E1-like enzyme which specifically catalyzes the first step in ufmylation (PubMed:15071506, PubMed:18442052, PubMed:20368332, PubMed:25219498, PubMed:26929408, PubMed:27545674, PubMed:27545681, PubMed:27653677, PubMed:30412706, PubMed:30626644, PubMed:34588452). 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 (PubMed:20368332, PubMed:26929408, PubMed:27653677, PubMed:30412706). Activates UFM1 via a trans-binding mechanism, in which UFM1 interacts with distinct sites in both subunits of the UBA5 homodimer (PubMed:27653677). Trans-binding also promotes stabilization of the UBA5 homodimer, and enhances ATP-binding (PubMed:29295865). Transfer of UFM1 from UBA5 to the E2-like enzyme UFC1 also takes place using a trans mechanism (PubMed:27653677, PubMed:34588452). Ufmylation plays a key role in various processes, such as ribosome recycling, response to DNA damage, interferon response or reticulophagy (also called ER-phagy) (PubMed:30412706, PubMed:32160526, PubMed:35394863). Ufmylation is essential for erythroid differentiation of both megakaryocytes and erythrocytes (By similarity). {ECO:0000250|UniProtKB:Q8VE47, ECO:0000269|PubMed:15071506, ECO:0000269|PubMed:18442052, ECO:0000269|PubMed:20368332, ECO:0000269|PubMed:25219498, ECO:0000269|PubMed:26929408, ECO:0000269|PubMed:27545674, ECO:0000269|PubMed:27545681, ECO:0000269|PubMed:27653677, ECO:0000269|PubMed:29295865, ECO:0000269|PubMed:30412706, ECO:0000269|PubMed:30626644, ECO:0000269|PubMed:32160526, ECO:0000269|PubMed:34588452, ECO:0000269|PubMed:35394863}. This is the function of UBA5 (ubiquitin like modifier activating enzyme 5, ENSG00000081307).