Human Topoisomerase IIα Promotes Chromatin Condensation Via a Phase Transition

Topoisomerase II (topo II) enzymes are essential enzymes known to resolve topological entanglements during DNA processing. Curiously, while yeast expresses a single topo II, humans express two topo II isozymes, topo II and topo II{beta}, which share a similar catalytic domain but differ in their intrinsically disordered C-terminal domains (CTDs). During mitosis, topo II and condensin I constitute the most abundant chromosome scaffolding proteins essential for chromosome condensation. However, how topo II enables this function is poorly understood. Here, we discovered a new and functionally distinct role for human topo II - it condenses DNA and chromatin at a low topo II concentration (100 pM or less) during a polymer-collapse phase transition. The removal of the topo II CTDs effectively abolishes its condensation ability, indicating that the condensation is mediated by the CTDs. Although topo II{beta} can also perform condensation, it is about 4-fold less effective. During the condensation, topo II-DNA condensates form along DNA, working against a DNA tension of up to 1.5 pN, greater than that previously reported for yeast condensin. In addition, this condensation does not require ATP and thus is independent of topo IIs catalytic activity. We also found that condensation and catalysis can concurrently proceed with minimal mutual interference. Our findings suggest topo II may directly participate in chromosome condensation during mitosis.