ABSTRACT The Spindle Assembly Checkpoint (SAC) is a signaling pathway responsible for the fidelity of chromosome segregation. Disruption of this process leads to catastrophic cellular consequences, such as aneuploidy and cancer. The primary effector of the SAC is the inhibitory complex known as Mitotic Checkpoint Complex (MCC). MCC is responsible for binding and inhibiting the 1.2 MDa ubiquitin ligase Anaphase-Promoting Complex/Cyclosome (APC). Once all of the sister chromatids achieve proper bipolar orientation, MCC is dismantled. This restores APC activity to trigger the ubiquitin-mediated proteasomal destruction of key mitotic regulators, e.g. Cyclin B and Securin, permitting mitotic exit. The mechanisms of release and disassembly of APC-bound MCC (BUBR1, MAD2, BUB3, and CDC20- an APC coactivator) by a triad of large multiprotein enzymes remain poorly understood. I aim to dissect this process using an innovative technological approach involving enzyme kinetics, chemical crosslinking, protein engineering, electron microscopy, NMR, crystallography, and cell-based assays. Information generated from the proposed research will have a long- lasting impact on the cell cycle field and may enable the development of novel cancer therapeutics.