ANAPC16

ANAPC16 (APC16) is a metazoan-specific subunit of the APC/C E3 ubiquitin ligase that contributes to the ubiquitination of cell-cycle substrates required for mitotic and meiotic progression (PMID:20392738, PMID:21775471). It is a bona fide component of the human APC/C present throughout the cell cycle, and its depletion reduces APC/C ubiquitin ligase activity toward mitotic substrates, phenocopying loss of other APC/C subunits (PMID:20392738). Structurally, APC16 resides in the Arc Lamp subcomplex together with CDC26, APC13, and the TPR proteins APC7, APC3, APC6, and APC8, where a single APC16 molecule binds asymmetrically to the symmetric APC3 homodimer and bridges APC3 to APC7, recruiting APC7 into the assembly (PMID:25490258). APC16 is conserved across metazoans but absent in fungi, with functional equivalents identified in C. elegans (emb-1/K10D2.4) and zebrafish (PMID:20392738); in C. elegans, loss of emb-1 arrests one-cell embryos in metaphase of meiosis I and is genetically suppressed and enhanced by known APC/C regulators, and it is additionally required for mitotic germline proliferation, placing APC16 firmly within the APC/C pathway for both meiotic and mitotic cell division (PMID:21775471).

1. 2010 High

   Established that APC16 is a genuine, constitutive subunit of the human APC/C rather than a transient interactor, and that it is functionally required for APC/C ligase activity.

   Evidence Tandem-affinity purification, reciprocal co-IP, and siRNA knockdown coupled to APC/C activity assays in human cells

   PMID:20392738

   Open questions at the time

   • Did not resolve where APC16 sits structurally within the complex
   • Mechanism by which depletion lowers ligase activity not defined
2. 2010 Medium

   Defined APC16 as a metazoan-specific APC/C subunit by showing functional equivalence of C. elegans and zebrafish orthologs, distinguishing it from fungal APC/C architecture.

   Evidence Sequence homology analysis with depletion/complementation in C. elegans and zebrafish (single lab)

   PMID:20392738

   Open questions at the time

   • Functional equivalence based on depletion phenotype only
   • No biochemical demonstration that orthologs occupy the same APC/C position
3. 2011 High

   Placed APC16/EMB-1 within the APC/C pathway controlling meiotic progression, answering whether its loss specifically disrupts cell-division control.

   Evidence Genetic epistasis with temperature-sensitive mutants, double-mutant analysis, and suppressor/enhancer screens in C. elegans

   PMID:21775471

   Open questions at the time

   • Does not identify the molecular step in meiosis I that APC16 enables
   • Substrate(s) whose ubiquitination requires APC16 not defined
4. 2011 Medium

   Extended APC16's role beyond meiosis by showing a requirement in mitotic germline proliferation, consistent with a general APC/C function.

   Evidence Loss-of-function genetic analysis in C. elegans germline (single lab)

   PMID:21775471

   Open questions at the time

   • Mitotic phenotype characterized at cellular but not molecular level
   • Relationship between meiotic and mitotic requirements not dissected
5. 2014 High

   Provided the structural basis for APC16 function, showing how it physically assembles part of the APC/C by bridging APC3 to APC7.

   Evidence X-ray crystallography of APC3Δloop alone and bound to the APC16 C-terminal domain, with biochemical mapping and mutagenesis

   PMID:25490258

   Open questions at the time

   • Structural work used isolated APC3-APC16 module, not the holo-APC/C
   • Functional consequence of disrupting the asymmetric APC3-APC16 interface not tested in cells

• How APC16-mediated APC7 recruitment quantitatively shapes APC/C substrate selectivity and activity in vivo remains unresolved.
• No defined set of substrates dependent specifically on the APC16-APC7 module
• Regulation of APC16 across cell-cycle phases not characterized