors to recruitment sites on X (rex sites)--is independent of SUMOylation, but robust binding of the complete complex requires SUMOylation. SUMOylated DCC subunits are enriched at rex sites, and SUMOylation enhances interactions between X-targeting factors and condensin subunits that facilitate DCC binding beyond the low level achieved without SUMOylation. DCC subunits also participate in condensin complexes essential for chromosome segregation, but their SUMOylation occurs only in the context of the DCC. Our results reinforce a newly emerging theme in which multiple proteins of a complex are SUMOylated in response to a specific stimulus, leading to accelerated complex formation and enhanced function.剂量补偿的核心过程通过一类类似凝集素（condensin）的剂量补偿复合物（dosage compensation complex, DCC），使秀丽隐杆线虫（C. elegans）XO型雄性与XX型雌雄同体的X染色体基因表达水平趋于均衡。该复合物可结合雌雄同体的两条X染色体，将转录活性抑制至原有水平的一半。本研究证实，经由SUMO结合通路（SUMO conjugation pathway）介导的翻译后修饰，对于DCC以性别特异性方式组装至X染色体至关重要。在活体中耗竭SUMO肽段会严重破坏特定DCC亚基的结合能力，并引发X连锁基因表达的改变，其表型与敲除编码DCC亚基的基因所导致的结果一致。有三个DCC亚基本身会发生SUMO化修饰，而耗竭SUMO会优先降低这些亚基与X染色体的结合能力，这表明DCC亚基的SUMO化对于其与X染色体的稳定结合不可或缺。DCC的SUMO化由启动DCC组装至X染色体的信号所触发。组装的初始步骤——即X靶向因子结合至X染色体上的招募位点（rex sites）——不依赖于SUMO化修饰，但完整复合物的稳定结合则需要SUMO化修饰。发生SUMO化的DCC亚基在rex位点处富集，且SUMO化能够增强X靶向因子与凝集素亚基之间的相互作用，从而使DCC的结合水平超越无SUMO化时的低水平状态。DCC亚基同时也参与染色体分离所必需的凝集素复合物，但其SUMO化仅发生在DCC复合物的语境中。本研究结果进一步印证了一个新兴的研究范式：复合物中的多种蛋白质可响应特定刺激而发生SUMO化，从而加速复合物的组装并增强其功能。