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

Yumi Lee,Sang-Wook Kang,Youngsup Song,Hongsik Eum,Sohee Lee,Duri Lee

doi:10.1038/s41418-019-0354-1

Yumi Lee, Sang-Wook Kang + Show 4 more

Open Access

https://doi.org/10.1038/s41418-019-0354-1

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Abstract

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.

Highlights

One-third of proteins, including secreted and transmembrane proteins, in mammalian cells are synthesized in the endoplasmic reticulum (ER)
To produce ctmPrP, we employed a well-characterized construct expressing a pathogenic prion protein (PrP) carrying a fatal mutation within its internal hydrophobic region
We equipped it with an N-terminal signal sequence that is persistently controlled by the pre-emptive quality control pathway (that is, Fig. 1 Analyses of the unique topology of ctmPrP. a Various mutations in the signal sequence (SP) and internal hydrophobic domain (HD) used in this study, the functional domain, and the antigenic determinants are illustrated

Summary

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One-third of proteins, including secreted and transmembrane proteins, in mammalian cells are synthesized in the endoplasmic reticulum (ER). These topologic outcomes are achieved during translocation, in which the N-terminal signal sequence delivers the ribosome-associated PrP nascent chain to the ER (ER targeting) and opens the protein channel, Sec translocon, to pass the PrP (translocon gating) [9]. Of these two steps, translocon gating is the functional step that is coordinated by PrP mature domain and is influenced by the luminal environment. This study provides insight into how MSTC is regulated to influence the outcome of prion topology and proteotoxicity

Materials and methods

Results and discussion

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