Functional Analysis of the Drosophila melanogaster condensin subunit Cap-G

• Applicant: Professor Dr. Stefan Heidmann
• Subject Area: Cell Biology
• Term: from 2009 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 157904188

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.

• DFG Programme: Research Grants