CDC16

CDC16 (APC6/ANAPC6) is a core subunit of the anaphase-promoting complex/cyclosome (APC/C), a regulated ubiquitin-protein ligase that catalyzes mitosis-specific ubiquitination of cyclin B to drive its proteasomal destruction and enable anaphase progression (PMID:7736580). Structurally, CDC16 is a TPR-repeat protein that folds into a contiguous 14-TPR superhelix: its N-terminal TPR block mediates homo-dimerization through an interface conserved with CDC27, while its C-terminal TPR block engages CDC26/Hcn1, which stabilizes APC6 through an intermolecular TPR mimic required for APC/C assembly and integrity and which simultaneously sequesters the CDC26 N-acetyl-Met to protect it from ubiquitin-dependent degradation (PMID:19668213, PMID:20924356). Stable incorporation of CDC16 (and CDC27) into the APC/C is further reinforced by Swm1/Apc13, whose loss reduces CDC16 association and abolishes ligase activity (PMID:15060174). Through this scaffolding role, CDC16 enforces several cell-cycle controls: it restricts DNA replication to once per cycle by directing targeted proteolysis upstream of origin firing, such that loss-of-function permits Cdc6- and Mcm5-dependent rereplication despite elevated mitotic CDK activity (PMID:8620535, PMID:9660930), and it is required for the onset of anaphase (PMID:7798319). In fission yeast CDC16 has an additional APC/C-independent function: together with Byr4 it forms a two-component GTPase-activating protein for the Spg1 GTPase that negatively regulates septation, with CDC16 alone lacking GAP activity but cooperating through a defined Byr4 Cdc16-binding site to stimulate Spg1 GTP hydrolysis (PMID:8334988, PMID:9742395, PMID:10196225). In human cancer contexts, CDC16's APC/C-associated ubiquitination activity is modulated by competitive binding: DEPDC1B competes with the substrate SCUBE3 for CDC16 to block its degradation in melanoma, and c-Jun competes for CDC16 to reduce APC/C formation and induce G1 arrest in neuroblastoma (PMID:35088579, PMID:40149013).

1. 1993 High

   Established the first cellular function for CDC16 by showing its fission-yeast ortholog limits septum formation and sustains mitotic CDK activity, framing it as a negative regulator of cytokinesis.

   Evidence Gene cloning, deletion and temperature-sensitive mutant analysis with genetic epistasis in S. pombe

   PMID:8334988

   Open questions at the time

   • Molecular mechanism of septation control not defined
   • Connection to ubiquitination machinery not yet made
2. 1994 High

   Identified CDC16/Cut9 as a TPR-repeat protein required for the onset of anaphase, providing the structural and phenotypic basis later unified with APC/C function.

   Evidence Temperature-sensitive mutant characterization, TPR domain analysis and genetic suppressor analysis in S. pombe

   PMID:7798319

   Open questions at the time

   • Biochemical activity of the TPR protein not established
   • Anaphase substrate not identified
3. 1995 High

   Placed CDC16 within the 20S APC and defined the complex's biochemical activity, showing it is a ubiquitin ligase that targets cyclin B for destruction to enable anaphase.

   Evidence Biochemical fractionation of mitotic Xenopus extracts, immunodepletion/immunopurification and in vitro reconstituted ubiquitination assays

   PMID:7736580

   Open questions at the time

   • Specific catalytic contribution of CDC16 versus other subunits not resolved
   • Substrate-recognition mechanism not defined
4. 1996 High

   Extended CDC16 function to genome stability by showing it restricts DNA replication to once per cell cycle, implicating it in proteolysis of an S-phase initiator.

   Evidence Genetic overreplication screen, flow cytometry DNA-content analysis and conditional allele characterization in S. cerevisiae

   PMID:8620535

   Open questions at the time

   • Identity of the proteolytic target controlling rereplication not defined
   • Direct biochemical link to APC/C in this role not shown
5. 1998 High

   Resolved how CDC16 controls replication licensing, placing it upstream of origin firing where its loss permits Cdc6- and Mcm5-dependent reinitiation despite high CDK activity.

   Evidence 2D DNA electrophoresis, density-transfer experiments and genetic epistasis with cdc6 and cdc46/mcm5 in S. cerevisiae

   PMID:9660930

   Open questions at the time

   • Direct ubiquitination substrate at origins not identified
6. 1998 High

   Revealed an APC/C-independent function by reconstituting CDC16 as part of a two-component GAP with Byr4 that inactivates the Spg1 GTPase to negatively regulate septation.

   Evidence Yeast two-hybrid, in vitro coprecipitation, in vitro GTPase assays with GTPase specificity controls in S. pombe

   PMID:9742395

   Open questions at the time

   • How CDC16 contributes catalytically to GAP activity not defined
   • Relationship between GAP role and APC/C role unresolved
7. 1999 High

   Defined the minimal functional unit of the Cdc16-Byr4 GAP, showing a single Cdc16-binding site plus one Spg1-binding site suffices to reconstitute full GAP activity.

   Evidence In vitro mutagenesis, GTPase assays and binding-affinity measurements of Byr4 mutants in S. pombe

   PMID:10196225

   Open questions at the time

   • Structural basis of the Cdc16-Byr4 interaction not determined
8. 2004 High

   Identified Swm1/Apc13 as a structural stabilizer that maintains CDC16 incorporation into the APC/C and is required for ligase activity.

   Evidence Reciprocal Co-IP, in vitro ubiquitination assays and cross-species complementation

   PMID:15060174

   Open questions at the time

   • Atomic detail of how Swm1 stabilizes CDC16 not resolved at this stage
9. 2004 Medium

   Linked APC/C subunit function to replication licensing in fission yeast by showing cut9 alleles are required for the rereplication process independently of cyclin B.

   Evidence Genetic screen for rereplication-defective ts mutants and complementation cloning in S. pombe

   PMID:15466421

   Open questions at the time

   • No detailed mechanistic follow-up
   • Single lab, screen-based identification
10. 2010 High

    Solved the molecular architecture of CDC16, defining a contiguous 14-TPR superhelix with an N-terminal homo-dimerization interface and a C-terminal CDC26/Hcn1-binding chamber that protects the partner from degradation.

    Evidence X-ray crystallography of the Cut9-Hcn1 complex with analysis of dimerization and binding interfaces (and CDC26-APC6 structure, PMID 19668213)

    PMID:19668213 PMID:20924356

    Open questions at the time

    • Structure of full APC/C-incorporated CDC16 not resolved here
    • Dynamics of dimerization during catalysis not addressed
11. 2018 Low

    Began extending CDC16 into human cancer biology by identifying YDJC as a binding partner and CDC16 as a negative regulator of cytoskeletal remodeling, migration and invasion.

    Evidence Co-IP, siRNA knockdown and overexpression with migration/invasion assays in A549 lung cancer cells

    PMID:29796162

    Open questions at the time

    • Single Co-IP without reciprocal/structural validation
    • Mechanism connecting binding to functional output not reconstituted
12. 2019 Low

    Proposed that CDC16 targets YDJC for ubiquitination and suppresses EMT via PP2A/ERK2 modulation, suggesting a substrate-directed role in cancer signaling.

    Evidence Co-IP, overexpression/knockdown, ubiquitination assays and EMT marker analysis in lung cancer cells

    PMID:31485224

    Open questions at the time

    • Single-lab data without rigorous reconstitution
    • Direct CDC16-mediated ubiquitination of YDJC not biochemically isolated
13. 2022 Medium

    Showed that CDC16 substrate ubiquitination can be controlled by competitive partner binding, with DEPDC1B competing against SCUBE3 to stabilize SCUBE3 and promote melanoma angiogenesis and metastasis.

    Evidence Reciprocal Co-IP, ubiquitination and protein-stability assays with gain/loss-of-function in melanoma cells and mouse models

    PMID:35088579

    Open questions at the time

    • Single lab
    • Whether SCUBE3 is a direct APC/C substrate not structurally confirmed
14. 2025 Medium

    Demonstrated regulation of APC/C assembly through CDC16 in cancer, with c-Jun competitively binding CDC16 to reduce complex formation and induce G1 arrest in neuroblastoma.

    Evidence Co-IP, flow cytometry cell-cycle analysis, EdU proliferation and transwell assays in neuroblastoma cells

    PMID:40149013

    Open questions at the time

    • No structural or in vitro reconstitution of the c-Jun-CDC16 interaction
    • Single lab

• How the CDC16 TPR scaffold integrates its dual roles in APC/C-dependent proteolysis and in cancer-specific competitive binding to define context-dependent substrate choice remains unresolved.
• No unified structural model of CDC16 within human APC/C bound to competing partners
• Direct catalytic versus scaffolding contribution of CDC16 to substrate ubiquitination not isolated