Condensins, a class of multiprotein complexes, localise to the central axis of mitotic chromosomes and have essential roles in mitotic chromosome assembly and segregation. Studies have shown, however, that additional proteins must be involved in these processes, with Ki-67, a protein that localizes to the periphery of mitotic chromosomes, identified as a potential candidate. In their Research Article, Masatoshi Takagi and colleagues (Takagi et al., 2018) establish a set of cell lines in which Ki-67 and/or condensin subunits fused with an auxin-inducible degron can be depleted from human cells in order to investigate how Ki-67 might cooperate with condensins to build individual chromosomes. The authors first establish that the dynamic behavior of Ki-67 on the periphery of chromosomes does not depend on SMC2, a core ATPase subunit that is present in both condensins I and II, or on the chromatin structure supported by condensins. In the absence of both Ki-67 and SMC2, however, chromosomes rapidly lose their structural integrity following breakdown of the nuclear envelope. The authors report that the chromatin forms a ball-like cluster in which the shape and border of individual chromosomes is indiscernible. This phenotype is distinct from that observed in cells depleted of either Ki-67 or condensins alone, and the altered chromosome morphology is further validated by a machine-learning algorithm. These findings suggest that Ki-67 and condensins, which function independently, cooperate to support the integrity of mitotic chromosomes.
