#APC/C

Cell Cycle Regulation

Which regulatory factor is crucial for the metaphase-to-anaphase transition?

Metaphase to anaphase transition represents irreversible commitment executed by APC/C bound to Cdc20 coactivator. Throughout prometaphase chromosomes attach via kinetochores, generating unattached kinetochore signal converting Mad2 to active inhibitor forming mitotic checkpoint complex with BubR1, Bub3, and Cdc20 that binds APC/C core to block E2 recruitment. Upon achievement of biorientation of all chromosomes and establishment of tension, checkpoint protein recruitment stops, complex disassembles, and free Cdc20 associates with phosphorylated APC/C previously primed by CDK1 and Plk1. Active

Ref: Peters et al., Cell 1998, APC/C Regulation. Musacchio, Nat Rev Mol Cell Biol 2011, Anaphase.

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Cell Cycle Regulation

Which phase transition requires the Anaphase-Promoting Complex (APC/C)?

The anaphase-promoting complex/cyclosome, APC/C, is a 1.2 MDa cullin-RING E3 ubiquitin ligase comprising over a dozen core subunits including APC2, APC11 catalytic core, and scaffolding proteins APC1, APC3, APC6, plus transient coactivators Cdc20 and Cdh1 that confer substrate specificity through D-box and KEN motif recognition. During early mitosis its activity is tightly repressed by spindle assembly checkpoint that sequesters Cdc20 in mitotic checkpoint complex composed of Mad2, BubR1, and Bub3. Priming phosphorylation by cyclin B-CDK1 and Polo-like kinase Plk1 on APC3 and APC1 enhances Cdc

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Cell Cycle Control, APC/C Regulation.

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Cell Cycle Regulation

APC/C-Cdh1 is required for:

After separase cleavage of cohesin and chromosome segregation, cells must eliminate remaining mitotic cyclins to establish low CDK environment permissive for origin licensing, transcription reactivation and cytokinesis in G1. Two sequential APC/C coactivators accomplish temporal ordering. APC/C-Cdc20 initiates anaphase by destroying securin and majority of cyclin B during metaphase to anaphase transition. Cdc20 itself then becomes substrate for APC/C-Cdh1. Cdh1, also known as Fzr1, associates with APC/C core from late anaphase through G1, recognizing extended degron repertoire including KEN-bo

Ref: Lindon, APC/C-Cdh1 Control of G1 Homeostasis, Biochem J 2008; Peters, Anaphase Promoting Complex Mechanism and Regulation.

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Cell Cycle Regulation

What happens if APC/C-Cdc20 is inhibited?

Anaphase-promoting complex bound to coactivator Cdc20 represents irreversible executor of metaphase exit, targeting securin and cyclin B. Inhibition of this ligase through multiple mechanisms arrests cells before chromosome separation. Endogenous inhibition occurs during spindle checkpoint activation where Mitotic Checkpoint Complex binds APC/C core blocking substrate recruitment. Exogenous inhibitors apcin occupying D-box binding pocket on Cdc20 and proTAME blocking APC/C activation, or depletion of Cdc20 protein itself, mimic checkpoint. Consequence is persistence of securin which continues

Ref: Pines, Mitotic Exit Regulation by APC/C-Cdc20, Nat Rev Mol Cell Biol 2011; NCBI Bookshelf, Consequences of APC/C Inhibition.

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Cell Cycle Regulation

During mitosis, which event is triggered by APC/C activation?

Transition into anaphase is driven by activation of anaphase promoting complex cyclosome bound to Cdc20, a large 1.2 MDa cullin-RING E3 ubiquitin ligase. When SAC is satisfied, MCC disassembly through p31comet and TRIP13 releases Cdc20 permitting association with APC/C core. Active APC/C-Cdc20 recognizes destruction box degrons on two pivotal substrates: securin and Cyclin B. Polyubiquitination of securin triggers rapid proteolysis via 26S proteasome, discharging separase cysteine protease previously held inactive as pseudosubstrate. Free separase cleaves Scc1 or Rad21 kleisin subunit of cohes

Ref: Pines, APC/C Regulation of Anaphase Onset, Nat Rev Mol Cell Biol 2011; NCBI Bookshelf, Anaphase Triggered by APC/C.

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Cell Cycle Regulation

Which component of the Mitotic Checkpoint Complex (MCC) directly inhibits APC/C?

Mitotic checkpoint complex inhibition of anaphase-promoting complex relies on multiple contacts that block substrate recognition and catalytic activity. MCC consists of BubR1, Bub3, Mad2, and Cdc20, assembled at unattached kinetochores. Closed conformer of Mad2, generated through Mad1-Mad2 template mechanism, wraps around Cdc20 as safety belt, sequestering it and presenting inhibitory surface. Although BubR1 contains KEN boxes that mimic APC/C substrates and directly blocks substrate binding via pseudo-substrate inhibition, Mad2 provides crucial initial locking of Cdc20 in MCC, preventing it f

Ref: Alfieri et al., Nature 2016, APC/C-MCC Structure; De Antoni et al., Curr Biol 2005, Mad2 Conformers.

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Cell Cycle Regulation

Which protein complex is responsible for delaying mitosis in case of spindle defects?

Accurate chromosome segregation requires that all kinetochores achieve amphitelic attachment to spindle microtubules from opposite poles and generate tension. Unattached or improperly attached kinetochores generate wait signal that delays onset of anaphase to prevent mis-segregation. This delay is enforced by mitotic checkpoint complex, abbreviated MCC, composed of Mad2, Bub3, BubR1 (Mad3 in yeast), and Cdc20. At unattached kinetochores, scaffold protein Knl1 is phosphorylated by Mps1, recruiting Bub1-Bub3 and Mad1-Mad2 complexes that catalyze conformational conversion of open Mad2 to closed f

Ref: Musacchio & Salmon, Nature Rev Mol Cell Biol 2007, Spindle Assembly Checkpoint; Lara-Gonzalez et al., Curr Biol 2012, MCC Functions.

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