Catalyzes the first step in ubiquitin conjugation to mark cellular proteins for degradation through the ubiquitin-proteasome system (PubMed:1606621, PubMed:1447181, PubMed:33108101). Activates ubiquitin by first adenylating its C-terminal glycine residue with ATP, and thereafter linking this residue to the side chain of a cysteine residue in E1, yielding a ubiquitin-E1 thioester and free AMP (PubMed:1447181). Essential for the formation of radiation-induced foci, timely DNA repair and for response to replication stress. Promotes the recruitment of TP53BP1 and BRCA1 at DNA damage sites (PubMed:22456334). ATP + ubiquitin + [E1 ubiquitin-activating enzyme]-L-cysteine = AMP + diphosphate + S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Monomer (By similarity). Interacts with GAN (via BTB domain). Detected in erythrocytes (at protein level). Ubiquitous. The first 11 amino acids are essential for phosphorylation and exclusive nuclear localization. ISGylated. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Somatic variants affecting the initiator methionine of isoform 2 are recurrently found in VEXAS patients. These variants cause loss of isoform 2 and production of a shorter isoform with strongly reduced enzymatic activity from a downstream methionine (Met-67). There are two active sites within the E1 molecule, allowing it to accommodate two ubiquitin moieties at a time, with a new ubiquitin forming an adenylate intermediate as the previous one is transferred to the thiol site. Belongs to the ubiquitin-activating E1 family.