The APC/C is a multisubunit E3 ubiquitin ligase complex that becomes activated during mitosis through Cdc20 binding and targets D-box–containing substrates for proteasome-mediated degradation that ends mitosis. Cdc20 binding to the D-box motif of the substrate can strengthen existing APC/C and Cdc20 interactions. Tosyl-L-arginine methyl ester (TAME) is a small molecule known to disrupt the interaction between APC and Cdc20 and transiently arrest cells in mitosis. Sackton et al. thought that it might be possible to cause permanent mitotic arrest and eventual cell death by disrupting APC/C–mediated interactions with substrates. One potential small-molecule candidate was apcin, which was previously described as an inhibitor of cyclin degradation, but the identity of its targets remained unknown. The authors incubated Xenopus laevis extracts with apcin-coupled beads and found that Cdc20 was specifically removed from the extract, resulting in the loss of ubiquitinylation and eventual stabilization of substrates such as cyclin B1 and securin. Apcin binding to Cdc20 could not be disrupted by the addition of TAME, suggesting distinct binding interactions for the two compounds. They determined the crystal structure of the apcin–Cdc20 complex, which revealed that apcin bound to the D-box–binding site of Cdc20, preventing access for substrate binding. The combination of apcin and TAME exhibited synergistic effects in stabilizing APC/C substrates and increased mitotic duration compared to single drug treatments, confirming that blocking multiple protein-protein interactions of a large protein machine may be an effective targeting strategy.
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Miura, G. Cell cycle: Mitotic tag team. Nat Chem Biol 10, 876 (2014). https://doi.org/10.1038/nchembio.1675