Page "Spindle checkpoint" Paragraph 33

The leading model of MCC formation is the " MAD2-template model ", which depends on the kinetochore dynamics of MAD2 to create the MCC.

MAD1 localizes to unattached kinetochores while binding strongly to MAD2.

The localization of MAD2 and BubR1 to the kinetochore may also be dependent on the Aurora B kinase.

Cells lacking Aurora B fail to arrest in metaphase even when chromosomes lack microtubule attachment.

Unattached kinetochores first bind to a MAD1-C-MAD2-p31 < sup > comet </ sup > complex and releases the p31 < sup > comet </ sup > through unknown mechanisms.

The resulting MAD-C-MAD2 complex recruits the open conformer of Mad2 ( O-Mad2 ) to the kinetochores.

This O-Mad2 changes its conformation to closed Mad2 ( C-Mad2 ) and binds Mad1.

This Mad1 / C-Mad2 complex is responsible for the recruitment of more O-Mad2 to the kinetochores, which changes its conformation to C-Mad2 and binds Cdc20 in an auto-amplification reaction.

Since MAD1 and CDC20 both contain a similar MAD2-binding motif, the empty O-MAD2 conformation changes to C-MAD2 while binding to CDC20.

This positive feedback loop is negatively regulated by p31 < sup > comet </ sup >, which competitively binds to C-MAD2 bound to either MAD1 or CDC20 and reduces further O-MAD2 binding to C-MAD2.

Further control mechanisms may also exist, considering that p31 < sup > comet </ sup > is not present in lower eukaryotes.

The ' template model ' nomenclature is thus derived from the process where MAD1-C-MAD2 acts as a template for the formation of C-MAD2-CDC20 copies.

This sequestration of Cdc20 is essential for maintaining the spindle checkpoint.