Protocol for cytoskeleton staining of the semi-adherent multiple myeloma cell line RPMI 8226 by immunofluorescence.

Preservation of fine cellular details of semi-adherent or suspension cells for imaging by immunofluorescence is challenging. This protocol enables staining of floating cells with minimal morphological distortions, as we demonstrate with the semi-adherent multiple myeloma cell line RPMI 8226. We describe steps to better preserve structural details by fixing, permeabilizing, and staining cells in solution, while minimizing the number of centrifugation steps and centrifugation g-force. For complete details on the use and execution of this protocol, please refer to Osei-Amponsa et al.1.

hRpn13 shapes the proteome and transcriptome through epigenetic factors HDAC8, PADI4, and transcription factor NF-κB p50.

The anti-cancer target hRpn13 is a proteasome substrate receptor. However, hRpn13-targeting molecules do not impair its interaction with proteasomes or ubiquitin, suggesting other critical cellular activities. We find that hRpn13 depletion causes correlated proteomic and transcriptomic changes, with pronounced effects in myeloma cells for cytoskeletal and immune response proteins and bone-marrow-specific arginine deiminase PADI4. Moreover, a PROTAC against hRpn13 co-depletes PADI4, histone deacetylase HDAC8, and DNA methyltransferase MGMT. PADI4 binds and citrullinates hRpn13 and proteasomes, and proteasomes from PADI4-inhibited myeloma cells exhibit reduced peptidase activity. When off proteasomes, hRpn13 can bind HDAC8, and this interaction inhibits HDAC8 activity. Further linking hRpn13 to transcription, its loss reduces nuclear factor κB (NF-κB) transcription factor p50, which proteasomes generate by cleaving its precursor protein. NF-κB inhibition depletes hRpn13 interactors PADI4 and HDAC8. Altogether, we find that hRpn13 acts dually in protein degradation and expression and that proteasome constituency and, in turn, regulation varies by cell type.