PrP-containing aggresomes are cytosolic components of an ER quality control mechanism

Limited detoxification capacity often directs aggregation-prone, potentially hazardous, misfolded proteins to be deposited in designated cytosolic compartments known as 'aggresomes'. The roles of aggresomes as cellular quality control centers, and the cellular origin of the deposits contained within these structures, remain to be characterized. Here, we utilized the observation that the prion protein (PrP, also known as PRNP) accumulates in aggresomes following the inhibition of folding chaperones, members of the cyclophilin family, to address these questions. We found that misfolded PrP molecules must pass through the endoplasmic reticulum (ER) in order to be deposited in aggresomes, that the Golgi plays no role in this process and that cytosolic PrP species are not deposited in pre-existing aggresomes. Prior to their deposition in the aggresome, PrP molecules lose the ER localization signal and have to acquire a GPI anchor. Our discoveries indicate that PrP aggresomes are cytosolic overflow deposition centers for the ER quality control mechanisms and highlight the importance of these structures for the maintenance of protein homeostasis within the ER.