Silencing of the proteasome and oxidative stress impair endoplasmic reticulum targeting and signal cleavage of a prostate carcinoma antigen.

The endoplasmic reticulum (ER) is an organelle with high protein density and therefore prone to be damaged by protein aggregates. One proposed preventive measure is a pre-emptive quality control pathway that attenuates ER import during protein folding stress. ER resident proteins are targeted into the ER via signal peptides cleaved rapidly upon ER insertion by the ER signal peptidase. Here we show that the ER insertion and cleavage of the ER-targeting peptide of the prostate carcinoma antigen prostate stem cell antigen (PSCA) is retarded and strongly reduced when the proteasome is inhibited or genetically silenced. Also overexpression of the C-terminally extended ubiquitin variant Ub-UBB or oxidative stress attenuated signal peptide processing. Proteasome inhibition likewise protracted ER signal processing of the ER targeted hormone leptin and the MHC class I molecule H-2D. These findings, which are consistent with a pre-emptive ER quality control pathway, may explain why an immunodominant MHC class I peptide ligand of PSCA spanning its ER signal peptidase cleavage site is efficiently generated in the cytoplasm from PSCA precursors that fail to reach the ER.