Wild-type PrP and a mutant associated with prion disease are subject to retrograde transport and proteasome degradation.

The cytoplasm seems to provide an environment that favors conversion of the prion protein (PrP) to a form with the physical characteristics of the PrP(Sc) conformation, which is associated with transmissible spongiform encephalopathies. However, it is not clear whether PrP would ever exist in the cytoplasm under normal circumstances. We report that PrP accumulates in the cytoplasm when proteasome activity is compromised. The accumulated PrP seems to have been subjected to the normal proteolytic cleavage events associated with N- and C-terminal processing in the endoplasmic reticulum, suggesting that it arrives in the cytoplasm through retrograde transport. In the cytoplasm, PrP forms aggregates, often in association with Hsc70. With prolonged incubation, these aggregates accumulate in an "aggresome"-like state, surrounding the centrosome. A mutant (D177N), which is associated with a heritable and transmissible form of the spongiform encephalopathies, is less efficiently trafficked to the surface than wild-type PrP and accumulates in the cytoplasm even without proteasome inhibition. These results demonstrate that PrP can accumulate in the cytoplasm and is likely to enter this compartment through normal protein quality-control pathways. Its potential to accumulate in the cytoplasm has implications for pathogenesis.