A comparative study of the cryo-EM structures of and human anaphase-promoting complex/cyclosome (APC/C).

The anaphase-promoting complex/cyclosome (APC/C) is a large multi-subunit E3 ubiquitin ligase that controls progression through the cell cycle by orchestrating the timely proteolysis of mitotic cyclins and other cell cycle regulatory proteins. Although structures of multiple human APC/C complexes have been extensively studied over the past decade, the APC/C has been less extensively investigated. Here, we describe medium resolution structures of three APC/C complexes: unphosphorylated apo-APC/C and the ternary APC/C-substrate complex, and phosphorylated apo-APC/C. Whereas the overall architectures of human and APC/C are conserved, as well as the mechanism of CDH1 inhibition by CDK-phosphorylation, specific variations exist, including striking differences in the mechanism of coactivator-mediated stimulation of E2 binding, and the activation of APC/C by phosphorylation. In contrast to human APC/C in which coactivator induces a conformational change of the catalytic module APC2:APC11 to allow E2 binding, in apo-APC/C the catalytic module is already positioned to bind E2. Furthermore, we find no evidence of a phospho-regulatable auto-inhibitory segment of APC1, that in the unphosphorylated human APC/C, sterically blocks the CDC20 binding site of APC8. Thus, although the functions of APC/C are conserved from to humans, molecular details relating to their regulatory mechanisms differ.