Ubiquitin (Ub) is a 76 amino acid protein widely expressed in the cytoplasmic and nucleus of cells. Ub is posttranslationally conjugated to proteins by the E1, E2, E3 protein ubiquitination cascade. Ub can be conjugated on proteins as monoUb or polyUb chains. Protein ubiquitination plays both proteolytic and nonproteolytic functions. Usually, polyubiquitinated proteins are targeted to the 26S proteasome for proteolysis. Typical concentration to support in vitro ubiquitination is 50-100 μM. In Ub(K29R), lysine 29 was substituted with an arginine, thereby it cannot form lysine 29-linked polyubiquitin chains.
