The NuRD nucleosome remodelling complex and NHK-1 kinase are required for chromosome condensation in oocytes

Chromosome condensation during cell division is one of the most dramatic events in the cell cycle. Condensin and topoisomerase II are the most studied factors in chromosome condensation. However, their inactivation leads to only mild defects and little is known about the roles of other factors. Here, we took advantage of Drosophila oocytes to elucidate the roles of potential condensation factors by performing RNA interference (RNAi). Consistent with previous studies, depletion of condensin I subunits or topoisomerase II in oocytes only mildly affected chromosome condensation. In contrast, we found severe undercondensation of chromosomes after depletion of the Mi-2-containing NuRD nucleosome remodelling complex or the protein kinase NHK-1 (also known as Ballchen in Drosophila). The further phenotypic analysis suggests that Mi-2 and NHK-1 are involved in different pathways of chromosome condensation. We show that the main role of NHK-1 in chromosome condensation is to phosphorylate Barrier-to-autointegration factor (BAF) and suppress its activity in linking chromosomes to nuclear envelope proteins. We further show that NHK-1 is important for chromosome condensation during mitosis as well as in oocytes.