Tom Kucharski from the Teodoro Lab presented a talk at the recent cell cycle meeting at Cold Spring Harbor Laboratory entitled: “53BP1 is a novel anaphase promoting complex/cyclosome mitotic substrate and regulator”. 


Cells respond to DNA damage through highly conserved mechanisms that result in the rapid recruitment of repair factors to sites known as ionizing radiation induced foci (IRIFs). Among such factors is the repair protein53BP1, which is a tumour suppressor known for its role in promoting DNA repair through the Non-homologous End Joining pathway (NHEJ).  The Anaphase Promoting Complex/Cyclosome is an E3 Ubiquitin Ligase whose activity is essential for proper mitotic progression. APC/C activity is therefore tightly regulated by a combination of post-translational modifications, co-activator proteins, and inhibitory proteins such as EMI and RASSF1A. Here we show that 53BP1 is itself a target of the APC/C during mitosis and that this regulation is important in the control of mitotic progression. 53BP1 contains three highly conserved KEN boxes that are required for its APC/C and proteasome dependent degradation during prometaphase, which occurs both during unperturbed mitosis and also under an active Spindle Assembly Checkpoint. Accordingly, 53BP1 is heavily ubiquitinated during mitosis through K11 linked Ubiquitin chains, a recently discovered hallmark of APC/C substrates. We also show that 53BP1 is constitutively bound to the APC/C co-activator cdc20 and crucially, inhibits its activity. However, binding of 53BP1 to cdc20 in interphase cells does not require the KEN boxes, but instead requires the tandem BRCT repeats of 53BP1. Silencing of 53BP1 expression results in a decrease in the levels of most APC/C substrates, which does not disturb cell proliferation under normal conditions, but is lethal when the cells are faced with the spindle poisons Nocodazole or Taxol. We therefore propose that 53BP1 may be a novel APC/C inhibitor. Consistent with this hypothesis, the half-lives of APC/C substrates are shortened in the absence of 53BP1, and the substrates are more extensively ubiquitinated. The degradation of 53BP1 in early mitosis, therefore appears to allow proper temporal activation of the APC/C and prevent premature mitotic entry, and thus likely contributes to the tumour suppressor status of 53BP1.