Functional Analysis of the Drosophila melanogaster condensin subunit Cap-G

在细胞周期中染色体的精确复制和分离是确保个体生物和整个种群遗传稳定性的先决条件。在参与调节这些过程的许多成分中，一种称为凝聚蛋白的蛋白质复合物在有丝分裂染色体的形成中起着至关重要的作用，从而使它们能够忠实地分布。许多生物体含有两种凝缩蛋白复合物（凝缩蛋白I和凝缩蛋白II），它们都被证明是精确的染色体分布所必需的。在果蝇黑腹果蝇中，凝缩素II似乎缺少一种叫做Cap-G2的成分。我们已经表明，缩合蛋白I的相应成分（Cap-G）似乎并不接管Cap-G2的作用，也不参与缩合蛋白II的组装。由于我们的研究结果进一步质疑了苍蝇体内可溶凝缩蛋白II复合物的存在，因此果蝇体内的凝缩蛋白I是否接管了其他生物体中凝缩蛋白II的所有功能，这是一个悬而未决的问题。在脊椎动物中，凝缩蛋白II已被证明主要存在于细胞周期间期的细胞核中。它的功能之一是在DNA合成阶段启动已复制的姐妹染色单体的分解。由于在果蝇中，Cap-G是间期唯一核富集的凝缩蛋白i特异性亚基，我们的目的是分析Cap-G单独或与其他凝缩蛋白亚基结合，是否有助于分离已经在s期重复的姐妹染色单体。此外，我们最近对Cap-G与其他蛋白的关联研究表明，Brahma （Brm）和Moira （Moi）蛋白与Cap-G特异性富集。Brm和Moi是SWI/SNF染色质重塑复合体的组成部分，这是一种分子机器，有助于染色质的重组，使其更容易被转录。我们打算阐明在果蝇中SWI/SNF作用是否对凝缩蛋白有功能依赖性，反之亦然。令人惊讶的是，即使缺少374个氨基酸，即使这种变体不再局限于细胞核，但基本的Cap-G的c端截断变体仍然支持活蝇的发育。然而，仅表达这种截断的Cap-G变体的母亲产生的胚胎表现出严重降低的生存能力。我们的目的是进一步剖析这种表型，以明确回答Cap-G的核定位是否需要通过早期胚胎周期的忠实过渡。

The accurate duplication and segregation of chromosomes during cell cycles is a prerequisite for ensuring genetic stability within an individual organism and entire populations. Among the many components involved in regulating these processes, a protein complex called condensin plays a crucial role in shaping mitotic chromosomes, so that they can be faithfully distributed. Many organisms contain two of these condensin complexes (condensin I and II), which both have been shown to be required for accurate chromosome distribution. In the fly Drosophila melanogaster, condensin II appears to lack one of its components, called Cap-G2. We have shown that the corresponding component of condensin I (Cap-G) does not seem to take over the role of Cap-G2 and does not participate in the assembly of condensin II. As our results furthermore questioned the very existence of a soluble condensin II complex in flies, it is an open question whether condensin I in Drosophila takes over all the functions of condensin II described in other organisms. In vertebrates, condensin II has been shown to reside primarily in the nucleus during interphase of the cell cycle. One of its functions is to initiate resolution of replicated sister chromatids already during the phase of DNA synthesis. As in Drosophila Cap-G is the only clearly nuclearly enriched condensin I-specifc subunit during interphase, we aim to analyze whether Cap-G, alone or in combination with other condensin subunits, helps to disengage the intertwined sister chromatids after their duplication already in S-phase. Furthermore, our recent association studies of Cap-G with other proteins have revealed that the proteins Brahma (Brm) and Moira (Moi) are specifically enriched together with Cap-G. Brm and Moi are components of the SWI/SNF chromatin remodelling complex, a molecular machine, which helps to restructure chromatin in order to make it more accessible for transcription. We propose to elucidate whether there is a functional dependence of SWI/SNF action on condensin, or vice versa, in Drosophila. Surprisingly, a substantially C-terminally truncated variant of the essential Cap-G still supports development of living flies, even though 374 amino acids are lacking and even though this variant no longer localizes to the nucleus. However, embryos produced by mothers expressing solely this truncated Cap-G variant exhibit a severely reduced viability. We aim at dissecting this phenotype further to specifically answer the question whether it is the nuclear localization of Cap-G which is required for a faithful transition through the early embryonic cycles.