The Mad1/Mad2 Complex as a Template for Mad2 Activation in the Spindle Assembly Checkpoint

BACKGROUND The spindle assembly checkpoint (SAC) imparts fidelity to chromosome segregation by delaying anaphase until all sister chromatid pairs have become bipolarly attached. Mad2 is a component of the SAC effector complex that sequesters Cdc20 to halt anaphase. In prometaphase, Mad2 is recruited to kinetochores with the help of Mad1, and it is activated to bind Cdc20. These events are linked to the existence of two distinct conformers of Mad2: a closed conformer bound to its kinetochore receptor Mad1 or its target in the checkpoint Cdc20 and an open conformer unbound to these ligands. RESULTS We investigated the mechanism of Mad2 recruitment to the kinetochore during checkpoint activation and subsequent transfer to Cdc20. We report that a closed conformer of Mad2 constitutively bound to Mad1, rather than Mad1 itself, is the kinetochore receptor for cytosolic open Mad2 and show that the interaction of open and closed Mad2 conformers is essential to sustain the SAC. CONCLUSIONS We propose that closed Mad2 bound to Mad1 represents a template for the conversion of open Mad2 into closed Mad2 bound to Cdc20. This simple model, which we have named the "Mad2 template" model, predicts a mechanism for cytosolic propagation of the spindle checkpoint signal away from kinetochores.