APC/C-mediated degradation of cell cycle proteins

The Anaphase Promoting Complex or Cyclosome (APC/C) functions during mitosis to promote sister chromatid separation and mitotic exit through the degradation of mitotic cyclins and securin. This complex is also active in interphase insuring the appropriate length of the G1 phase (reviewed in Peters, 2002). The APC/C contains at least 12 subunits and functions as an ubiquitin-protein ligase (E3) promoting the multiubiquitination of its target proteins (see Gieffers et al., 2001). <br>In the ubiquitination reaction, ubiquitin is activated by the formation of a thioester bond with the (E1) ubiquitin activating enzyme then transferred to a cysteine residue within the ubiquitin conjugating enzyme (E2) and ultimately to a lysine residue within the target protein, with the aid of ubiquitin-protein ligase activity of the APC/C. The ubiquitin chains generated are believed to target proteins for destruction by the 26S proteasome (Reviewed in Peters, 1994 ) <br>The activity of the APC/C is highly periodic during the cell cycle and is controlled by a combination of regulatory events. The APC/C is activated by phosphorylation and the regulated recruitment of activating subunits and is negatively regulated by sequestration by kinetochore-associated checkpoint proteins. The Emi1 protein associates with Cdh1 and Cdc20, inhibiting the APC/C between G1/S and prophase. RSSA1 may play a similar role in ihibiting the APC during early mitosis.<br>Following phosphorylation of the APC/C core subunits by mitotic kinases, the activating subunit, Cdc20 is recruited to the APC/C and is responsible for mitotic activities, including the initiation of sister chromatid separation and the timing of exit from mitosis (See Zachariae and Nasmyth, 1999). Substrates of the Cdc20:APC/C complex, which are recognized by a motif known as the destruction box (D box) include Cyclin A, Nek2, Securin and Cyclin B. Degradation of Securin and Cyclin B does not occur until the mitotic spindle checkpoint has been satisfied (see Castro et al. 2005).<br>Cdc20 is degraded late in mitosis (Reviewed in Owens and Hoyt, 2005). At this time the activating subunit, Cdh1, previously maintained in an inactive phosphorylated state by mitotic kinases, is dephosphorylated and associates with and activates the APC/C. The APC/C:Cdh1 complex recognizes substrates containing a D box, a KEN box (Pfleger and Kirschner, 2000) or a D box activated (DAD) domain (Castro et al., 2002) sequence and promotes the ordered degration of mitotic cyclins and other mitotic proteins culminating with its own ubiquitin-conjugating enzyme (E2) subunit UbcH10 (Rape et al., 2006). This ordered degradation promotes the stability of Cyclin A at the end of G1. This stabilization, in turn, promotes the phosphorylation of Cdh1 and its abrupt dissociation from the APC/C, allowing accumulation of cyclins for the next G1/S transition (Sorensen et al., 2001). <br><br>

Anaphase Promoting Complex 或 Cyclosome (APC/C) 在有丝分裂过程中发挥作用，通过降解有丝分裂周期蛋白和Securin来促进姐妹染色单体分离和有丝分裂退出。此复合物在间期也保持活跃，确保G1期的适当长度（参见Peters，2002年的综述）。APC/C至少包含12个亚基，作为泛素-蛋白连接酶（E3）促进其靶蛋白的多泛素化（参见Gieffers等，2001年的研究）。在泛素化反应中，泛素通过与(E1)泛素激活酶形成硫酯键而被激活，然后转移到泛素连接酶（E2）的半胱氨酸残基上，最终在APC/C的泛素-蛋白连接酶活性帮助下转移到靶蛋白的赖氨酸残基上。生成的泛素链被认为靶向蛋白质，由26S蛋白酶体进行降解（参见Peters，1994年的综述）。APC/C在细胞周期中的活性高度周期性，并由一系列调节事件共同控制。APC/C通过磷酸化和激活亚基的调节招募而被激活，并通过与着丝粒相关检查点蛋白的隔离进行负调节。Emi1蛋白与Cdh1和Cdc20结合，在G1/S期和前期抑制APC/C。RSSA1可能在早期有丝分裂中抑制APC的作用。在有丝分裂激酶磷酸化APC/C核心亚基后，激活亚基Cdc20被招募到APC/C，并负责有丝分裂活动，包括姐妹染色单体分离的启动和有丝分裂退出的时机（参见Zachariae和Nasmyth，1999年的研究）。Cdc20:APC/C复合物的底物，通过称为破坏盒（D盒）的基序被识别，包括Cyclin A、Nek2、Securin和Cyclin B。Securin和Cyclin B的降解不会发生，直到有丝分裂纺锤体检查点得到满足（参见Castro等，2005年的研究）。Cdc20在有丝分裂晚期被降解（参见Owens和Hoyt，2005年的综述）。在此期间，激活亚基Cdh1，之前由有丝分裂激酶保持在不活化的磷酸化状态，被去磷酸化并与APC/C结合并激活。APC/C:Cdh1复合物识别含有D盒、KEN盒（参见Pfleger和Kirschner，2000年的研究）或D盒激活（DAD）结构域（参见Castro等，2002年的研究）序列的底物，并促进有丝分裂周期蛋白和其他有丝分裂蛋白的有序降解，最终包括其自身的泛素连接酶（E2）亚基UbcH10（参见Rape等，2006年的研究）。这种有序的降解促进了G1期末Cyclin A的稳定性。这种稳定性反过来又促进了Cdh1的磷酸化和其与APC/C的突然解离，允许积累周期蛋白进行下一个G1/S转换（参见Sorensen等，2001年的研究）。