AFM images of open and collapsed states of yeast condensin suggest a scrunching model for DNA loop extrusion

Structural Maintenance of Chromosome (SMC) protein complexes are the key organizers of the spatiotemporal structure of chromosomes. The condensin SMC complex, which compacts DNA during mitosis, was recently shown to be a molecular motor that extrudes large loops of DNA. The mechanism of this unique motor, which takes large steps along DNA at low ATP consumption, remains elusive however. Here, we use Atomic Force Microscopy (AFM) to visualize the structure of yeast condensin and condensin-DNA complexes. Condensin is found to exhibit mainly open O-shapes and collapsed B-shapes, and it cycles dynamically between these two states over time. Condensin binds double-stranded DNA via a HEAT subunit and, surprisingly, also via the hinge domain. On extruded DNA loops, we observe a single condensin complex at the loop stem, where the neck size of the DNA loop correlates with the width of the condensin complex. Our results suggest that condensin extrudes DNA by a fast cyclic switching of its conformation between O and B shapes, consistent with a scrunching model.