Regulatory subunit of the dimeric Uba3-APP-BP1 E1 enzyme. E1 activates Nedd8 by first adenylating its C-terminal glycine residue with ATP, thereafter linking this residue to the side chain of the catalytic cysteine, yielding a Nedd8-Uba3 thioester and free AMP. E1 finally transfers Nedd8 to the catalytic cysteine of UbcE2M. Required for Cul1 and Cul3 neddylation. Appl and APP-BP1 interact antagonistically during development. Protein modification; protein neddylation. Heterodimer of Uba3 and APP-BP1. The complex binds Nedd8 and UbcE2M. Interacts with Appl (via the intracellular domain, ICD). Expressed throughout development. Hinders tissue development, causes apoptosis in imaginal disk cells, and blocks the Nedd8 conjugation pathway. Belongs to the ubiquitin-activating E1 family. ULA1 subfamily.