Anaphase-promoting Complex - Additional APC/C Regulation

Additional APC/C Regulation

APC/CCdc20 inactivation during early stages of the cell cycle is partially achieved by the protein Emi1. Initial experiments have shown that addition of Emi1 to Xenopus cycling exracts can prevent the destruction of endogenous cyclin A, cyclin B, and mitotic exit, suggesting that Emi1 is able to counteract the activity of the APC. Furthermore, depletion of Emi1 in somatic cells leads to the lack of accumulation of cyclin B. The lack of Emi1 likely leads to a lack of inhibition of the APC preventing cyclin B from accumulating.

From these early observations, it has been confirmed that in G2 and early mitosis, Emi1 binds and inhibits Cdc20 by preventing its association with APC substrates. Cdc20 can still be phosphorylated and bind to APC/C, but bound Emi1 blocks Cdc20’s interaction with APC targets. Emi1 association with Cdc20 allows for the stabilization of various cyclins throughout S and G2 phase, but Emi1’s removal is essential for progression through mitosis. Thus, in late prophase, Emi1 is phosphorylated by Polo-like kinase, Plk. Plk is activated during early mitosis by Cdk1 activity, and its phosporylation of Emi1’s BTRC (gene) βTrCP binding site makes it a target for SCF, leading to its subsequent destruction in prometaphase. Emi1’s destruction leads APC/CCdc20 activation, allowing for the destruction of cyclin A in early mitosis. Emi1 levels begin to rise again in G, which help inhibit APC/CCdh1.

Regulation of APC/CCdc20 activity towards metaphase substrates like securin and cyclin B may be a result of intracellular localization. It is hypothesized that spindle checkpoint proteins that inhibit APC/CCdc20 only associate with a subset of the Cdc20 population localized near the mitotic spindle. In this manner, cyclin A can be degraded while cyclin B and securin are degraded only once sister chromatids have achieved bi-orientation.