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| Andrea Musacchio - | |
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Mad2 adopts a closed (C) conformation when bound to Mad1 or Cdc20, and an
open (O) conformation when unbound to these ligands. O-Mad2 is the
predominant form of Mad2 in the cytoplasm, and we have recently discovered
that Mad2 is recruited to the kinetochore as O-Mad2. Until now, the
spindle checkpoint protein Mad1 has been regarded as the Mad2 receptor at
the kinetochore. This view, however, is challenged by data from our
laboratory showing that an O-Mad2 mutant of Mad2 (Mad2DC) that is
completely unable to interact with Mad1 is effectively recruited to the
kinetochore. Because Mad1 and Mad2 form a tight complex that locks Mad2
in the C-Mad2 conformation, we have asked whether the Mad1-Mad2 complex,
rather than Mad1 itself, acted as the O-Mad2 kinetochore receptor. We
present compelling in vitro and in vivo data confirming this hypothesis.
In particular, we discovered that O-Mad2 and C-Mad2 engage in a tight
complex, which is responsible for the recruitment of O-Mad2 to the
Mad1-bound C-Mad2 at the kinetochore. Using appropriate mutants of Mad2,
we have also shown that the O-Mad2/C-Mad2 interaction is required to
sustain the checkpoint in HeLa cells. Thus, the dimerization of
conformationally distinct monomers of Mad2 is essential for propagation
of the checkpoint signal. Mad1, which is absolutely required to sustain
Mad2-Cdc20 complex formation, acts as a docking site for C-Mad2 at the
kinetochore. We also suggest that Mad1-bound C-Mad2 is a structural
template for the conversion of a cytoplasmic ligand-free form of O-Mad2
into Cdc20-bound C-Mad2, and that the latter acts as a structural copy
of Mad1/Mad2 designed to elicit further conversion of O-Mad2 into C-Mad2
away from kinetochores, leading to signal amplification.
Musacchio, A. & Hardwick, K. G. The spindle checkpoint: structural
insights into dynamic signalling. Nat Rev Mol Cell Biol 3, 731-41. (2002).
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