Drosophila CAP-D2 is required for condensin complex stability and resolution of sister chromatids

The precise mechanism of chromosome condensation and decondensation remains a mystery, despite progress over the last 20 years aimed at identifying components essential to the mitotic compaction of the genome. In this study, we analyse the localization and role of the CAP-D2 non-SMC condensin subunit and its effect on the stability of the condensin complex. We demonstrate that a condensin complex exists in Drosophila embryos, containing CAP-D2, the anticipated SMC2 and SMC4 proteins, the CAP-H/Barren and CAP-G (non-SMC) subunits. We show that CAP-D2 is a nuclear protein throughout interphase, increasing in level during S phase, present on chromosome axes in mitosis, and still present on chromosomes as they start to decondense late in mitosis. We analysed the consequences of CAP-D2 loss after dsRNA-mediated interference, and discovered that the protein is essential for chromosome arm and centromere resolution. The loss of CAP-D2 after RNAi has additional downstream consequences on the stability of CAP-H, the localization of DNA topoisomerase II and other condensin subunits, and chromosome segregation. Finally, we discovered that even after interfering with two components important for chromosome architecture (DNA topoisomerase II and condensin), chromosomes were still able to compact, paving the way for the identification of further components or activities required for this essential process.