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Description

Ubiquitin-conjugating (E2) enzymes are characterized by the presence of a highly conserved ubiquitin-conjugating domain which accommodates ATP-activated ubiquitin (Ub) via a covalently linked thioester onto its active-site residue. E2 enzymes act via selective protein-protein interactions with the ubiquitin-activating E1 enzyme and ubiquitin ligase E3 enzymes and are able to differentiate effects on downstream substrates, either with a single Ub molecule or a Ub chain. While E3s are involved in substrate selection, E2s are the main determinants for selection of the lysine to construct Ub chains, which thereby directly control the cellular fate of the substrate. Typically, a polyubiquitin chain that targets a protein for degradation by the proteasome is linked through lysine-48 (K48) of ubiquitin, however ubiquitin chains linked through other lysines such as K63 have also been found. K63 linked chains are non-degradative and play a role in signalling and protein trafficking. The heterodimeric E2 complex of Ubc13 and Uev1A has been shown to be involved in the formation of K63 linkages in conjunction with a number of E3 ligases including Parkin, CHIP and TRAF6.