DEG_APCC_TPR_1

Progress through the cell division cycle depends on ubiquitin-mediated proteasomal degradation of specific proteins at specific times. An essential regulator of this process is the Anaphase Promoting Complex or Cyclosome (APC/C), an E3 ubiquitin ligase that catalyses covalent linking of polyubiquitin chains to crucial cell cycle regulators and thereby marks them for 26S proteasome-mediated proteolysis. The APC/C is a multiprotein complex that contains at least 13 different subunits, most of which are conserved in Eukaryotes. These include the catalytic Apc2 and Apc11 subunits, the tetratricopeptide repeat (TPR)-containing Apc3/Cdc27, Apc8/Cdc23 and Apc6/Cdc16 subunits, which form homodimers and are thus present in two copies in a single APC/C complex, and the Apc10/Doc1 subunit, which is involved in substrate recognition (Barford,2011, Primorac,2013).
Timely and sequential targeting of its substrates for degradation depends on tight temporal regulation of the activity and substrate specificity of the APC/C. This regulation is partly mediated by the two mitotic co-activators Cdc20 and Cdh1, which activate the APC/C at different stages of the cell cycle: Cdc20 during early mitosis and Cdh1 during late mitosis and G1 phase. They both contain a WD40-repeat domain, which acts as a binding platform for APC/C substrates that contain D-box (LIG_APCC_Dbox_1) or KEN-box (LIG_APCC_KENbox_2) degradation motifs. D-box and KEN-box motifs bind to distinct sites on the WD40-repeat domain and are thought to preferentially interact with Cdc20 and Cdh1, respectively, however, efficient recruitment to and ubiquitylation by the APC/C likely depends on cooperative binding of multiple motifs (Chao,2012). In addition, the specificity and affinity for D-box-containing substrates is enhanced by cooperative binding of the motif with co-activator and Apc10/Doc1, which together form a multivalent co-receptor for the D-box (Matyskiela,2009, da Fonseca,2011). Another important determinant of APC/C activity and specificity is the Mitotic Checkpoint Complex (MCC), which imposes the Spindle Assembly Checkpoint (SAC) to ensure that chromosomes are correctly attached to the spindle apparatus before chromosome segregation is initiated. The MCC includes Cdc20 and binds to the APC/C to inhibit recruitment of substrates by Cdc20, thereby preventing metaphase to anaphase transition. Important substrates that are stabilised by the SAC are securin and B-type cyclins, which both inhibit the separase activity that is required for chromosome segregation (Barford,2011).
Binding of the co-activators to the APC/C is also mediated by multiple motifs, allowing multivalent cooperative binding (Matyskiela,2009). Only recently, a Cdc20-specific motif was described as a site for binding to the APC/C, possibly via the Apc8/Cdc23 subunit, and related to the specific role of this co-activator in the SAC (Izawa,2012). Both Cdc20 and Cdh1 share an internal C-box motif (LIG_APCC_Cbox_1 and LIG_APCC_Cbox_2), located in the N-terminal region, that most likely binds to the Apc2 subunit (da Fonseca,2011). In addition, both co-activators contain an IR sequence at their extreme C-terminus that interacts with the TPRs of the Apc3/Cdc27 subunit (Vodermaier,2003). This C-terminal sequence is also present in the Apc10/Doc1 subunit, where it also mediates binding to Apc3/Cdc27 (Wendt,2001). In addition, some substrates, including Nek2 kinase and the kinesin-like protein Kif18A, use a similar C-terminal hydrophobic-arginine motif to bind directly to the APC/C, probably also via Apc3/Cdc27, independently of a co-activator (Barford,2011, Sedgwick,2013).