Mutant-selective topologic conversion facilitates selective degradation of a pathogenic prion isoform.

Regulating protein import across the endoplasmic reticulum (ER) membrane occasionally results in the synthesis of topologically unnatural variants, and their accumulation often leads to proteotoxicity. However, since this is a regulated process, it is questionable whether the topological rearrangement really has adverse consequences. In the present study, we provide an insight into the functional benefit of translocational regulation by illustrating mutant-selective topologic conversion (MSTC) and demonstrate that MSTC contributes to selective degradation of a membrane-anchored prion protein isoform (ctmPrP). We find that ctmPrP is inherently short-lived and topologically competent for degradation rather than accumulation. MSTC achieves, cotranslationally, the unique topology of ctmPrP during translocation, facilitating selective ctmPrP degradation from the ER via the proteasome-dependent pathway before entering the secretory pathway. At this time, the N-terminal polycationic cluster is essential for MSTC, and its cytosolic exposure acquires "ERAD-degron"-like activity for ctmPrP. Bypassing MSTC delays ctmPrP degradation, thus increasing prion proteotoxicity. Thus, topological rearrangement is used for the MSTC as a part of the protein quality control pathway to ensure the safety of the secretory pathway from misfolded PrP.