Demonstration of two distinct high molecular weight proteases in rabbit reticulocytes, one of which degrades ubiquitin conjugates.

Reticulocytes contain a nonlysosomal proteolytic pathway that requires ATP and ubiquitin. By DEAE chromatography and gel filtration, we were able to fractionate the ATP-dependent system into a 30-300-kDa fraction that catalyzes the ATP-dependent conjugation of ubiquitin to substrates ("Conjugation Fraction") and a high mass fraction (greater than 450 kDa) necessary for hydrolysis of the conjugated proteins. The latter contains two distinct proteases. One protease is unusually large, approximately 1500 kDa, and degrades proteins only when ATP and the conjugating fractions are added. This activity precipitates at 0-38% (NH4)2SO4 saturation and is essential for ATP-dependent proteolysis. Like crude extracts, it is labile in the absence of nucleotides and is inhibited by heparin, poly(Glu-Ala-Tyr), 3,4-dichloroisocoumarin, hemin, decavanadate, N-ethylmaleimide, and various peptide chloromethyl ketones. It lacks amino-peptidase and insulin-degrading activities and does not require tRNA for activity. The ubiquitin-conjugate degrading enzyme, which we suggest be named UCDEN, is inactive against substrates that cannot undergo ubiquitin conjugation. The smaller protease (670 kDa), which precipitates at 40-80% (NH4)2SO4 saturation, does not require ATP or ubiquitin and is therefore not required for ATP-dependent proteolysis. It is stimulated by N-ethylmaleimide and 3,4-dichloroisocoumarin and is stable at 37 degrees C. It hydrolyzes fluorometric tetrapeptides and proteins, including proteins which cannot be conjugated to ubiquitin. Thus, reticulocytes contain two large cytosolic proteases: one is essential for the degradation of ubiquitin conjugates, while the function of the other is uncertain.