The checkpoint protein Mad2 inhibits the activity of the anaphase promoting
complex by sequestering Cdc20 until all chromosomes are aligned at the metaphase
plate. We report the solution structure of human Mad2 and its interaction
with Cdc20. Mad2 possesses a novel three-layered / fold with
three -helices packed between two -sheets. Using deletion mutants
we identified the minimal Mad2-binding region of human Cdc20 as a 40-residue
segment immediately N-terminal to the WD40 repeats. Mutagenesis and NMR titration
experiments show that a C-terminal flexible region of Mad2 is required for
binding to Cdc20. Mad2 and Cdc20 form a tight 1:1 heterodimeric complex in
which the C-terminal segment of Mad2 becomes folded. These results provide
the first structural insight into mechanisms of the spindle assembly checkpoint.
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fold with
three 
