APC

APC (Anaphase-Promoting Complex) 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. The APC/C's main function is to trigger the transition from metaphase to anaphase by tagging specific proteins for degradation. The two proteins of most importance that get degraded in this process as substrates of the APC/C are securin and S and M cyclins. Securin releases separase, a protease, after being degraded which in turn 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.