The post-translational attachment of ubiquitin () to proteins (ubiquitinylation) alters the function, location or trafficking of a protein, or targets it to the 26S proteasome for degradation [, , ]. Ubiquitinylation is an ATP-dependent process that involves the action of at least three enzymes: a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2, ), and a ubiquitin ligase (E3, , ), which work sequentially in a cascade []. The E1 enzyme is responsible for activating ubiquitin, the first step in ubiquitinylation. The E1 enzyme hydrolyses ATP and adenylates the C-terminal glycine residue of ubiquitin, and then links this residue to the active site cysteine of E1, yielding a ubiquitin-thioester and free AMP. To be fully active, E1 must non-covalently bind to and adenylate a second ubiquitin molecule. The E1 enzyme can then transfer the thioester-linked ubiquitin molecule to a cysteine residue on the ubiquitin-conjugating enzyme, E2, in an ATP-dependent reaction.This entry includes Ubiquitin-activating enzyme E1 (Uba1), SUMO-activating enzyme subunit 1 (Sae1) and similar proteins from eukaryotes. Sae1 is an heterodimer that acts as an E1 ligase for SUMO1, SUMO2, SUMO3, and probably SUMO4 and mediates ATP-dependent activation of SUMO proteins [, , ].
