C, In vitro deubiquitylation UbiTest of endogenously ubiquitylated MED24. HCT116 cells were treated with MG132 in the presence or absence of shRNA targeting USP22. Lysates were generated using buffer containing protease cocktail inhibitor, pan-DUB inhibitor PR619, and the JAMM protease inhibitor o-phenanthroline. Ubiquitylated proteins were purified on ubiquitin binding resin and eluates were either undigested (lanes 1 and 2) or digested with USP2 to strip polyubiquitin (lanes 3 and 4) and reduce target proteins to unit length. Digestion reactions were subjected to immunoblotting with the indicated antibodies. 