Molecule of the Month: Anaphase-Promoting Complex / Cyclosome

As cells grow and divide, they must pass through several checkpoints to ensure that new cells are true copies of the original. Cells that fail to pass a checkpoint often undergo apoptosis (or programmed cell death) to prevent them from dividing and passing on deleterious mutations. Many different proteins guard the checkpoints so that the cell cycle runs smoothly. However, when these proteins malfunction and a cell skips a checkpoint, it can grow uncontrollably and become cancerous.

Anaphase-promoting complex / cyclosome (APC/C) is a central gatekeeper of the cell cycle. Although its name suggests that it promotes “anaphase” - a specific step in the cell cycle - APC/C is a large protein complex that monitors many stages of the cell cycle. It helps in removing regulatory proteins, allowing cells to move from one step to the next. These regulatory proteins include cyclins, which trigger each phase of the cell cycle, and proteins that control the mechanics of critical cell cycle tasks such as DNA replication and chromosome separation. As each cell cycle phase nears completion, APC/C binds to the regulatory proteins and attaches ubiquitin to them, targeting them for degradation by proteasomes.

APC/C is an E3 ubiquitin ligase, and like other E3 enzymes, it acts as a scaffold that brings together multiple proteins. The APC/C has three functional regions - the “TPR lobe” (named after the distinctive “tetratricopeptide repeat” folding pattern of several of its proteins), the platform region, and the catalytic subunit. The TPR lobe positions two different adapter proteins, Cdh1 and Cdc20, at different times in the cell cycle to specifically recognize the target protein destined for degradation. The target proteins contain a characteristic “degron” sequence recognized by the adapter protein. The structure of APC/C in PDB ID 5a31 shows the adapter protein Cdh1 and a peptide representing the degron sequence of the target protein. The platform region of APC/C positions the catalytic module optimally relative to the degrons, and regulates the binding of Cdc20 to the TPR lobe. Finally, the catalytic module of APC/C binds the E2 ubiquitin-conjugating enzyme that delivers ubiquitin, and catalyses the transfer of ubiquitin from E2 to the target protein. The ubiquitination reaction may be repeated many times to attach a linked chain of multiple ubiquitins to the target protein.

The other APC/C adaptor protein Cdc20 (shown here from PDB ID 5g04) helps monitor the anaphase stage of the cell cycle. Cdc20 binds to APC/C early in the process of cell division and triggers a pathway that degrades the protein holding replicated chromosomes together, allowing the chromosomes to separate into the two daughter cells. However, Cdc20 overexpression allows this phase to occur even without proper alignment, possibly creating cells with damaged chromosomes. This type of genomic instability can help generate abnormal growth characteristics often found in cancerous cells. Overexpression of Cdc20 has long been associated with a multitude of different cancers. Because of its central role in cancer biology, APC/C and its adapters are attractive targets for the development of anticancer therapies.