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). Serves as sulfur donor in tRNA 2-thiolation reaction by being thiocarboxylated (-COSH) at its C-terminus by the MOCS3 homolog UBA4. The sulfur is then transferred to tRNA to form 2-thiolation of mcm(5)S(2)U. Prior mcm(5) tRNA modification by the elongator complex is required for 2-thiolation. Also acts as a ubiquitin-like protein (UBL) that is covalently conjugated via an isopeptide bond to lysine residues of target proteins such as AHP1. The thiocarboxylated form serves as substrate for conjugation and oxidative stress specifically induces the formation of UBL-protein conjugates. tRNA modification; 5-methoxycarbonylmethyl-2-thiouridine-tRNA biosynthesis. Homodimer; homodimerization may provide an autoprotection to the highly active C-terminal residue before attacking its substrates. Interacts with NCS2 and NCS6. Forms a conjugate with the target protein AHP1. C-terminal thiocarboxylation occurs in 2 steps, it is first acyl-adenylated (-COAMP) via the hesA/moeB/thiF part of UBA4, then thiocarboxylated (-COSH) via the rhodanese domain of UBA4. Belongs to the URM1 family.