Untangling Topo II’s function at mitotic centromeres

Sister chromatids get tangled up during DNA replication and so, before they can be segregated during mitosis, they must fi rst be untangled by the enzyme topoisomerase II (Topo II). Surprisingly, even though it isn't required for the enzyme's localization or catalytic activity, Topo II's C-terminal domain (CTD) is also required for accurate chromosome segregation. Two groups now reveal that this may be because Topo II's C terminus helps recruit the checkpoint ki-nase Aurora B to mitotic centromeres (1, 2). As part of the chromosomal passenger complex (CPC), Aurora B localizes to mi-totic centromeres, where it helps to activate the spindle assembly checkpoint if spindle microtubules are improperly attached to ki-netochores and tension levels are abnormally low. Topo II accumulates at centromeres during mitosis, and studies have suggested that it can regulate Aurora B activity under certain conditions (3, 4). Yoshiaki Azuma's lab at the University of Kansas previously found that Topo II's CTD is SUMOylated during mitosis (5), allowing it to recruit a protein called Claspin to mi-totic centromeres (6). Claspin, in turn, binds to an activator of Aurora B called Chk1, and, accordingly, Aurora B's activity was reduced when Azuma and colleagues , led by graduate student Makoto Yoshida, inhibited the mitotic SUMOylation of Topo II's CTD in Xenopus egg extract. But Aurora B's centromeric localization was also reduced, suggesting that Topo II's CTD might bind to additional proteins involved in Aurora B recruitment (1). Yoshida et al. performed a series of pulldown experiments and found that, when SUMOylated during mitosis, Topo II's CTD can bind to Haspin, a kinase that phosphorylates histone H3 to create a binding site for the CPC on centromeric chromatin. This interaction depended on two SUMO-interacting motifs in Haspin's N-terminal domain, as well as the mitosis-specifi c phosphorylation of a nearby thre-onine residue. " So the interaction is cell cycle dependent because two modifi ca-tions—SUMOylation of Topo II and phos-phorylation of Haspin—are required to form a stable association between the two proteins, " Azuma explains. Blocking this association impaired Haspin's recruitment to mitotic centromeres in Xenopus egg extract, with a corresponding decrease in H3 phosphoryla-tion that would explain why Aurora B's centromeric local-ization is reduced in the absence of Topo II SUMOylation. Meanwhile, at the University of Minnesota, Duncan Clarke and colleagues found that Topo II's CTD, and its SUMOylation during mitosis, was also required to recruit Aurora B to mitotic centro-meres in …