1C4Z

Ubiquitination is mediated by three enzymes, E1 (PDB:3cmm), E2 (PDB:1ayz) and E3 (PDB:1c4z). The first, E1, is essential for ubiquitin activation and transferring the substrate onto the second cascade enzyme, E2, which responsible for mediating repeated ubiquitination at the eventual substrate, which is brought into proximity of E2 by the active site of E3, the enzyme which also regulates substrate specificity. This entry represents the first of the three reactions occurring in this cascade: ubiquitin:[E1 ubiquitin-activating enzyme] ligase (AMP-forming)

E6AP belongs to the HECT (homologous to E6AP COOH-terminus) class of E3s, which has at least 20 members in humans. The HECT E3s are so far unique among the known classes of E3s in that they form a ubiquitin-thioester intermediate and directly catalyse substrate ubiquitination.

HECT E3s share a conserved ~40-kD COOH-terminal catalytic domain, the HECT domain (IPR000569), that has at least four biochemical activities: (i) it binds specific E2s; (ii) it accepts ubiquitin from the E2, forming a ubiquitin-thioester intermediate with its active-site cysteine; (iii) it transfers ubiquitin to the eta-amino groups of lysine side chains on the substrate by catalysing the formation of an isopeptide bond; and (iv) it transfers additional ubiquitin molecules to the growing end of the multi-ubiquitin chain. The NH2-terminal sequences of HECT E3s are not conserved and contain the primary determinants for specific substrate recognition.

HECT E3s have been implicated in cancer, hypertension, neurological disorders, and other diseases. Moreover, some pathogenic bacteria have evolved HECT-like E3s as virulence factors to manipulate host cell signalling.