RING type dimers are generally formed in one of
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 JWH 015 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 .
Multi-subunit RINGs There are RING-type E3s that exist as multi-subunit assemblies (see Fig. 3B). A striking example is the Cullin RING Ligase (CRL) superfamily , which exhibits enormous plasticity in substrate specificity. Each CRL subfamily is characterized by a cullin protein (Cul-1, 2, 3, 4a, 4b, 5, or 7), a small RING protein (in most cases Rbx1/Roc1/Hrt1), and either an adaptor protein(s) that binds interchangeable substrate recognition elements or, in the case of CRL3, proteins that bind both to the cullin protein (Cul-3) and to substrate . The CRL superfamily is exemplified by the well-studied Skp1-Cul1-F-box protein (SCF) family (Fig. 3B), in which one of ~69 (in humans) interchangeable F-box proteins can potentially recognize substrates  (reviewed in this issue by Bassermann et al.). Exchange of F-box proteins within the SCF scaffold takes place through a complex cycle that includes dynamic attachment and removal of the ubiquitin-like modifier, Nedd8 . While the CRL superfamily overwhelmingly exhibits the greatest range of substrate recognition, other multi-subunit E3s exhibit even greater structural complexity. The anaphase promoting complex/cyclosome (APC/C) is a highly complex E3 that in humans contains 13 core subunits including a cullin-like protein and a small RING protein. It also has two interchangeable co-activator subunits, Cdc20 and Cdh1, which recognize distinct substrates and are active during different phases of the cell cycle  (reviewed in this issue by Bassermann et al.).