Kinetic analysis of Mad2-Cdc20 formation: conformational changes in Mad2 are catalyzed by a C-Mad2-ligand complex

Structural changes in the mitotic arrest deficient protein 2 (Mad2) have been proposed to be essential for spindle checkpoint function. Current models for checkpoint activation propose that a C-Mad2-Mad1 core complex at unattached kinetochores is required for the structural activation through a process involving the interaction of two Mad2 conformers: a closed conformer bound to Mad1 or Cdc20 and an open conformer unbound to these ligands. To gain a molecular understanding of the mechanisms that accelerate the structural transition between the open and closed Mad2 conformations, we constructed a unique in vitro homogeneous Mad2 activity assay that specifically reports C-Mad2-Cdc20 formation. Using this assay we were are able to directly establish that (a) O-Mad2 transforms into a C-Mad2-Cdc20 complex >300-fold slower than unliganded C-Mad2, (b) a stable C-Mad2-Mad1 core complex catalyzes the transformation of O-Mad2 into a Cdc20-bound C-Mad2 complex, (c) a C-Mad2-Cdc20 complex can promote its own transformation of O-Mad2 into a Cdc20-bound C-Mad2 complex, and (d) the binding interaction between unliganded C-Mad2 and Cdc20 cannot be catalyzed by a C-Mad2-Mad1 core complex. Our data are consistent with the "Mad2 template" catalytic model in which a C-Mad2 template facilitates the binding of O-Mad2 to Cdc20 and supports a mechanism of C-Mad2-Cdc20 formation away from Mad1 containing kinetochores. Furthermore, our unique homogeneous Mad2 assay could be translated into a screening platform to identify small molecule drug-like compounds that directly modulate C-Mad2-Cdc20 formation.