For many E3 ligases, a mobile RING (Really Interesting New Gene) domain stimulates ubiquitin (Ub) transfer from a thioester-linked E2~Ub intermediate to a lysine on a remotely bound disordered substrate. One such E3 is the gigantic, multisubunit 1.2-MDa anaphase-promoting complex/cyclosome (APC), which controls cell division by ubiquitinating cell cycle regulators to drive their timely degradation. Intrinsically disordered substrates are typically recruited via their KEN-box, D-box, and/or other motifs binding to APC and a coactivator such as CDH1. On the opposite side of the APC, the dynamic catalytic core contains the cullin-like subunit APC2 and its RING partner APC11, which collaborates with the E2 UBCH10 (UBE2C) to ubiquitinate substrates. However, how dynamic RING?E2~Ub catalytic modules such as APC11?UBCH10~Ub collide with distally tethered disordered substrates remains poorly understood. In this paper, we report structural mechanisms of UBCH10 recruitment to APC<
sup>
CDH1<
/sup>
and substrate ubiquitination. Unexpectedly, in addition to binding APC11?s RING, UBCH10 is corecruited via interactions with APC2, which we visualized in a trapped complex representing an APC<
sup>
CDH1<
/sup>
?UBCH10~Ub?substrate intermediate by cryo-electron microscopy, and in isolation by X-ray crystallography. To our knowledge, this is the first structural view of APC, or any cullin?RING E3, with E2 and substrate juxtaposed, and it reveals how tripartite cullin?RING?E2 interactions establish APC?s specificity for UBCH10 and harness a flexible catalytic module to drive ubiquitination of lysines within an accessible zone. Finally, we propose that multisite interactions reduce the degrees of freedom available to dynamic RING E3?E2~Ub catalytic modules, condense the search radius for target lysines, increase the chance of active-site collision with conformationally fluctuating substrates, and enable regulation.