Our work establishes a general framework of how the spindle checkpoint ensures the fidelity of chromosome segregation. In this framework, kinetochores of chromatids that are not properly captured by microtubules recruit and activate the Mad1-3 and Bub1-3 proteins, which collaborate to inhibit Cdc20, the mitotic activator of APC/C, and delay anaphase onset. A regulated conformational switch of Mad2 is crucial for its activation.
Using both NMR and X-ray crystallography, we have determined the structures of the multiple conformers of Mad2 and Mad2 in complex with its inhibitor p31comet and showed that the conformational switch of Mad2 is regulated by Mad1 binding and by phosphorylation. These studies have contributed significantly to our understanding of the mechanisms of chromosome segregation.
We would like to continue the structural and functional studies of other important spindle checkpoint proteins. For example, the checkpoint kinase Bub1 has multiple, important roles during mitosis. The major goal of our current project is to focus on the structure, regulation, and function of human Bub1, which will greatly advance our understanding of chromosome segregation.