Ubiquitin Chain Elongation Requires E3-Dependent Tracking of the Emerging Conjugate

• The APC/C activates its E2 Ube2S in a Cdc20-dependent manner
• APC/CCdc20 increases the affinity of Ube2S for an acceptor ubiquitin
• APC/CCdc20 and Ube2S track the tip of an emerging ubiquitin chain
• Tracking a ubiquitin chain’s tip by the APC/C is required for accurate mitosis

SummaryProtein modification with ubiquitin chains is an essential signaling event catalyzed by E3 ubiquitin ligases. Most human E3s contain a signature RING domain that recruits a ubiquitin-charged E2 and a separate domain for substrate recognition. How RING-E3s can build polymeric ubiquitin chains while binding substrates and E2s at defined interfaces remains poorly understood. Here, we show that the RING-E3 APC/C catalyzes chain elongation by strongly increasing the affinity of its E2 for the distal acceptor ubiquitin in a growing conjugate. This function of the APC/C requires its coactivator as well as conserved residues of the E2 and ubiquitin. APC/C’s ability to track the tip of an emerging conjugate is required for APC/C-substrate degradation and accurate cell division. Our results suggest that RING-E3s tether the distal ubiquitin of a growing chain in proximity to the active site of their E2s, allowing them to assemble polymeric conjugates without altering their binding to substrate or E2.

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