Structural Mad-ness

The spindle checkpoint is turned on when unattached kinetochores generate a signal that inhibits the anaphase-promoting complex (APC) preventing premature chromosome separation. Now, Hongtao Yu and colleagues (University of Texas Southwestern Medical Center, Dallas, TX) have shown that conformational changes in the protein Mad2 are important in triggering this checkpoint pathway.

Monomeric Mad2 is recruited to the kinetochore when it forms a complex with Mad1. Mad2 can also associate with Cdc20, an APC activator, thereby inhibiting Cdc20 and the APC.

Mad2 is constitutively present in the cell, but does not always inhibit Cdc20. Yu set out to determine the basis of inhibition by analyzing the structure of bound and unbound Mad2. NMR structures of free Mad2 and Mad2 bound to a Mad2-binding peptide (MBP1) revealed a striking shift in the conformation of the protein upon peptide binding. A single β strand and the COOH terminus of the protein translocated from one end of the main β sheet to the other.

Binding of either Mad1 or Cdc20 caused similar conformational shifts in Mad2. According to Yu, the results suggest that Mad1 binding to Mad2 might help convert the protein to a form more compatible for binding to Cdc20, and that protein complexes containing Mad2 and Cdc20 may spread through the cell, dispersing the checkpoint signal. Yu would now like to identify how Mad2 changes its binding partner from Mad1 to Cdc20, especially given the fact that the two bind Mad2 with similar affinities. ▪

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