The anaphase-promoting complex

In addition to driving the events of M phase, MPF also triggers its own destruction by activating the anaphase-promoting complex/cyclosome (APC/C), a protein complex that causes M cyclins to be destroyed starting in anaphase. The destruction of M cyclins pushes the cell out of mitosis, allowing the new daughter cells to enter Gstart subscript, 1, end subscript. The APC/C also causes destruction of the proteins that hold the sister chromatids together, allowing them to separate in anaphase and move to opposite poles of the cell.
How does the APC/C do its job? Like a Cdk, the APC/C is an enzyme, but it has different type of function than a Cdk. Rather than attaching a phosphate group to its targets, it adds a small protein tag called ubiquitin (Ub). When a target is tagged with ubiquitin, it is sent to the proteasome, which can be thought of as the recycle bin of the cell, and destroyed. For example, the APC/C attaches a ubiquitin tag to M cyclins, causing them to be chopped up by the proteasome and allowing the newly forming daughter cells to enter Gstart subscript, 1, end subscript phasestart superscript, 8, end superscript.
The APC/C also uses ubiquitin tagging to trigger the separation of sister chromatids during mitosis. If the APC/C gets the right signals at metaphase, it sets off a chain of events that destroys cohesin, the protein glue that holds sister chromatids togetherstart superscript, 8, comma, 9, end superscript.
  • The APC/C first adds a ubiquitin tag to a protein called securin, sending it for recycling. Securin normally binds to, and inactivates, a protein called separase.
  • When securin is sent for recycling, separase becomes active and can do its job. Separase chops up the cohesin that holds sister chromatids together, allowing them to separate.

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