Mutation of lysine 48 to arginine renders ubiquitin (Ub) unable to form poly-Ub chains via lysine 48 linkages with other Ub molecules. Ub K48R c form E1-catalyzed active thioester at the C-terminus allowing the molecule to be trsferred to the lysines of substrate proteins (monoubiquitination). Ideal for the reduction in poly-Ub chain length/conjugation rates d determining if poly-Ub chains are K48 linked.