A Multiubiquitin Chain Is Confined to Specific Lysine in a Targeted Short-Lived Protein

The ubiquitin-dependent degradation of a test protein beta-galactosidase (beta gal) is preceded by ubiquitination of beta gal. The many (from 1 to more than 20) ubiquitin moieties attached to a molecule of beta gal occur as an ordered chain of branched ubiquitin-ubiquitin conjugates in which the carboxyl-terminal Gly76 of one ubiquitin is jointed to the internal Lys48 of an adjacent ubiquitin. This multiubiquitin chain is linked to one of two specific Lys residues in beta gal. These same Lys residues have been identified by molecular genetic analysis as components of the aminoterminal degradation signal in beta gal. The experiments with ubiquitin mutated at its Lys48 residue indicate that the multiubiquitin chain in a targeted protein is essential for the degradation of the protein.