Fig. 7. Affinity labeling of proteasome catalytic subunits in the presence of REGγ and REGγ (K188E). (A) A representative PhosphorImage of proteasome subunits labeled by ¹²⁵I-YL3-VS. (B) Quantitation of proteasome subunit labeling. Bands of activity corresponding to the T-like subunit and a combination of the unresolved CT and PGPH subunits were quantitated with the ImageQuant software (Molecular Dynamics). Total counts for each band were plotted as a function of time (CT+PGPH subunits, left panel; T subunit, right panel). (C) Stimulation of the initial labeling velocity by mutant and wild-type REGγ. The affinity labeling experiment has been repeated six times and the average stimulation by REG molecules is calculated as following: initial velocity in the presence of REG/initial velocity of proteasome.