Calpain and other cytosolic proteases can contribute to the degradation of retro-translocated prion protein in the cytosol.

PrP, a cell surface-localized N-linked glycoprotein, is required for the pathogenesis of prion diseases. Recent studies have revealed that prion protein (PrP) becomes neurotoxic and prone to aggregation when it is in the cytosol, suggesting that cytosolic PrP may play a role in the pathogenesis of prion disease. Retro-translocation of PrP from the endoplasmic reticulum to the cytosol for proteasome degradation offers a natural route for PrP to enter the cytosol, but whether PrP is subject to retrotranslocation is controversial. In this study, we investigated the metabolism of endogenous wild-type PrP in several cell lines and in primary mouse cortical neurons. Our results suggest that a portion of the endogenous wild-type PrP is retro-translocated to the cytosol and degraded by the proteasome. Moreover, we also found that calpain and other cytosolic proteases could degrade PrP in the cytosol when the proteasome activity is compromised. These results provide the foundation for the hypothesis that cytosolic PrP may be involved in the pathogenesis of prion disease.