E1-like enzyme which specifically catalyzes the first step in ufmylation. Activates ufm1 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 ufm1-E1 thioester and free AMP. Activates ufm1 via a trans-binding mechanism, in which ufm1 interacts with distinct sites in both subunits of the uba5 homodimer. Trans-binding also promotes stabilization of the uba5 homodimer, and enhances ATP-binding. Transfer of ufm1 from uba5 to the E2-like enzyme UFC1 also takes place using a trans mechanism. Ufmylation is involved in reticulophagy (also called ER-phagy) induced in response to endoplasmic reticulum stress. Homodimer; homodimerization is required for ufm1 activation. Interacts (via UIS motif) with ufm1; binds ufm1 via a trans-binding mechanism in which ufm1 interacts with distinct sites in both subunits of the uba5 homodimer. Interacts (via C-terminus) with ufc1. Belongs to the ubiquitin-activating E1 family. UBA5 subfamily.