SAE1, also known as AOS1, belongs to the ubiquitin-activating E1 family of proteins and plays an important role in the first step of the UBL1 conjugation pathway. Proteins conjugated to Ub are marked for progressive degradation by the 26S Proteasome. SAE1, which is a dimeric enzyme, functions as a UBLI E1 ligase mediating the ATP-dependent activation of UBL1.