Condensin Smc2-Smc4 Dimers Are Flexible and Dynamic

Abstract

SummaryStructural maintenance of chromosomes (SMC) protein complexes, including cohesin and condensin, play key roles in the regulation of higher-order chromosome organization. Even though SMC proteins are thought to mechanistically determine the function of the complexes, their native conformations and dynamics have remained unclear. Here, we probe the topology of Smc2-Smc4 dimers of the S. cerevisiae condensin complex with high-speed atomic force microscopy (AFM) in liquid. We show that the Smc2-Smc4 coiled coils are highly flexible polymers with a persistence length of only ∼4 nm. Moreover, we demonstrate that the SMC dimers can adopt various architectures that interconvert dynamically over time, and we find that the SMC head domains engage not only with each other, but also with the hinge domain situated at the other end of the ∼45-nm-long coiled coil. Our findings reveal structural properties that provide insights into the molecular mechanics of condensin complexes.

Of Rings and Rods: Regulating Cohesin Entrapment of DNA to Generate Intra- and Intermolecular Tethers

“…Indeed, almost universal evidence from EM, FRET, chemical cross-linking, and atomic force microscopy (AFM) documents rodlike condensin complexes [22,37,38,42,43]. A notable exception comes from liquid AFM performed on in vivo–assembled but extracted and purified yeast condensins [44]. In that study, condensins were predominantly folded over to promote hinge–head associations, but rings and lassos (dissociated heads in which only one appeared bound to the hinge) were also observed.…”

“…In that study, condensins were predominantly folded over to promote hinge–head associations, but rings and lassos (dissociated heads in which only one appeared bound to the hinge) were also observed. Whether such intermediate structures are predicated on the dynamics of partially disrupted and flexible complexes (having survived extractions and enrichments) or represent functional cycles (despite conformation changes that occur in the absence of DNA and ATP) remains unknown [44]. Regardless, the similar degree of conservation among coiled coil domains within Smc1,3 cohesin, and Smc2,4 condensin family members is consistent with the notion that mutation is limited or constrained to preserve intermolecular binding along the entire coiled coil domain [45,46].…”

“…For instance, TEM images of tethered sister minichromosomes include rods that are wider than a single flattened cohesin complex, suggesting that cohesins oligomerize side by side [41]. Higher-order coiled coil assemblies are strongly supported by numerous AFM studies in which both cohesins and condensins clearly fold back on themselves (Fig 2), likely stabilized through tetrameric intermolecular coiled coil associations to produce complexes of roughly 25 nm in length with heads and hinges closely apposed [43,44,49]. Biochemical and cytological analyses document both Pds5 and SA1/Scc3 binding proximal to both ATPase head and hinge domains, consistent with cohesin folding and intermolecular coiled coil associations [21,23].…”

RotoStep: A Chromosome Dynamics Simulator Reveals Mechanisms of Loop Extrusion

“…3E). It is reported that condensin is very flexible (Eeftens et al 2016) (short persistence length, 4 nm), and thus may show bursts of directed motion in vivo, depending on the distribution of tethers along the chromosome.…”

“…There are 11 beads in the condensin holocomplex; each bead is ∼10 nm in diameter. Condensin is a very flexible molecule, showing a persistence length of ∼4 nm (Eeftens et al 2016), about one-tenth that of DNA (50 nm). The chains of the antiparallel coiled-coils fold back on one another to length of 45 nm.…”