Kinetics of 125I-ubiquitin conjugation with and liberation from rabbit reticulocyte stroma.

The breakdown of mitochondria-containing stroma of rabbit reticulocytes is an ATP- and ubiquitin-dependent process and there is no evidence for an ATP-dependent but ubiquitin-independent proteolysis in these cells. The ubiquitin conjugate formation with heat-denatured stroma proteins is about one-fifth of that with native stroma. In reticulocytes there exist two mechanisms of ubiquitin liberation from its conjugates with stroma proteins: an ATP-dependent and hemin-resistant release of ubiquitin, which is assumed to be the first step in the degradation of ubiquitin conjugates by the protease system, and a release of ubiquitin catalyzed by an isopeptidase activity.