ANAPC16

ANAPC16 is a metazoan-specific subunit of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that controls mitotic and meiotic progression by ubiquitinating cell-cycle substrates. Present in APC/C complexes throughout the cell cycle, ANAPC16 resides in the Arc Lamp subcomplex where its C-terminal domain binds asymmetrically to the APC3 homodimer and recruits APC7 to APC3, as defined by crystal structure and cryo-EM (PMID:25490258, PMID:26083744). Depletion of ANAPC16 phenocopies loss of other APC/C subunits, abolishing APC/C ubiquitination activity toward mitotic substrates in human cells, and temperature-sensitive mutants of the C. elegans ortholog emb-1 arrest in meiosis I metaphase and fail in germline mitotic proliferation (PMID:20392738, PMID:21775471). ANAPC16 is conserved across metazoans but absent in fungi, and cross-species complementation confirms functional equivalence of vertebrate and invertebrate orthologs (PMID:20392738).

1. 2010 High

   The identity of ANAPC16 as a bona fide APC/C subunit was established, answering whether the holoenzyme contained additional uncharacterized components; its depletion abolished APC/C ubiquitination activity and phenocopied loss of core subunits, demonstrating functional necessity.

   Evidence TAP-MS identification, reciprocal co-IP, RNAi knockdown with mitotic substrate ubiquitination readouts, and cross-species complementation in human, C. elegans, and zebrafish cells

   PMID:20360068 PMID:20392738

   Open questions at the time

   • Binding interface between APC16 and other APC/C subunits was unresolved
   • Whether APC16 has a specific role beyond structural integrity of the complex was unknown
2. 2011 High

   Genetic epistasis in C. elegans placed APC16 definitively within the APC/C pathway, revealing that it is required for both meiotic progression and mitotic germline proliferation — extending the functional requirement beyond somatic mitosis.

   Evidence Temperature-sensitive emb-1/APC16 mutants, double mutant combinations with known APC/C subunits and suppressors, meiosis I arrest and germline proliferation assays

   PMID:21775471

   Open questions at the time

   • Whether APC16 contributes to substrate recognition or only to complex assembly was unknown
   • Structural basis of APC16 integration into the holoenzyme remained unresolved
3. 2014 High

   Crystal structure and cryo-EM resolved how APC16 integrates into the APC/C: one APC16 binds asymmetrically to the symmetric APC3 homodimer via its C-terminal domain and recruits APC7, defining a specific architectural role within the Arc Lamp subcomplex.

   Evidence X-ray crystallography of APC3–APC16 C-terminal domain complex, 7.4 Å cryo-EM reconstruction of the full 1.2 MDa APC/C, mutagenesis, and in vitro ubiquitination assays

   PMID:25043029 PMID:25490258

   Open questions at the time

   • How APC16's asymmetric binding relates to the asymmetric catalytic cycle of the APC/C was not established
   • Whether APC16 influences substrate selectivity or only holoenzyme assembly was unresolved
4. 2015 High

   Atomic-resolution cryo-EM of APC/C–coactivator complexes placed APC16 in full mechanistic context, revealing the overall architecture that enables Emi1-mediated inhibition and UbcH10-dependent ubiquitin transfer.

   Evidence Atomic-resolution cryo-EM of APC/C–Cdh1–Emi1 and APC/C–UbcH10–Ub complexes

   PMID:26083744

   Open questions at the time

   • Direct contribution of APC16 to catalytic steps (E2 positioning, substrate handoff) versus purely structural scaffolding was not dissected
5. 2016 High

   The phosphorylation-driven activation mechanism of APC/C was elucidated in the context of the Arc Lamp subcomplex containing APC16: Cdk–cyclin–Cks recruited by the APC3 phospho-loop hyperphosphorylates an auto-inhibitory segment of APC1 to enable Cdc20 binding.

   Evidence Cryo-EM, biochemical phosphorylation assays, and mutagenesis of APC3 loop and APC1 auto-inhibitory segment

   PMID:27120157

   Open questions at the time

   • Whether APC16 modulates the phosphorylation-dependent conformational switch in APC3/APC1 is unknown
   • No post-translational modifications of APC16 itself have been functionally characterized

• It remains unknown whether APC16 plays any role beyond structural scaffolding — for example, whether it directly influences substrate recognition, E2 engagement, or processivity of polyubiquitin chain elongation.
• No separation-of-function mutations distinguishing APC16's architectural role from potential regulatory roles have been reported
• No post-translational regulation of APC16 has been characterized
• Functional consequence of the asymmetric APC16 stoichiometry on the symmetric APC3 homodimer is unexplained