APF-1/ubiquitin is shifted to high-molecular-mass compound(s) following incubation in ATP-containing crude cell extract.

¹²⁵I-labeled APF-1/ubiquitin was incubated with reticulocyte crude fraction II in the absence (open circles) or presence (closed circles) of ATP, and the reaction mixtures were resolved via gel filtration chromatography. Shown is the radioactivity measured in each fraction. As can be seen, following addition of ATP, APF-1/ubiquitin becomes covalently attached to some component(s) in fraction II, which could be another enzyme of the system or its substrate(s) (with permission from Proceedings of the National Academy of the USA; published originally in Ciechanover et al.³⁹).