E2s: structurally economical and functionally replete

Abstract

Synopsis Ubiquitination is a post-translational modification pathway involved in myriad cellular regulation and disease pathways. The ubiquitin (Ub) transfer cascade requires three enzyme activities: a Ub-activating (E1) enzyme, a Ub-conjugating (E2) enzyme, and a Ub ligase (E3). Because the E2 is responsible both for E3 selection and substrate modification, E2s function at the heart of the Ub transfer pathway and are responsible for much of the diversity of Ub cellular signaling. There are currently over ninety three-dimensional structures of E2s, both alone and in complex with protein binding partners, providing a wealth of information regarding how E2s are recognized by a wide variety of proteins. In this review, we describe the prototypical E2/E3 interface and discuss limitations of current methods to identify cognate E2/E3 partners. We present non-canonical E2-protein interactions and highlight the economy of E2s in their ability to facilitate many protein-protein interactions at nearly every surface on their relatively small, compact catalytic domain. Lastly, we compare the structures of conjugated E2~Ub species, their unique protein interactions, and the mechanistic insights provided by species that are poised to transfer Ub.

Reconstruction of an active SOCS3-based E3 ubiquitin ligase complexin vitro: identification of the active components and JAK2 and gp130 as substrates

“…Both of these were shown to be active. There are approximately fifty E2 enzymes encoded in the human genome 32; 33 . We tested a panel of 13 of these and discovered four that were active.…”

The MDM2 gene family

“…Upon activation, ubiquitin is transferred from E1 to a catalytic cysteine on E2, forming a thioester-linked conjugate. The E2-ubiquitin conjugate engages E3 and, together, they transfer ubiquitin from E2 to the ε -amino group of a lysine side chain on the target protein (48). In many instances, RING domains within E3s are interaction sites for E2s, and the presence of RING domains is assumed to be indicative of E3 ubiquitin ligase function (49, 50).…”

Association of the Disordered C-terminus of CDC34 with a Catalytically Bound Ubiquitin

“…The E2 Ub-conjugating enzyme is a key protein in this cascade as it accomplishes at least three important tasks: (1) recognizing both E1 and E3 enzymes, (2) forming a covalent thiolester with Ub, and, in the case of RING E3-ligases, (3) labeling the protein substrate. [2][3][4] Cell division cycle protein 34 (CDC34; E.C. 6.3.2.19) is a class II E2-conjugating enzyme that participates in the Ub-dependent degradation pathway of cell cycle proteins.…”