br RING dimerization RING type domains
RING dimerization RING-type domains are found in many different structural contexts. While many exist as single-chain Nutlin 3 (Fig. 3A), a notable feature of RING-type E3s is their tendency to form homodimers and heterodimers (Fig. 3C–F). Homodimeric RING-type E3s include cIAP, RNF4, BIRC7 (shown in Fig. 3D), IDOL, and the U-box proteins CHIP and Prp19 , , , , , , . Examples of well-characterized heterodimeric E3s include BRCA1–BARD1 (shown in Fig. 3F), Mdm2–MdmX (or HdmX/Hdm4 in humans), and RING1B–Bmi1. While for homodimeric RING E3s both RINGs have the intrinsic capacity to functionally interact with E2s, this appears not to be the case for some heterodimeric RINGs. BRCA1 and RING1B each function with E2, while their partners serve to enhance activity, potentially interact with substrates, and, in the case of BRCA1–BARD1, to stabilize the complex in vivo, , , . For the RING of BARD1, its lack of E2 binding activity can be attributed, at least in part, to the absence of a portion of the conserved central α-helix necessary for E2 interactions , . In contrast, there is evidence that MdmX, the ‘inactive’ partner, does physically interact with E2, in addition to having the capacity to bind the best-characterized Mdm2–MdmX substrate, p53. Importantly, while Mdm2 can homodimerize and is active, MdmX has little tendency to form a homodimer and is inactive, and the two RINGs can form an active heterodimer , . This underscores the important role played by RING dimerization (discussed further below). Strikingly, MdmX\'s lack of in vitro activity can be restored by mutating a single residue at the RING dimerization interface to that found in the analogous position of Mdm2 . However, additional mutations of MdmX to mimic the nucleolar localization sequence of Mdm2, found in its RING:E2 interface, are required for in vivo activity of MdmX towards p53 . Interestingly, since p53 exists largely as a homo-tetramer , there is the potential to assemble four Mdm2–MdmX heterodimers in close proximity. RING dimerization may also be a mode of cellular regulation of ubiquitination, as occurs for cIAP1, where its RING homodimerization interface is sequestered in a ‘closed’ inactive form until activation by IAP antagonists, such as SMAC (second mitochondrial activator of caspases) or DIABLO (direct IAP-binding protein with low isoelectric point) . Binding of SMAC or DIABLO to cIAP stabilizes it in an ‘open’ conformation that allows RING dimerization and thus, presumably, E2 binding and ubiquitin transfer. RING-type dimers are generally formed in one of two ways: 1) sequences outside the RING are primarily responsible for dimerization; or 2) the RING per se is responsible for dimerization. In both types, the two RINGs are positioned such that the E2 binding surfaces face away from each other (see Fig. 3D and F, surface highlighted in red), indicating that a direct cooperative interaction between the two RING-bound E2s in the context of a dimer is unlikely. At present, there is more structural characterization of RING dimers of the second type (Fig. 3C and D), which includes Mdm2–MdmX and homodimers of RNF4, IDOL, BIRC7, and cIAP. These RINGs are all found at the extreme C-terminus of the proteins containing them and the structures reveal that dimers are formed via interleaved C-termini, explaining previous reports of the importance of the C-termini in dimer stability and E3 ligase activity , , , , . In contrast, dimers of the first type are formed via interactions involving (usually) α-helical regions that flank each of the two RINGs (Fig. 3E and F). Proteins that form RING dimers in this manner include Rad18, BRCA1–BARD1, and RING1B–Bmi1 , , ; these dimers have their RINGs near their N- or C-termini. These two manners of dimerization are not necessarily mutually exclusive and, in fact, the available U-box structures of CHIP and Prp19 homodimers reveal distinct dimerization interfaces involving the U-box as well as regions N- and/or C-terminal to the U-box domain , . Some RING-type E3s have been shown to dimerize or form oligomers through domains that are structurally distinct and remote from the RING. Interestingly, proteins belonging to this group, such as Cbl family members and gp78 , , , , contain RINGs that are neither at the N- or C-terminus of the E3. Higher-order oligomers that bring together multiple RING-type dimers have also been reported for Prp19, which is active as a tetramer .