MM is dependent on DDI2/NRF1 for survival. (A) NRF1 processing in the face of adequate (left panel) or inadequate (right panel) proteasome activity. The ER localization domain, transactivating domain (TAD), and DNA binding domain (DBD) of NRF1, as well as deglycanase NGLY1, aspartic protease DDI2 and p97/VCP, are shown. Cleaved NRF1 translocates to the nucleus where it dimerizes with small MAF proteins, binds to antioxidant responsive elements (ARE), and induces transcription of proteasome subunit genes (PSM). (B) Western blot showing full length (FL), deglycosylated (DG), and processed (P) NRF1 (top blots); DDI2 (second blot); and polyUb proteins (third blot) across a panel of MM, AL amyloidosis (AL), B cell lymphoma (B-CELL LYMPH), and T acute lymphoblastic leukemia (T-ALL) cell lines. In comparison with SU-DHL4 and SU-DHL10 cell lines (red box), all other cell lines are characterized by higher polyUb protein accumulation and increased expression of NRF1. GAPDH served as a loading control (bottom blot). (C) CERES score for DDI2 across 342 cancer cell lines assessed via genomic CRISPR screening as part of the DepMap project. Using this approach, a CERES score of −1 identifies an essential gene. MM (green stars indicated by a black arrow) is the cancer cell type that is most highly dependent on DDI2. (D-E) Relative percentage of living cells (Annexin V⁻ and PI⁻ on flow cytometry) after pulse treatment with specific doses of bortezomib (D) or carfilzomib (E), normalized against cells treated with DMSO (control).³³  Data are the average of 3 biological replicates with standard deviation.