A Role for the CAL1-Partner Modulo in Centromere Integrity and Accurate Chromosome Segregation in Drosophila

The relationship between the nucleolus and the centromere, although documented, remains one of the most elusive aspects of centromere assembly and maintenance. Here we identify the nucleolar protein, Modulo, in complex with CAL1, a factor essential for the centromeric deposition of the centromere-specific histone H3 variant, CID, in Drosophila. Notably, CAL1 localizes to both centromeres and the nucleolus. Depletion of Modulo, by RNAi, results in defective recruitment of newly-synthesized CAL1 at the centromere. Furthermore, depletion of Modulo negatively affects levels of CID at the centromere and results in chromosome missegregation. Interestingly, examination of Modulo localization during mitosis reveals it localizes to the chromosome periphery but not the centromere. Combined, the data suggest that rather than a direct regulatory role at the centromere, it is the nucleolar function of modulo which is regulating the assembly of the centromere by directing the localization of CAL1. We propose that a functional link between the nucleolus and centromere assembly exists in Drosophila, which is regulated by Modulo.核仁 (nucleolus)与着丝粒 (centromere)之间的关联尽管已有文献记载，但仍是着丝粒组装与维持过程中最难以捉摸的方面之一。本研究鉴定出核仁蛋白Modulo与CAL1形成复合物，其中CAL1是果蝇中着丝粒特异性组蛋白H3变体 (centromere-specific histone H3 variant) CID的着丝粒沉积所必需的因子。值得注意的是，CAL1同时定位于着丝粒与核仁。通过RNA干扰（RNAi）敲低Modulo的表达，会导致新合成的CAL1在着丝粒处的招募出现缺陷。此外，敲低Modulo会降低着丝粒处的CID水平，并引发染色体分离错误。有趣的是，对有丝分裂过程中Modulo的定位分析显示，其仅定位于染色体周边，而非着丝粒。综合上述实验数据，本研究表明，Modulo并非在着丝粒处发挥直接调控作用，而是通过其核仁功能，调控CAL1的定位，进而参与着丝粒组装过程。我们提出，果蝇中核仁与着丝粒组装之间存在功能性关联，且该关联受Modulo调控。