This enzyme is responsible for the first step in ubiquitin-protein isopeptide bond formation, by forming a high-energy thioester bond with ubiquitin. The ubiquitin is activated by first adenylating with ATP its C-terminal glycine residue (Gly76) and thereafter linking this residue to the side chains of a cysteine residue in UBE1, yielding an ubiquitin-UBE1 thiolester and free AMP. The activated ubiquitin is then transferred to a lysine of the targeted protein via the E2-E3 conjugation cascade. UBE1 is a critical component for the initiation of any in vitro conjugation reactions.