Fig. 1. Discovery of the ligation of ubiquitin to lysozyme, a substrate of the proteolytic system.
Reaction products were separated by SDS–PAGE. Lane 1, incubation of 125I-labeled
ubiquitin with fraction 1 in the absence of ATP; ubiquitin remains free and migrates at the
front. Lanes 2–5, incubation of 125I-labeled ubiquitin with fraction 1 in the presence of ATP.
Lane 2, ubiquitin becomes covalently linked to many high-molecular-weight derivatives,
presumably endogenous protein substrates present in fraction 2. Lanes 3–5, several new …