Editing Anaphase-promoting complex (section)

== Subunits ==
There is not a vast amount of extensive investigation on APC/C subunits, which serve mostly as adaptors. Studies of APC subunits are mainly conducted in yeast, and studies show that the majority of yeast APC subunits are also present in vertebrates, this suggests conservation across eukaryotes. Eleven core APC subunits have been found in vertebrates versus thirteen in yeast.<ref name= "Morgan_2007" /> Activator subunits bind to APC at varying stages of the cell cycle to control its ubiquitination activity, often by directing APC to target substrates destined for ubiquitination. The specificity of APC ligases is proposed to be controlled by the incorporation of specificity factors into the ligase complex, instead of substrate phosphorylation. i.e.: The subunit, CDC20 allows APC to degrade substrates such as anaphase inhibitors (Pdsp1) at the beginning of anaphase, on the other hand when CDC20 is substituted for specificity factor Hct1, APC degrades a different set of substrates, particularly mitosis cyclins in late anaphase. Activators CDC20 and Cdh1 are of particular significance and are the most widely studied and familiar of the APC/C subunits.

The catalytic core of the APC/C consists of the cullin subunit Apc2 and RING H2 domain subunit Apc11. These two subunits catalyze ubiquitination of substrates when the C-terminal domain of Apc2 forms a tight complex with Apc11. RING/APc11 binds to the E2-ubiquitin conjugate that  catalyzes the transfer of ubiquitin to an active site in E2.<ref name= "Morgan_2007" />
In addition to the catalytic functionality, other core proteins of the APC are composed multiple repeat motifs with the main purpose of providing molecular scaffold support. These include Apc1, the largest subunit which contains 11 tandem repeats of 35–40 amino acid sequences, and Apc2, which contains three [[cullin]] repeats of approximately 130 amino acids total.<ref name =Barford>{{cite journal | vauthors = Barford D | title = Structural insights into anaphase-promoting complex function and mechanism | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 366 | issue = 1584 | pages = 3605–24 | date = December 2011 | pmid = 22084387 | pmc = 3203452 | doi = 10.1098/rstb.2011.0069 }}</ref> The major motifs in APC subunits include [[Tetratricopeptide repeat|tetratricopeptide (TPR) motifs]] and WD40 repeats 1.<ref name= "Morgan_2007" /> C-termini regions of CDC20 and Cdh1 have a WD40 domain that is suggested to form a binding platform that binds APC substrates, thus contributing to APCs ability to target these substrates, although the exact mechanism through which they increase APC activity is unknown.<ref>{{cite journal | vauthors = Castro A, Bernis C, Vigneron S, Labbé JC, Lorca T | title = The anaphase-promoting complex: a key factor in the regulation of cell cycle | journal = Oncogene | volume = 24 | issue = 3 | pages = 314–25 | date = January 2005 | pmid = 15678131 | doi = 10.1038/sj.onc.1207973 | s2cid = 29467714 | doi-access =  }}</ref> It is also suggested that variations in these WD40 domains result in varying substrate specificity, which is confirmed by recent results suggesting  that different APC substrates can directly and specifically bind to Cdc20 and Cdh1/Hct1  Ultimately, the specificity differences are responsible for the timing of the destruction of several APC targets during mitosis. With CDC20 targeting a few major substrates at metaphase and Cdh1 targeting a broader range of substrates towards late mitosis and G1.<ref>{{cite journal | vauthors = Schwab M, Neutzner M, Möcker D, Seufert W | title = Yeast Hct1 recognizes the mitotic cyclin Clb2 and other substrates of the ubiquitin ligase APC | journal = The EMBO Journal | volume = 20 | issue = 18 | pages = 5165–75 | date = September 2001 | pmid = 11566880 | pmc = 125620 | doi = 10.1093/emboj/20.18.5165 }}</ref>

Most notably, 4 subunits of yeast APC/C consist almost entirely of multiple repeats of the 34 amino acid tetratricopeptide residue (TPR) motif. These TPR subunits, Cdc16,<ref>{{cite journal | vauthors = Wang J, Dye BT, Rajashankar KR, Kurinov I, Schulman BA | title = Insights into anaphase promoting complex TPR subdomain assembly from a CDC26-APC6 structure | journal = Nature Structural & Molecular Biology | volume = 16 | issue = 9 | pages = 987–9 | date = September 2009 | pmid = 19668213 | pmc = 2759704 | doi = 10.1038/nsmb.1645 }}</ref> [[Cdc27]],<ref>{{cite journal | vauthors = Yamaguchi M, Yu S, Qiao R, Weissmann F, Miller DJ, VanderLinden R, Brown NG, Frye JJ, Peters JM, Schulman BA | display-authors = 6 | title = Structure of an APC3-APC16 complex: insights into assembly of the anaphase-promoting complex/cyclosome | journal = Journal of Molecular Biology | volume = 427 | issue = 8 | pages = 1748–64 | date = April 2015 | pmid = 25490258 | pmc = 4444369 | doi = 10.1016/j.jmb.2014.11.020 }}</ref> [[Cdc23]], and Apc5, mainly provide scaffolding and support to mediate other protein-protein interactions. Cdc27 and Cdc23 have been shown to support the binding of Cdc20 and Cdh1, as mutations in key residues of these subunits led to increased dissociation of the activators. Apc10/Doc1, has been shown to promote substrate binding by mediating their interactions with Cdh1 and Cdc20.<ref name=Passmore>{{cite journal | vauthors = Passmore LA, McCormack EA, Au SW, Paul A, Willison KR, Harper JW, Barford D | title = Doc1 mediates the activity of the anaphase-promoting complex by contributing to substrate recognition | journal = The EMBO Journal | volume = 22 | issue = 4 | pages = 786–96 | date = February 2003 | pmid = 12574115 | pmc = 145444 | doi = 10.1093/emboj/cdg084 | author-link1 = Lori Passmore }}</ref>

In particular, CDC20 (also known as p55CDC, Fizzy, or Slp1) inactivates CDK1 via ubiquitination of B-type cyclins. This results in activation of Cdh1(a.k.a. Fizzy-related, Hct1, Ste9, or Srw1), which interacts with APC during late mitosis and G1/G0. Cdh1 is inactivated via phosphorylation during S, G2 and early M phase. During these points in the cycle, it is not able to be assembled.<ref>{{cite journal | vauthors = Kramer ER, Scheuringer N, Podtelejnikov AV, Mann M, Peters JM | title = Mitotic regulation of the APC activator proteins CDC20 and CDH1 | journal = Molecular Biology of the Cell | volume = 11 | issue = 5 | pages = 1555–69 | date = May 2000 | pmid = 10793135 | pmc = 14867 | doi = 10.1091/mbc.11.5.1555 }}</ref>

Evidence shows that APC3 and APC7 serve to recruit Cdh1 to the anaphase-promoting complex.<ref>{{cite journal | vauthors = Vodermaier HC, Gieffers C, Maurer-Stroh S, Eisenhaber F, Peters JM | title = TPR subunits of the anaphase-promoting complex mediate binding to the activator protein CDH1 | journal = Current Biology | volume = 13 | issue = 17 | pages = 1459–68 | date = September 2003 | pmid = 12956947 | doi = 10.1016/s0960-9822(03)00581-5 | s2cid = 5942532 | doi-access = free }}</ref> This further supports  that Cdh1 is responsible for maintaining APC activity during G1. Cdh1 does not require APC to be phosphorylated in order to bind, in fact, phosphorylation of Cdh1 by Cdks prevents it from binding to APC from S to M phase. With destruction of M-Cdk, release of CDC20 from the APC and binding of Cdh1 can now occur, allowing APC activity to continue on during G1 entry.<ref name= "Morgan_2007" />  While Cdh1 recognizes M and S cyclins, allowing for their destruction until the entire cell commits to proceed to a new cycle, it does not recognize G1/S cyclins, and during G1/S phase, their cyclin activity can rise unhindered and phosphorylate and thus inactivating Cdh1 and therefore APC.

The subunit Apc15 plays an important role in APC/C<sup>Cdc20</sup> activation following the bi-orientation of sister chromatids across the metaphase plate. When kinetochores are unattached to spindles, mitotic checkpoint complexes (MCC) and inhibit APC. In the absence of Apc15, MCCs and Cdc20 remain locked on the APC/C preventing its activity once the spindle checkpoint requirements are met. Apc15 mediates the turnover of Cdc20 and MCCs to provide information on the attachment state of kinetochores.<ref>{{cite journal | vauthors = Mansfeld J, Collin P, Collins MO, Choudhary JS, Pines J | title = APC15 drives the turnover of MCC-CDC20 to make the spindle assembly checkpoint responsive to kinetochore attachment | journal = Nature Cell Biology | volume = 13 | issue = 10 | pages = 1234–43 | date = September 2011 | pmid = 21926987 | pmc = 3188299 | doi = 10.1038/ncb2347 }}</ref>

===CDC27/APC3===

One of the subunits that exhibit the TPR motif, CDC27 has been identified to interact with mitotic checkpoint proteins such as Mad2, p55CDC and BUBR1, suggesting that it may have involvement in the timing of M phase.<ref>"Entrez Gene: CDC27 cell division cycle 27 homolog (S. cerevisiae)".</ref>  Evidence shows that CDC27 is involved in a ternary complex with SMAD2/3 and Cdh1, which is created in response to TGFβ signalling. Because of its interaction with Cdh1 in particular, it has a potential role in determining affinity between APC and its activators Cdc20 and Cdh1. A study suggests that TGF-β-induced Cdc27 phosphorylation enhances interaction between cdc27 and Cdh1–which is directly involved in activating APC.<ref>{{cite journal | vauthors = Zhang L, Fujita T, Wu G, Xiao X, Wan Y | title = Phosphorylation of the anaphase-promoting complex/Cdc27 is involved in TGF-beta signaling | journal = The Journal of Biological Chemistry | volume = 286 | issue = 12 | pages = 10041–50 | date = March 2011 | pmid = 21209074 | pmc = 3060455 | doi = 10.1074/jbc.M110.205518 | doi-access = free }}</ref> CDC27 can serve as  a vehicle through which TGFβ signalling can activate APC. Induced CDC27 hyperphosphorylation by TGFβ showed elevated APC activity.

===CDC23, CDC16, CDC27===

CDC23, another TPR subunit interacts with SWM1, binding to the D-box of CLB2. Based upon hybrid assays in vivo and co-immunoprecipitation in vitro, it is suggested that Cdc16p, Cdc23p and Cdc27p (analogs in Sacchyromyces cerevisiae) interact and form a macromolecular complex. Their common theme of TPR is suggested to mediate these interactions.<ref>{{cite journal | vauthors = Lamb JR, Michaud WA, Sikorski RS, Hieter PA | title = Cdc16p, Cdc23p and Cdc27p form a complex essential for mitosis | journal = The EMBO Journal | volume = 13 | issue = 18 | pages = 4321–8 | date = September 1994 | pmid = 7925276 | doi = 10.1002/j.1460-2075.1994.tb06752.x | pmc = 395359 }}</ref>  As for Cdc27 and Cdc16 in drosophila, their functions have been tested via [[RNA interference]] (RNAi).<ref>{{cite journal | vauthors = Huang JY, Raff JW | title = The dynamic localisation of the Drosophila APC/C: evidence for the existence of multiple complexes that perform distinct functions and are differentially localised | journal = Journal of Cell Science | volume = 115 | issue = Pt 14 | pages = 2847–56 | date = July 2002 | doi = 10.1242/jcs.115.14.2847 | pmid = 12082146 | url = http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=12082146 | url-access = subscription }}</ref> Results suggest that they may mediate activity of the entire complex via different mechanisms at different sites. In further drosophila studies, Cdk16 and cdk23 appear to be activated via phosphorylation by Polo-like kinase 1 (Plk1) and its fission yeast counterpart, appear to bind particularly to Cdc23.<ref>{{cite journal | vauthors = Deak P, Donaldson M, Glover DM | title = Mutations in mákos, a Drosophila gene encoding the Cdc27 subunit of the anaphase promoting complex, enhance centrosomal defects in polo and are suppressed by mutations in twins/aar, which encodes a regulatory subunit of PP2A | journal = Journal of Cell Science | volume = 116 | issue = Pt 20 | pages = 4147–58 | date = October 2003 | pmid = 12953067 | doi = 10.1242/jcs.00722 | doi-access = free }}</ref>

The complex is understood to be regulated by activators CDC20 and Cdh1 during mitosis. Their role in degradation for cyclin B is demonstrated by a screen of Saccharomyces cerevisiae mutants defective for cyclin B degradation, which were found to have mutations in CDC16 and CDC23 genes. Mutants for CDC27, CDC23 and CDC 27 all resulted in a cell-cycle arrest at metaphase.<ref>{{cite journal | vauthors = Hartwell LH, Smith D | title = Altered fidelity of mitotic chromosome transmission in cell cycle mutants of S. cerevisiae | journal = Genetics | volume = 110 | issue = 3 | pages = 381–95 | date = July 1985 | doi = 10.1093/genetics/110.3.381 | pmid = 3894160 | pmc = 1202570 }}</ref>