URM1: ProductsUbiquitin Related Modifier 1 (URM1) is a conserved, ubiquitously expressed cytoplasmic Ubiquitin-like protein. It is 101 amino acids (aa) in length with a predicted molecular weight of 11.4 kDa. Human URM1 shares 93% and 94% aa sequence identity with the mouse and rat orthologs, respectively. URM1 contains a core beta-grasp fold and a C-terminal glycine-glycine motif characteristic of Ubiquitin and Ubiquitin-like modifiers. URM1 is activated by the Ubiquitin-activating (E1)-like enzyme MOCS3, Uba4 in yeast, via the formation of a thiocarboxylate group at its C-terminus. More specifically, MOCS3 and Uba4 mediate the adenylation and subsequent addition of sulfur to the carboxyl group of the C-terminal glycine residue of URM1. Thiocarboxylated URM1 functions as a sulfur carrier in the thiolation of eukaryotic cytoplasmic transfer RNA. URM1 can also be conjugated to lysine residues of several proteins including yeast Ahp1 and human MOCS3, CTU1, and CTU2. URM1 has been implicated in oxygen stress tolerance and in nutrient sensing, invasive growth, and budding in yeast.