In most metazoans two isoforms of the condensin complex, condensin I and II, are responsible for mitotic chromosome condensation. Condensin I and II are pentameric complexes, sharing structural maintenance of chromosome proteins SMC2 and SMC4, but differing for the composition of the kleisin and heat family proteins. Condensin I is more similar to the archetypical yeast condensin at a sequence level, but only accesses the genome during M phase when the nuclear envelop breaks down. In contrast, condensin II has a more divergent sequence and is present in the nucleus throughout the entire cell cycle. Despite the general architectural conservation, whether condensin I and II display different biochemical and biophysical properties remain to be seen.
We have reconstituted recombinant human condensin I and II holocomplexes and have assayed their specific biochemical properties. Using single molecule fluorescence approaches, we have visualised and quantified DNA loop formation, using different nuclei acid scaffolds. Furthermore, we obtained electron microscopy reconstructions of both human condensin I and II, unveiling the general architecture of the holocomplexes. Using this integrated structural biology approach we were able to highlight key similarities and differences in condensin I and II activity and structure, and provide new insights as to how condensin complexes interacts with chromatin.