Reductive methylation of lysine residues renders ubiquitin unable to form poly-ubiquitin chains via lysine linkages with other Ub molecules. Methylated Ub can form an E1-catalyzed active thioester at the C-terminus allowing the molecule to be transferred to the lysines of substrate proteins (which can be mono-ubiquitinated). Ideal for the reduction in poly-ubiquitin chain length and rates of ubiquitin conjugation.