Regulatory subunit of the dimeric ECR1-AXL1 E1 enzyme. E1 activates RUB1/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 RUB1-ECR1 thioester and free AMP. E1 finally transfers RUB1 to the catalytic cysteine of RCE1 (Probable). May function redundantly with AXR1 in the RUB conjugating pathway (PubMed:17655650). Seems not to be functionally equivalent to AXR1 in vivo (PubMed:21311953). Protein modification; protein neddylation. Heterodimer of ECR1 and AXL1. The complex binds to RUB1/NEDD8 and RCE1. A number of isoforms are produced. According to EST sequences. Belongs to the ubiquitin-activating E1 family. ULA1 subfamily.