NEDD8 Activating Enzyme 1 (APPBP1/UBA3) is a heterodimeric NEDD8-activating (E1) enzyme with a predicted molecular weight of 112 kDa. It is responsible for the first step in the conjugation of NEDD8 to protein substrates. The NEDD8 activating enzyme heterodimer is composed of a regulatory subunit, APPBP1 and a catalytic subunit, ubiquitin-like modifier activating enzyme 3 (UBA3). Human APPBP1 is a 534 amino acid (aa) protein with a predicted molecular weight of 60 kDa that is expressed ubiquitously in fetal tissues and in the adult brain. APPBP1 is required for UBA3 neddylation activity, regulates enzyme specificity and is expressed as two isoforms, the full length protein and a second isoform with an alternate N-terminal, aa 1-17 sequence. APPBP1 has been shown to drive cell cycle progression and its expression is increased in the hippocampus of Alzheimer's disease brains. Human UBA3 is a 463 aa protein with a predicted molecular weight of 52 kDa. It is ubiquitously expressed and shares high aa sequence identity with the C-terminal domain of human UBE1. UBA3 contains an ATP-binding domain and an active site cysteine residue, Cys237 in humans, which are both common to E1 enzymes. Like APPBP1, two isoforms of UBA3 have been identified in humans, the full length protein and a truncated isoform, which lacks aa 8-21. UBA3 is required for cell cycle progression and has been shown to downregulate steroid receptor activation. Neddylation and its associated enzymes have been implicated in the progression of Alzheimer's disease, via neddylation of APP, and cancer via post-translational modification of oncogenes.