p31^comet Blocks Mad2 Activation through Structural Mimicry

The status of spindle checkpoint signaling depends on the balance of two opposing dynamic processes that regulate the highly unusual two-state behavior of Mad2. In mitosis, a Mad1-Mad2 core complex recruits cytosolic Mad2 to kinetochores through Mad2 dimerization and converts Mad2 to a conformer amenable to Cdc20 binding, thereby facilitating checkpoint activation. p31^comet inactivates the checkpoint through binding to Mad1- or Cdc20-bound Mad2, thereby preventing Mad2 activation and promoting the dissociation of the Mad2-Cdc20 complex. Here, we report the crystal structure of the Mad2-p31^comet complex. The C-terminal region of Mad2 that undergoes rearrangement in different Mad2 conformers is a major structural determinant for p31^comet binding, explaining the specificity of p31^comet toward Mad1- or Cdc20-bound Mad2. p31^comet adopts a fold strikingly similar to that of Mad2 and binds at the dimerization interface of Mad2. Thus, p31^comet exploits the two-state behavior of Mad2 to block its activation by acting as an "anti-Mad2.".