Figure 3: Interactions between ubiquitin (Ub) and E3.

(a–c) Close-up views of the E3-Ub interface from previously reported crystal structures. For the homodimeric E3s, Ub (orange) makes contacts with the E2-distal monomer (green). E3 residues shown in cyan mediate interactions with Ub. For the monomeric CBL-B, phosphorylation mediates the E3-Ub interaction. (d–g) Previous crystal structures were combined using structural overlays to generate models of E3–E2∼Ub complexes. A close-up view of the modelled E3-Ub interface is shown in each case (with RING monomers coloured as at top). Residues from the E3 that are expected to contact Ub are shown in cyan. (d) Model of cIAP2–UbcH5∼Ub complex. The experimental structure of cIAP2–UbcH5b (3EB6 (ref. 45)) was overlaid with that of BIRC7–UbcH5b∼Ub (4AUQ³⁰) by aligning the E2s. (e) Model of MDM2–MDMX–UbcH5∼Ub generated by aligning the MDM2 monomer from the MDM2–MDMX structure (2VJE⁷⁰) with the E2 proximal BIRC7 monomer from 4AUQ³⁰. (f,g) Models of PCGF5– and PCGF4–RING1B-UbcH5∼Ub generated by overlaying the structure reported here (f) or 3RPG (g) with 4AUQ³⁰ by aligning the E2s.