These structural features suggest that CC0651 acts as a molecular bridge between Cdc34A and ubiquitin. == Figure 3. spatial and temporal control Biperiden of protein stability, activity and/or localization1, and is frequently RAB11FIP4 dysregulated in cancer and other diseases2,3. The conserved E1-E2-E3 enzyme cascade activates and transfers ubiquitin through step-wise thioester linkages that culminate in covalent conjugation of ubiquitin to free amino groups on substrate proteins. The resultant mono- or polyubiquitination of the substrate typically leads to altered protein interactions or destruction by the 26S proteasome, respectively1,4,5. The E2 enzymes lie at a crucial nexus in the UPS hierarchy as they exhibit specific interactions with E1 enzymes, E3 enzymes, deubiquitinating enzymes and substrates. E2 enzymes contain an essential catalytic cysteine that forms the ubiquitin thioester and an Biperiden adjacent invariant asparagine residue that stabilizes the oxyanion transition state6,7. Weak protein interactions between the E2 and ubiquitin are important for catalysis. In particular, the donor site tethers the thioesterified ubiquitin to prevent steric occlusion of the reaction centre and allow efficient attack of the thioester by the incoming substrate nucleophile, whereas the acceptor site orients the incoming ubiquitin to guide formation of the appropriate ubiquitin chain linkage8-10. The detailed structural understanding of the ubiquitin transferase reaction has been hampered by the transient and structurally complex nature of these non-covalent catalytic intermediates. The cullin-RING ligases (CRLs) form the largest family of E3 enzymes and are built on a core cullin-based architecture that recruits many hundreds of substrates through cohorts of different adaptor proteins11-13. The Rbx1 RING domain subunit provides the docking site for Cdc34A (Ube2R1) and Cdc34B (Ube2R2), which are the principal E2s for the CRL family. Weak electrostatic interactions between the acidic C-terminus of Cdc34A and a basic cleft on the cullin subunit facilitate rapid cycles of E2 loading/unloading in the complex14and stabilize the E2-cullin interaction15. CRL enzyme activity depends on the reversible modification of the cullin subunit by the ubiquitin-like modifier Nedd8, which triggers a conformational release of the Rbx1 subunit and the docked E2 enzyme to enable the E2 to access the bound substrate16. Global CRL activity has been validated as a cancer target through development of a Nedd8 activating enzyme (NAE1) inhibitor called MLN4924 that traps NAE1 in a stable intermediate with Nedd8 and drives all CRLs into inactive non-neddylated forms17,18. MLN4924 potently inhibits cancer cell proliferation in pre-clinical models, primarily through perturbation of cell cycle, DNA replication and DNA damage/repair functions3. As a parallel strategy to inhibit CRL activity, we recently identified a small molecule called CC0651 as a specific Biperiden inhibitor of the human E2 enzyme Cdc34A19. Like MLN4924, CC0651 stabilizes the Biperiden CDK inhibitor p27 in cultured cells and inhibits the proliferation of human cancer cell lines. A previous structure of the CC0651-Cdc34A complex showed that CC0651 binds a cryptic pocket on the Cdc34A surface that is far removed from the active site cysteine but did not explain the mechanism of inhibition19. Here, we show that CC0651 unexpectedly traps the weak interaction between ubiquitin and the donor site of Cdc34A and thereby impedes catalysis. == Results == == Interactions between CC0651, Cdc34A and free ubiquitin == A partial overlap between the CC0651 binding site and a predicted donor ubiquitin binding surface on Cdc34A19lead us to investigate the interactions between CC0651, Cdc34A and free ubiquitin. We developed a synthetic route for CC0651 in order to produce sufficient quantities for structural and biophysical studies, and showed that the preparations used were of virtually identical purity.