Additional in vivo ubiquitination experiments related to Fig. 4. (A) In vivo ubiquitination assay. Extracts from MCF7 cells treated in the presence or absence of KU-55933 and expressing Flag-MDM2, myc-FBXO31, and HA-ubiquitin (HA-Ub) were immunoprecipitated with a Flag antibody and probed for HA-ubiquitin. GFP expressed from a cotransfected plasmid was used as transfection/loading control. (B and C) In vivo ubiquitination assays. H1299 cells were transfected with plasmids expressing His-ubiquitin, Flag-MDM2, either myc-FBXO31 or myc-FBXO31∆F, and either GFP alone (B) or GFP-p53 (C). Proteins bound to His-ubiquitin were purified by Ni-NTA pull-down, washed, and eluted in imidazole. Ubiquitinated Flag-MDM2 and GFP-p53 were detected using an anti-Flag or anti-GFP antibody, respectively.