RING-Between-RING E3 Ligases: Emerging Themes amid the Variations

摘要

Abstract Covalent, reversible, post-translational modification of cellular proteins with the small modifier, ubiquitin (Ub), regulates virtually every known cellular process in eukaryotes. The process is carried out by a trio of enzymes: a Ub-activating (E1) enzyme, a Ub-conjugating (E2) enzyme, and a Ub ligase (E3) enzyme. RING-in-Between-RING (RBR) E3s constitute one of three classes of E3 ligases and are defined by a RING-HECT-hybrid mechanism that utilizes a E2-binding RING domain and a second domain (called RING2) that contains an active site Cys required for the formation of an obligatory E3~Ub intermediate. Albeit a small class, RBR E3s in humans regulate diverse cellular process. This review focuses on non-Parkin members such as HOIP/HOIL-1L (the only E3s known to generate linear Ub chains), HHARI and TRIAD1, both of which have been recently demonstrated to work together with Cullin RING E3 ligases. We provide a brief historical background and highlight, summarize, and discuss recent developments in the young field of RBR E3s. Insights reviewed here include new understandings of the RBR Ub-transfer mechanism, specifically the role of RING1 and various Ub-binding sites, brief structural comparisons among members, and different modes of auto-inhibition and activation. Graphical abstract Display Omitted Highlights ? Intramolecular contacts cause auto-inhibition of RBRs; release requires domain rearrangements. ? Mechanism of activation differs among RBR E3s. ? RBR RING1 domains bind an E2~Ub in an open conformation. ? Open E2~Ubs have low aminolysis activity ensuring transfer of Ub from E2~Ubs to the E3 active site. ? Contacts between the Ub of E2~Ub and RBR domains are required to generate the E3~Ub. ? RBR E3s (and not their E2s) dictate the type of Ub product made to the substrate.