E3 (ubiquitin) ligases interact with both ubiquitin-E2 (ubiquitin-conjugating) enzymes and the target protein, transferring the ubiquitin from the E2 to the substrate. U-box ubiquitin ligases (UULs) possess a domain structurally related the RING finger domain found in many E3s, but lacks Zn²⁺-co-ordinating residues. Some UULs have been found to possess E4 activity, co-ordinating with E1, E2 and E3 enzymes to transfer polyubiquitin to target proteins. (Adapted from PMID:20979629 and PMID:16250894).