The E2 binds to the HECT-E3 domain and transfers ubiquitin to the active site cysteine via a thioester bond from which it is transferred to the substrate protein. HECT domains use different mechanisms of chain elongation either by transferring individual ubiquitin molecules to a growing substrate-linked chain (A) or by pre-assembling chains on its active site cysteine before transferring the whole chain to a substrate (B).