Single-DNA Detection of Stepwise DNA Compaction by Cohesin SMC Protein Complexes

Abstract

Structural Maintenance of Chromosomes (SMC) protein complexes are essential for the precise folding and segregation of chromosomes during cell proliferation in both prokaryotes and eukaryotes. Cohesin SMC complexes are believed to hold sister chromatids together during chromosome segregation. However, how cohesins interact with DNA at the molecular level is still poorly understood. In our current study, we report that yeast cohesin SMC heterodimers condense single-DNA molecules by distinct steps in a force-dependent manner in the absence of ATP. The rate of condensation is supercoiling-dependent, and positive supercoiling of the DNA accelerates the condensation. We also observed stepwise DNA condensation by cohesin SMC complexes (with Scc1 kleisin subunits). Moreover, the DNA compaction by cohesin complexes is regulated by ATP, and requires the torsional rigidity of dsDNA. Our results suggest that cohesins may play a broader role as organizers of chromatin, possibly by defining “cross-linkers” or “loops” through the whole cell cycle.