The maintenance of genome stability during mitosis is coordinated by the spindle assembly checkpoint (SAC) through its effector the mitotic checkpoint complex (MCC), an inhibitor of the anaphase-promoting complex (APC/C, also known as the cyclosome)^,. Unattached kinetochores control MCC assembly by catalysing a change in the topology of the β-sheet of MAD2 (an MCC subunit), thereby generating the active closed MAD2 (C-MAD2) conformer^{, –}. Disassembly of free MCC, which is required for SAC inactivation and chromosome segregation, is an ATP-dependent process driven by the AAA+ ATPase TRIP13. In combination with p31^comet, an SAC antagonist, TRIP13 remodels C-MAD2 into inactive open MAD2 (O-MAD2)^{, , –}. Here, we present a mechanism that explains how TRIP13–p31^comet disassembles the MCC. Cryo-electron microscopy structures of the TRIP13–p31^comet–C-MAD2–CDC20 complex reveal that p31^comet recruits C-MAD2 to a defined site on the TRIP13 hexameric ring, positioning the N terminus of C-MAD2 (MAD2^NT) to insert into the axial pore of TRIP13 and distorting the TRIP13 ring to initiate remodelling. Molecular modelling suggests that by gripping MAD2^NT within its axial pore, TRIP13 couples sequential ATP-driven translocation of its hexameric ring along MAD2^NT to push upwards on, and simultaneously rotate, the globular domains of the p31^comet–C-MAD2 complex. This unwinds a region of the αA helix of C-MAD2 that is required to stabilize the C-MAD2 β-sheet, thus destabilizing C-MAD2 in favour of O-MAD2 and dissociating MAD2 from p31^comet. Our study provides insights into how specific substrates are recruited to AAA+ ATPases through adaptor proteins and suggests a model of how translocation through the axial pore of AAA+ ATPases is coupled to protein remodelling.