Plays a central role in 2-thiolation of mcm(5)S(2)U at tRNA wobble positions of tRNA(Lys), tRNA(Glu) and tRNA(Gln). May act by forming a heterodimer with NCS6 that ligates sulfur from thiocarboxylated URM1 onto the uridine of tRNAs at wobble position. Prior mcm(5) tRNA modification by the elongator complex is required for 2-thiolation. May also be involved in protein urmylation. tRNA modification; 5-methoxycarbonylmethyl-2-thiouridine-tRNA biosynthesis. Interacts with NCS6 and URM1. May act by forming a heterodimer with NCS6. Belongs to the CTU2/NCS2 family.