Once a cell has successfully replicated its DNA, it must correctly segregate its chromosomes, a process guarded by the spindle assembly checkpoint. Sudakin et al. (page 925) have isolated and characterized a protein complex that appears to transmit an important signal in this checkpoint, findings that necessitate revising an earlier model. Tang et al. (Developmental Cell, 1:227–237) also examined the regulation of this checkpoint; the two papers are compared and discussed in a Comment by Hoyt (page 909).
Previous work suggested that unattached kinetochores caused the conversion of the Mad2 checkpoint protein to an activated form, which then bound to the anaphase-promoting complex or cyclosome (APC). In this model, interaction with activated Mad2 blocked the ubiquitin ligase activity of the APC, which is required for cells to complete mitosis. In the new work, Sudakin and colleagues purified an APC inhibitory factor, the MCC (mitotic checkpoint complex), from HeLa cells. The MCC, which includes Mad2 as well as BubR1, Bub3, and Cdc20, inhibits APC activity ∼3,000-fold more effectively than purified Mad2 alone, apparently by binding stoichiometrically to the APC. Tang and colleagues found that recombinant BubR1 alone is also a more potent inhibitor of the APC than Mad2 alone. Interestingly, Sudakin and colleagues found that active MCC is present in interphase cells as well as mitotic cells, but MCC only inhibits APC from mitotic cells.
The results suggest a new model in which unattached kinetochores sensitize the APC to inhibition by the MCC, rather than directly generating the inhibitor of the APC as previously thought. The nature of the signal generated by unattached kinetochores remains to be determined. ▪