This was done as previously described (44 (link)). Briefly, newly synthesized proteins were labeled using the Click-iT Protein Labeling kit (Invitrogen). Isogenic Nalm-6 cells were cultured in fresh medium for 24 hrs, washed twice with PBS, and resuspended in methionine-free RPMI 1640 medium (Gibco) supplemented with 10% FCS for 30 min, at which point the methionine analog L-azidohomoalanine (AHA; Invitrogen) was added (50 μM) to allow incorporation of AHA into nascent proteins. MG-132 (Sigma, M7449; 5 μM) was added at 4 hrs before the methionine replacement. Protein was extracted from the cells and 150 μg of total protein was used in the cross-linking of AHA-labeled nascent proteins to alkyne-derivatized biotin in Click-iT Protein Reaction Buffer (Invitrogen) according to the manufacturer’s instructions. Biotin-cross-linked nascent protein was captured overnight with streptavidin-coated Dynabeads (M-280, Invitrogen) and eluted. The whole volume of AHA-labeled, biotin-cross-linked, streptavidin-precipitated protein was separated by SDS–PAGE.