The Structural Maintenance of Chromosomes (SMC) complex family forms an essential part of the genome’s organisational machinery in all domains of life. In eukaryotes condensin is one such complex that mediates the packaging of DNA for segregation during cell division. This pentameric ATPase is comprised of two elongated Smc proteins (Smc2 and Smc4), a kleisin type protein (Brn1) and two HEAT repeat proteins (Ycg1 and Ycs4).

Despite the important role of condensin in such fundamental cell processes its overall structure, and consequently its molecular mechanism, are currently unknown. This study describes the processes leading to production of the first atomic resolution model of the Saccharomyces cerevisiae condensin complex in the ATP free apo state by cryo electronmicroscopy. This model demonstrates that several features such as a folded conformation in the Smc protein arms are common to multiple members of the SMC complex family. Furthermore, previously described structural features such as the location of the Ycs4 subunit in contact with the Smc protein head domains, and a discontinuity in the arms of the Smc proteins called the joint, are also described as part of the whole complex. Finally, the position of the Ycg1 subunit is also described as sitting flexibly under the Smc head domains, anchored by the Brn1 subunit to the rest of the complex.

Also described in this study is preliminary work to further characterise the biochemical properties of the condensin complex in the context of chromatin using the Lumix C-trap optical tweezer system. Condensins’ ability to compact DNA in real time and to translocate on a fixed DNA substrate in the C-trap system are demonstrated, as is ongoing work to chromatinise the same DNA substrate in situ. This chromatinisation involves the purification of recombinant S. cerevisiae histone octamers and histone chaperone Nap1 which have been successfully applied to DNA within the C-trap to create a chromatin substrate suitable for future experimentation.