Abstract
Condensin, a conserved member of the SMC protein family of ring‐shaped multi‐subunit protein complexes, is essential for structuring and compacting chromosomes. Despite its key role, its molecular mechanism has remained largely unknown. Here, we employ single‐molecule magnetic tweezers to measure, in real time, the compaction of individual DNA molecules by the budding yeast condensin complex. We show that compaction can proceed in large steps, driving DNA molecules into a fully condensed state against forces of up to 2 pN. Compaction can be reversed by applying high forces or adding buffer of high ionic strength. While condensin can stably bind DNA in the absence of ATP, ATP hydrolysis by the SMC subunits is required for rendering the association salt insensitive and for the subsequent compaction process. Our results indicate that the condensin reaction cycle involves two distinct steps, where condensin first binds DNA through electrostatic interactions before using ATP hydrolysis to encircle the DNA topologically within its ring structure, which initiates DNA compaction. The finding that both binding modes are essential for its DNA compaction activity has important implications for understanding the mechanism of chromosome compaction.
Highlights
The structural maintenance of chromosome (SMC) complexes cohesin and condensin play central roles in many aspects of chromosome biology, including the successful segregation of mitotic chromosomes, chromatin compaction and regulation of gene expression
Our results indicate that the condensin reaction cycle involves two distinct steps, where condensin first binds DNA through electrostatic interactions before using ATP hydrolysis to encircle the DNA topologically within its ring structure, which initiates DNA compaction
We further show that ATP hydrolysis is required to render the association with DNA into a salt-resistant topological binding mode, where the DNA is fully encircled by the condensin ring
Summary
The structural maintenance of chromosome (SMC) complexes cohesin and condensin play central roles in many aspects of chromosome biology, including the successful segregation of mitotic chromosomes, chromatin compaction and regulation of gene expression (reviewed in Hirano, 2016; Aragon et al, 2013; Nasmyth & Haering, 2009). SMC protein complexes are characterized by their unique ring-like structure (Fig 1A). The head domains of the Smc and Smc subunits are connected by a protein of the kleisin family, completing the ring-like structure (Fig 1A). The condensin kleisin subunit recruits two additional subunits that consist mainly of HEAT-repeat motifs. Most metazoan cells express two condensin complexes, condensin I and II, which contain different non-SMC subunits and make distinct contributions to the formation of mitotic chromosomes (Ono et al, 2003). The genome of the budding yeast Saccharomyces cerevisiae, encodes only a single condensin complex, which contains the kleisin subunit Brn and the HEAT-repeat subunits Ycg and Ycs (Fig 1A)
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