Another multi subunit RING containing ligase

Another multi-subunit RING-containing ligase is the Fanconi anemia (FANC) E3. There are at least 13 complementation groups associated with this disease, and Ademetionine corresponding to eight complementation groups are components of the FANC ubiquitin ligase, including a RING-type protein (FancL). This E3 is recruited to sites of DNA damage to effect translesional repair. Despite its complexity, the role of the FANC E3, as we currently understand it, is limited to monoubiquitination of two associated proteins that are subsequently deubiquitinated as part of the DNA repair process. Degradation of a key FANC component, FANCM, via SCFβTrCP is responsible for inactivating the function of the FANC E3 during mitosis, thereby preventing chromosomal abnormalities [3], [5], [40]. Some multi-subunit RING-type E3s contain multiple RING proteins. The yeast GID (glucose-induced degradation deficient) complex, which targets fructose-1, 6 bisphosphatase for ubiquitination in response to glucose, consists of seven subunits including two interacting RINGs [41]. The yeast PEX ubiquitin ligase, which mono-ubiquitinates the peroxisome receptor, Pex5p, and possibly other substrates, includes three distinct RING proteins as part of a multi-subunit complex [42], [43]. The specific function of each of the RINGs in such complexes is currently unknown. Finally, there are single proteins that contain multiple RINGs. Mindbomb, involved in Notch signaling, has three RINGs in its C-terminal region, although to date only the most C-terminal of these has been studied and shown to be required for activity [44]. RING–IBR–RING (RBR) proteins are a class of ~13 proteins (in humans) that include a RING consensus sequence (RING1) followed by a Cys-rich ‘in between RING’ (IBR) region and a third domain, the RING2, that was originally characterized as a second RING-like domain. Although RBR proteins were thought to function as canonical RING E3s, recent studies have shown that they employ a RING–HECT hybrid mechanism [45], [46], [47], [48], [49]. The RING1 domain binds E2 (similar to the RING mechanism) but ubiquitin is transferred to a specific Cys within RING2 before being transferred to substrates (similar to the HECT mechanism). Well-known members of this family include Parkin, HHARI, HOIP, and HOIL-1L. The latter two are subunits of the Linear Ubiquitin Chain Assembly Complex (LUBAC) E3 consisting of HOIP, HOIL-1L, and Sharpin (a non-RING-containing protein). This complex plays critical roles in NF-κB activation (reviewed in this issue by Kazuhiro et al).
RING-type E3s and their substrates There is enormous diversity in substrate ubiquitination and its regulation, as the targets of RING-type E3s are incredibly varied. RING-type E3s are implicated as tumor suppressors, oncogenes, and mediators of endocytosis, and play critical roles in complex multi-step processes such as DNA repair and activation of NF-κB signaling. A RING-type E3 may have multiple substrates and several E3s can target the same substrate. Not surprisingly, the mechanisms of substrate recognition by RING-type E3s are highly varied, and occur in the context of networks of interactions that often also include HECT E3s and deubiquitinating enzymes (DUBs). Substrates may bind directly to a RING-type E3 or may associate indirectly. The capacity of RING-type E3s for self-ubiquitination, first utilized as a means of assessing their potential to function with E2s [50], frequently occurs in vivo, as does ubiquitination of RING E3s by heterologous RING or HECT-type E3s as part of regulatory networks [51].