RING and HECT-type mechanisms of ubiquitin transfer. (a) On the left, a RING E3 ligase (blue) is shown bound to a ubiquitin-conjugated E2, from which the ubiquitin is transferred to a lysine on the substrate. On the right, a HECT E3 ligase (orange) is shown bound to a ubiquitin-conjugated E2, from which ubiquitin is first transferred to the active-site cysteine of the E3, and is then transferred to a lysine on the substrate bound to the E3 (lower panel). (b) Proposed mechanism for RBR ubiquitin transfer. RBR ligases combine features of both RING- and HECT-type ligases. The ubiquitin-conjugated E2 binds to the RING1 domain of the RBR E3 ligase. The ubiquitin is then transferred from the E2 to the E3 RING2 domain from which it is transferred to the substrate.