Mapping the checkpoint

The spindle checkpoint blocks exit from mitosis until all kinetochores are bound by spindle microtubules. At that point, Cdc20 is released by the checkpoint proteins and activates the anaphase promoting complex (APC), a ubiquitin ligase, to initiate anaphase. On page 61, Casaletto et al. identify Xnf7 as the first APC regulator known to bind the core APC components, rather than Cdc20. On page 49, Kops et al. show that Zwint-1 acts as a physical bridge between the structural kinetochore proteins and other checkpoint proteins, including those implicated in sequestering Cdc20.

While looking for a regulator of cyclin B2, which is one of the major targets of the APC, Casaletto et al. identified Xnf7. Immunodepletion of Xnf7 accelerated meiotic exit after Ca²⁺ addition, which released a meiotic block in Xenopus egg extracts....