Anaphase-Promoting Complex and Cell Cycle Control

Core Idea

The anaphase-promoting complex/cyclosome (APC/C) is a multisubunit ubiquitin ligase controlling cell cycle progression by targeting mitotic cyclins and securin for proteasomal degradation. APC/C remains inactive until phosphorylated and activated by binding Cdc20 coactivator, triggering securin destruction and sister chromatid separation, followed by cyclin B degradation and mitotic exit. This irreversible step enforces unidirectional cell cycle progression and prevents rereplication within a single cell cycle.

How It's Best Learned

Measure APC/C activity in cell extracts using ubiquitination assays; track substrate degradation timing in live cells. Identify APC/C substrates and their degron sequences; test the sufficiency of minimal degrons.

Common Misconceptions

• APC/C destroys all mitotic cyclins at once; E1 and B cyclins are degraded sequentially with distinct thresholds. - APC/C activation is irreversible; its activity remains high until mitosis exit when Cdh1 remains bound.

Explainer

From your study of cell cycle regulation and checkpoints, you know that cyclin-CDK complexes drive the cell forward through each phase, and that checkpoints halt progression when conditions are not met. But the cell cycle also requires an irreversible switch — a mechanism that commits the cell to completing a transition with no turning back. The anaphase-promoting complex/cyclosome (APC/C) is that switch for the metaphase-to-anaphase transition, and it works by destroying key regulatory proteins through ubiquitin-mediated proteolysis.

The APC/C is a large, multisubunit E3 ubiquitin ligase — the enzyme that attaches chains of the small protein ubiquitin to target substrates, marking them for degradation by the 26S proteasome. Unlike phosphorylation, which is easily reversed by phosphatases, protein destruction is permanent. Once the proteasome degrades a substrate, that protein is gone until the gene is transcribed and translated again. This irreversibility is exactly what the cell needs at anaphase: once sister chromatids separate, they cannot be reattached.

The APC/C achieves its timing through regulated activation by coactivator proteins. During metaphase, the spindle assembly checkpoint keeps APC/C inactive by sequestering its coactivator Cdc20 through the mitotic checkpoint complex (MCC). Only when every chromosome achieves proper bipolar attachment to spindle microtubules does the checkpoint release Cdc20, which binds and activates APC/C. The first critical substrate is securin, the inhibitor of the protease separase. When APC/C-Cdc20 ubiquitinates securin, the proteasome degrades it, freeing separase to cleave the cohesin rings holding sister chromatids together. This is anaphase onset — the physical separation of chromosomes.

After securin destruction, APC/C targets cyclin B for degradation, which inactivates CDK1 (the mitotic kinase). Without CDK1 activity, the cell cannot maintain the mitotic state — the spindle disassembles, chromosomes decondense, the nuclear envelope reforms, and the cell exits mitosis. Later in G1, a second coactivator called Cdh1 replaces Cdc20 and keeps APC/C active, ensuring that mitotic cyclins remain suppressed throughout G1 and preventing premature re-entry into S phase. This sustained APC/C-Cdh1 activity must be switched off by rising cyclin levels before the cell can commit to a new round of DNA replication. The APC/C thus enforces unidirectional progression: by permanently destroying the proteins that drove the previous phase, it ensures the cell cycle moves only forward, never backward.