Qin L, Mizrak A, Guimaraes DSPSF, Tambrin HM, Morgan DO, Hall MC

The anaphase-promoting complex/cyclosome (APC/C) is a large, multi-subunit ubiquitin ligase involved in regulation of cell division. APC/C substrate specificity arises from binding of short degron motifs in its substrates to transient activator subunits, Cdc20 and Cdh1. The destruction box (D-box) is the most common APC/C degron and plays a crucial ...

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role in substrate degradation by linking the activator to the Doc1/Apc10 subunit of core APC/C to stabilize the active holoenzyme and promote processive ubiquitylation. Degrons are also employed as pseudosubstrate motifs by APC/C inhibitors, and pseudosubstrates must bind their cognate activators tightly to outcompete substrate binding while blocking their own ubiquitylation. Here, we examined how APC/C activity is suppressed by the small pseudosubstrate inhibitor Acm1 from budding yeast (Saccharomyces cerevisiae). Mutation of a conserved D-box converted Acm1 into an efficient ABBA motif-dependent APC/CCdh1 substrate in vivo, suggesting that this D-box somehow inhibits APC/C. We then identified a short, conserved sequence at the C terminus of the Acm1 D-box that was necessary and sufficient for APC/C inhibition. In several APC/C substrates, the corresponding D-box region proved important for their degradation, despite poor sequence conservation, redefining the D-box as a 12-amino acid motif. Biochemical analysis suggested that the Acm1 D-box extension inhibits reaction processivity by perturbing the normal interaction with Doc1/Apc10. Our results reveal a simple, elegant mode of pseudosubstrate inhibition that combines high-affinity activator binding with specific disruption of Doc1/Apc10 function in processive ubiquitylation.

Date: Sep. 27, 2019

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