p97 Inhibitor CB-5083 Blocks ERAD in Trypanosoma brucei

Misfolded proteins trapped in the endoplasmic reticulum (ER) are specifically recognized and retrotranslocated to the cytosol by the ER-Associated Degradation (ERAD) system and delivered to the proteasome for destruction. This process was recently described in Trypanosoma brucei (T. brucei) using the misfolded epitope tagged Transferrin Receptor subunits ESAG7:Ty and HA:ESAG6 (HA:E6). Critical to this work was the proteasomal inhibitor MG132. However, MG132 has off-target inhibitory effects on lysosomal Cathepsin L that could cause misinterpretation of turnover results. Here, we evaluate an orally bioavailable p97 inhibitor, CB-5083, for use in T. brucei. p97 is a ubiquitous protein involved in many cellular events including the membrane extraction step of ERAD. CB-5083 strongly inhibits turnover of HA:E6, with comparable protein recovery to MG132 treatment. Interestingly, little deglycosylated cytoplasmic species accumulates, though it normally emerges with MG132 treatment. This suggests that CB-5083 blocks ERAD upstream of the proteasome, as expected for inhibition of the trypanosomal p97 orthologue TbVCP. Under CB-5083 treatment, HA:E6 is also strongly membrane-associated, suggesting ER localization. Finally, we provide an experimental example where CB-5083 treatment offers clarity to the off-target effects of MG132 treatment.