SummaryCondensin protein complexes coordinate the formation of mitotic chromosomes and thereby ensure the successful segregation of replicated genomes. Insights into how condensin complexes bind to chromosomes and alter their topology are essential for understanding the molecular principles behind the large-scale chromatin rearrangements that take place during cell divisions. Here, we identify a direct DNA-binding site in the eukaryotic condensin complex, which is formed by its Ycg1Cnd3 HEAT-repeat and Brn1Cnd2 kleisin subunits. DNA co-crystal structures reveal a conserved, positively charged groove that accommodates the DNA double helix. A peptide loop of the kleisin subunit encircles the bound DNA and, like a safety belt, prevents its dissociation. Firm closure of the kleisin loop around DNA is essential for the association of condensin complexes with chromosomes and their DNA-stimulated ATPase activity. Our data suggest a sophisticated molecular basis for anchoring condensin complexes to chromosomes that enables the formation of large-sized chromatin loops.

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Highlights d Smc4 and Smc2 ATPase head structures reorganize upon ATP binding and dimerization d A Q-loop-mediated switch releases the Ycs4 HEAT-repeat subunit from the Smc4 head d The Smc2 head engages with the ATP-bound Smc4 head into an asymmetric heterodimer d Head dimerization releases the Brn1 kleisin from Smc2 via coiled-coil rotation

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