Pan-branch Ubiquitin TUBE-UBQLN1 (Biotinylated) #67535

Ubiquitin is a conserved polypeptide unit that plays an important role in the ubiquitin-proteasome pathway. Ubiquitin can be covalently linked to many cellular proteins by the ubiquitination process, which targets proteins for degradation by the 26S proteasome. Three components are involved in the target protein-ubiquitin conjugation process. Ubiquitin is first activated by forming a thiolester complex with the activation component E1; the activated ubiquitin is subsequently transferred to the ubiquitin-carrier protein E2, then from E2 to ubiquitin ligase E3 for final delivery to the epsilon-NH₂ of the target protein lysine residue (1-3). The ubiquitin-proteasome pathway has been implicated in a wide range of normal biological processes and in disease-related abnormalities. Several proteins such as IκB, p53, cdc25A, and Bcl-2 have been shown to be targets for the ubiquitin-proteasome process as part of regulation of cell cycle progression, differentiation, cell stress response, and apoptosis (4-7).

Substrate proteins are linked to ubiquitin using seven distinct ubiquitin lysine residues (Lys6, Lys11, Lys27, Lys29, Lys33, Lys48, and Lys63). Formation of a polyubiquitin chain occurs when a lysine residue of ubiquitin is linked to the carboxy-terminal glycine of another ubiquitin. Proteins polyubiquitinated at specific lysine residues display a tendency to be targeted for different processes; K48-linked polyubiquitin chains mainly target proteins for proteasomal degradation, while K63-linked polyubiquitin chains regulate protein function, subcellular localization, or protein-protein interactions (8). K63-linked polyubiquitin chains exert nonproteolytic functions in vivo, such as protein trafficking, kinase/phosphatase activation, and DNA damage control, all of which might be important in regulation of cancer survival and development (9,10).

Ubiquitin-associated (UBA) domains are protein regions that interact with ubiquitin. Tandem-repeated ubiquitin-binding entities (TUBEs) were designed by using four tandem UBA domains, based on the theory that tetraubiquitin chains are a minimum requirement for efficient proteasomal degradation (11). TUBEs designed with UBA domains from UBQLN1 and RAD23A bind to K48- and K63-linked tetraubiquitin chains and can be used to efficiently purify ubiquitylated proteins from cell extracts (12).