Anaphase-promoting complex

Anaphase-promoting complex (also called the cyclosome or APC/C) is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cullin (Apc2) and RING (Apc11) subunit much like SCF. Other parts of the APC/C have unknown functions but are highly conserved. It was the discovery of the APC/C (and SCF) and their key role in eukaryotic cell-cycle regulation that established the importance of ubiquitin-mediated proteolysis in cell biology. Once perceived as a system exclusively involved in removing damaged protein from the cell, ubiquitination and subsequent protein degradation by the proteasome is now perceived as a universal regulatory mechanism for signal transduction whose importance approaches that of protein phosphorylation. In 2014, the APC/C was mapped in 3D at a resolution of less than a nanometre, which also uncovered its secondary structure. This finding could improve understanding of cancer and reveal new binding sites for future cancer drugs. The APC/C's main function is to trigger the transition from metaphase to anaphase by tagging specific proteins for degradation. The three major targets for degradation by the APC/C are securin and S and M cyclins. Securin releases separase, a protease, when degraded. Separase then triggers the cleavage of cohesin, the protein complex that binds sister chromatids together. During metaphase, sister chromatids are linked by intact cohesin complexes. When securin undergoes ubiquitination by the APC/C and releases separase, which degrades cohesin, sister chromatids become free to move to opposite poles for anaphase. The APC/C also targets the mitotic cyclins for degradation, resulting in the inactivation of M-CDK (mitotic cyclin-dependent kinase) complexes, promoting exit from mitosis and cytokinesis Unlike the SCF, activator subunits control the APC/C. Cdc20 and Cdh1 are the two activators of particular importance to the cell cycle.