Molecular dissection of condensin II-mediated chromosome assembly using in vitro assays

Chromosome segregation: Brushing up on centromeres

Turning centromere DNA into a mechanical spring is central to the fidelity of chromosome segregation. A recent study shows how centromere DNA loops and partitioning cohesin and condensin convert centromeres and pericentromeres into bipartite bottlebrushes.

Regulation of condensin II by self-suppression and release mechanisms

In vertebrates, two distinct condensin complexes, condensin I and condensin II, cooperate to drive mitotic chromosome assembly. It remains largely unknown how the two complexes differentially contribute to this process at a mechanistic level. We have previously dissected the role of individual subunits of condensin II by introducing recombinant complexes into Xenopus egg extracts. Here we extend these efforts by introducing a modified functional assay using extracts depleted of topoisomerase IIα (topo IIα), which allows us to further elucidate the functional similarities and differences between condensin I and condensin II. The intrinsically disordered C-terminal region of the CAP-D3 subunit (the D3 C-tail) is a major target of Cdk1 phosphorylation, and phosphorylation-deficient mutations in this region impair condensin II functions. We also identify a unique helical structure in CAP-D3 (the D3 HEAT docker) that is predicted to directly interact with CAP-G2. Deletion of the D3 HEAT docker, along with the D3 C-tail, enhances the ability of condensin II to assemble mitotic chromosomes. Taken together, we propose a self-suppression mechanism unique to condensin II that is released by mitotic phosphorylation. Evolutionary implications of our findings are also discussed.

Regulation of the mitotic chromosome folding machines

Over the last several years enormous progress has been made in identifying the molecular machines, including condensins and topoisomerases that fold mitotic chromosomes. The discovery that condensins generate chromatin loops through loop extrusion has revolutionized, and energized, the field of chromosome folding. To understand how these machines fold chromosomes with the appropriate dimensions, while disentangling sister chromatids, it needs to be determined how they are regulated and deployed. Here, we outline the current understanding of how these machines and factors are regulated through cell cycle dependent expression, chromatin localization, activation and inactivation through post-translational modifications, and through associations with each other, with other factors and with the chromatin template itself. There are still many open questions about how condensins and topoisomerases are regulated but given the pace of progress in the chromosome folding field, it seems likely that many of these will be answered in the years ahead.

Cell cycle-specific loading of condensin I is regulated by the N-terminal tail of its kleisin subunit

Condensin I is a pentameric protein complex that plays an essential role in mitotic chromosome assembly in eukaryotic cells. Although it has been shown that condensin I loading is mitosis specific, it remains poorly understood how the robust cell cycle regulation of condensin I is achieved. Here, we set up a panel of in vitro assays to demonstrate that cell cycle-specific loading of condensin I is regulated by the N-terminal tail (N-tail) of its kleisin subunit CAP-H. Deletion of the N-tail accelerates condensin I loading and chromosome assembly in Xenopus egg mitotic extracts. Phosphorylation-deficient and phosphorylation-mimetic mutations in the CAP-H N-tail decelerate and accelerate condensin I loading, respectively. Remarkably, deletion of the N-tail enables condensin I to assemble mitotic chromosome-like structures even in interphase extracts. Together with other extract-free functional assays in vitro, our results uncover one of the multilayered mechanisms that ensure cell cycle-specific loading of condensin I onto chromosomes.