Rapid topo IIα movement revealed by photobleach recovery over time (left to right).

The world of topoisomerase II (topo II) is changing. Until now, topo II has been thought to be a major structural protein. But on page 31, Christensen et al. show that this is an unlikely possibility. Then, in another shot to the field, they demonstrate that the two isoforms of mammalian topo II (IIα and IIβ), argued to be functionally redundant, probably make different contributions during mitosis.

Topo II passes DNA strands through each other during DNA replication and separation of sister chromatids during mitosis. It has also been considered the major nonhistone component of the mitotic chromosome scaffold. In vivo evidence to support or refute the latter role has been lacking, in part because it has been tricky to localize mammalian topo II. For example, topo IIβ has been seen in association with chromosomes in some studies, but with a diffuse cytoplasmic localization in others. Disparate results have been blamed on antibody identity or differences in staining or fixation techniques.Christensen et al. have devised a method to view topo IIα and IIβ in living cells, thus avoiding artifacts introduced during the preparation of chromosome spreads. By coexpressing an mRNA with both a GFP chimera of the isoforms (which is deleterious when overexpressed) and a selection marker (which is deleterious when underexpressed), the group selected cell lines expressing topo II species at approximately physiological levels.

The GFP–topo II chimeras are surprisingly mobile during both interphase and mitosis. This argues against the view that topo II is an immobile support protein of a chromosome scaffold. Although mobile proteins can still sustain a dynamic scaffold, the extent of topo II turnover (50% of all topo IIα and IIβ molecules replaced every 1.8 and 3.0 s, respectively) suggests that this is unlikely.

The results also argue against redundancy. Althogh both isoforms accumulate in nucleoli during interphase, only topo IIα strongly associates with chromosomes during mitosis. Thus, topo IIβ may not be required for mitotic cell division. ▪