Acts as a sulfur carrier required for 2-thiolation of mcm(5)S(2)U at tRNA wobble positions of cytosolic tRNA(Lys), tRNA(Glu) and tRNA(Gln) (PubMed:19017811). Serves as sulfur donor in tRNA 2- thiolation reaction by being thiocarboxylated (-COSH) at its C-terminus by MOCS3 (PubMed:19017811, PubMed:22453920). The sulfur is then transferred to tRNA to form 2-thiolation of mcm(5)S(2)U (PubMed:19017811, PubMed:22453920). Also acts as a ubiquitin-like protein (UBL) that is covalently conjugated via an isopeptide bond to lysine residues of target proteins such as MOCS3, ATPBD3, CTU2, USP15 and CAS (PubMed:21209336). The thiocarboxylated form serves as substrate for conjugation and oxidative stress specifically induces the formation of UBL-protein conjugates (PubMed:21209336). {ECO:0000255|HAMAP-Rule:MF_03048, ECO:0000269|PubMed:19017811, ECO:0000269|PubMed:21209336, ECO:0000269|PubMed:22453920}. This is the function of Ensembl gene identifier ENSG00000167118 (URM1, ubiquitin related modifier 1).