Multiubiquitin chains linked through lysine 48 are abundant in vivo and are competent intermediates in the ubiquitin proteolytic pathway

Abstract

A prerequisite for the selective degradation of intracellular proteins by the ubiquitin-dependent proteolytic pathway is the attachment of a chain of ubiquitin monomers to the targeted protein. In these multiubiquitin chains, the carboxyl-terminal glycine 76 of ubiquitin is linked via an isopeptide bond to the epsilon-amino group of lysine 48 in the adjacent ubiquitin. It remains to be determined whether these chains are preassembled and attached en masse to the target, are made by sequential conjugation of ubiquitin monomers to ubiquitins already linked to the protein, or both. Using the 20-kDa ubiquitin-conjugating enzyme TaUBC7 from wheat, we have generated free, glycine 76-->lysine 48-linked multiubiquitin chains (Ubqn), and have individually purified Ubqn species (n < or = 6) by anion-exchange, high pressure liquid chromatography. The migration of these chains during SDS-polyacrylamide gel electrophoresis was indistinguishable from those of major ubiquitin immunoreactive proteins in cell lysates from a variety of eukaryotes suggesting that free, multiubiquitin chains are abundant in vivo. One of these chain members (Ubq2) was purified from wheat and was demonstrated via amino acid sequence analysis of tryptic fragments to consist of two ubiquitin monomers joined via a glycine 76-->lysine 48 linkage. We also show in vitro that purified Ubq2 and Ubq4 are competent in activation by ubiquitin-activating enzyme (E1), in transfer to E2s, and in ubiquitin-protein ligase (E3)-independent conjugation to other Ubqn species and to histones H2A/B. These data demonstrate that multiubiquitin chains exist as free, functional structures in vivo and support the possibility that at least a portion of free ubiquitin is preassembled into multiubiquitin chains prior to its attachment to proteolytic substrates.