Purification and characterization of a multiprotein component of the Drosophila 26 S (1500 kDa) proteolytic complex

A multiprotein complex, referred to as the mu particle, was purified to apparent homogeneity from Drosophila melanogaster embryos. This multiprotein complex has no protease activity, but it can be incorporated into an even larger multiprotein complex which exhibits strong and selective protease activity, i.e. it degrades only ubiquitin-conjugated proteins. Incorporation of the mu particle into the ubiquitin conjugate-degrading larger complex is absolutely ATP-dependent. On these criteria the larger complex corresponds to the 26 S (1500 kDa) proteolytic complex partially purified and characterized from reticulocytes. A procedure is described for the purification of the Drosophila 26 S (1500 kDa) proteolytic complex. It was found to be a stoichiometric complex of the mu particle and the 20 S proteosome. Although no other polypeptide was present in stoichiometric amount in the 26 S (1500 kDa) proteolytic complex besides the mu particle and the 20 S proteosome, an additional protein factor(s) is required for its assembly, the ubiquitin conjugate-degrading activity cannot be reconstituted from the purified mu particle and the 20 S proteosome. Synthesis of the mu particle is developmentally regulated; its concentration is highest in embryos. This is probably connected with massive degradation of yolk proteins during embryogenesis. In chicken, rabbit and human cells a high molecular weight multiprotein complex can be detected, which is immunologically related to the Drosophila mu particle.