Mad2 exists in inactive (left) and active (right) forms.


Sequence dictates structure, but for Mad2, one structure is not enough. New results from groups led by Hongtao Yu and Josep Rizo (University of Texas Southwestern, Dallas, TX) show that Mad2 adopts two distinct conformations and that checkpoint activation may consist of switching Mad2 to the right form.Mad2 holds up anaphase until every chromosome is properly attached to the spindle. Past structural studies of Mad2 showed that Mad1 (its activator), and Cdc20 (its anaphase-halting target) share the same binding site. “It would seem,” says Rizo, “that [Mad1] would be a competitor rather than an activator because they bind in the same place.” But the newly identified second conformation suggests an answer to this puzzle–Mad1 may put Mad2 in an active conformation that is maintained even when the Mads separate.

The two forms can be distinguished based on column chromatography and NMR. The active form has a higher affinity for Cdc20 than does the other form and blocks anaphase in oocyte extracts. In vivo, sequestration of only the active form thwarts the checkpoint.

The forms interchange in vitro very slowly. A fragment of Mad1, however, accelerates transformation of the inactive into the active form. Some Mad2 is always complexed with Mad1 in cells, so the question now is what tells Mad1 to toss off Mad2 (in its active form) so it can bind to Cdc20. ▪


Luo, X., et al. 2004. Nat. Struct. Mol. Biol. 10.1038/nsmb748.