Fig. 8: Working model of the RBR mechanism.
From: The unifying catalytic mechanism of the RING-between-RING E3 ubiquitin ligase family
RBR E3 ligases (green) are activated by allosteric Ub or UBLs which bind a conserved site in the RING1-IBR helix. Allosteric Ub or UBL binding at this site induces a conformational change that promotes E2-Ub binding. E2-Ub binds to the RBR via a canonical E2 interface in the RING1 domain and extensive interactions between donor Ub and the IBR-RING2 helix, establishing a conserved transthiolation complex in which E2-Ub is stabilised in the open conformation and E2/E3 active sites (yellow circles) are aligned. Ub transfer from E2 to the RING2 active site (transthiolation) permits release of E2, and binding of incoming substrate molecules (including acceptor Ub) to the RING2 domain. Structural adaptations within the RING2 domain confer substrate and Ub chain type specificity to the RBR E3 ligases. Ub transfer to the bound substrate and subsequent release of the ubiquitinated substrate completes the cycle. The poly-Ub product of catalysis may function as an allosteric activator itself, providing feed-forward amplification of the ubiquitination cycle. The model combines this work and previous studies on HOIP, HHARI, Parkin and RNF216 as detailed in the main text.
