-!- In the first step the enzyme transfers ubiquitin from the E2 ubiquitin-conjugating enzyme (EC 2.3.2.23) to a cysteine residue in its HECT domain (which is located in the C-terminal region), forming a thioester bond. -!- In a subsequent step the enzyme transfers the ubiquitin to an acceptor protein, resulting in the formation of an isopeptide bond between the C-terminal glycine residue of ubiquitin and the epsilon- amino group of an L-lysine residue of the acceptor protein. -!- Cf. EC 2.3.2.27 and EC 2.3.2.31.