The kleisin and HEAT-repeat subunits have recently been shown to bind DNA in a unique safety belt arrangement ( Kschonsak et al., 2017), and the complexes can progressively move on DNA as motors in vitro ( Terakawa et al., 2017), which is consistent with the hypothesis that they actively form and stabilize DNA loops ( Nasmyth, 2001 Alipour and Marko, 2012 Goloborodko et al., 2016a, b).Įxtensive structural, biochemical, cell biological, and molecular biological research over the last two decades led to numerous models about how Condensins may shape mitotic chromosomes ( Cuylen and Haering, 2011 Hirano, 2012, 2016 Kschonsak and Haering, 2015 Piskadlo and Oliveira, 2016 Uhlmann, 2016 Kalitsis et al., 2017 Kinoshita and Hirano, 2017). The ATPase domains are bridged by the kleisin and associated HEAT-repeat subunits to form a pentameric ring-like architecture with an estimated length of overall ∼60 nm for the human complexes ( Anderson et al., 2002). SMC2 and SMC4 are backfolded into long coiled-coils, bringing their N and C termini together into two ATPase domains, and are connected at their central domains, creating a “hinge” between the two subunits.

Condensins consist of two shared subunits (SMC2 and SMC4) and three isoform-specific subunits: a kleisin (CAP-H or CAP-H2) and two HEAT-repeat proteins (CAP-D2 or CAP-D3 and CAP-G or CAP-G2). This process of mitotic chromosome condensation is essential for faithful genome partitioning ( Hudson et al., 2009) and involves two conserved structural maintenance of chromosomes (SMC) protein complexes, Condensins I and II ( Hirano and Mitchison, 1994 Strunnikov et al., 1995 Hirano et al., 1997 Ono et al., 2003 Yeong et al., 2003). Based on our comprehensive quantitative data, we propose a three-step hierarchical loop model of mitotic chromosome compaction: Condensin II initially fixes loops of a maximum size of ∼450 kb at the chromatid axis, whose size is then reduced by Condensin I binding to ∼90 kb in prometaphase and ∼70 kb in anaphase, achieving maximum chromosome compaction upon sister chromatid segregation.Ī fundamental structural and functional change of the human genome is the compaction of replicated interphase chromatin into rod-shaped mitotic chromosomes. The two Condensins rarely colocalize at the chromatid axis, where Condensin II is centrally confined, but Condensin I reaches ∼50% of the chromatid diameter from its center. Although ∼35,000 Condensin II complexes are stably bound to chromosomes throughout mitosis, ∼195,000 Condensin I complexes dynamically bind in two steps: prometaphase and early anaphase.

We used homozygous genome editing to fluorescently tag Condensin I and II subunits and mapped their absolute abundance, spacing, and dynamic localization during mitosis by fluorescence correlation spectroscopy (FSC)–calibrated live-cell imaging and superresolution microscopy.

The two Condensin complexes in human cells are essential for mitotic chromosome structure.

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