The role of the unusual threonine string in the conversion of prion protein.

Romany Abskharon,Fei Wang,Kayla J Vander Stel,Jiyan Ma,Kumar Sinniah

doi:10.1038/srep38877

Abstract

The conversion of normal prion protein (PrP) into pathogenic PrP conformers is central to prion disease, but the mechanism remains unclear. The α-helix 2 of PrP contains a string of four threonines, which is unusual due to the high propensity of threonine to form β-sheets. This structural feature was proposed as the basis for initiating PrP conversion, but experimental results have been conflicting. We studied the role of the threonine string on PrP conversion by analyzing mouse Prnpa and Prnpb polymorphism that contains a polymorphic residue at the beginning of the threonine string, and PrP mutants in which threonine 191 was replaced by valine, alanine, or proline. The PMCA (protein misfolding cyclic amplification) assay was able to recapitulate the in vivo transmission barrier between PrPa and PrPb. Relative to PMCA, the amyloid fibril growth assay is less restrictive, but it did reflect certain properties of in vivo prion transmission. Our results suggest a plausible theory explaining the apparently contradictory results in the role of the threonine string in PrP conversion and provide novel insights into the complicated relationship among PrP stability, seeded conformational change, and prion structure, which is critical for understanding the molecular basis of prion infectivity.

Highlights

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are a group of fatal neurodegenerative diseases that includes Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle, and chronic wasting disease (CWD) in cervids[1]
protein misfolding cyclic amplification (PMCA) was carried out as described previously[38,43]; the PMCA substrate consisted of recombinant PrP (recPrP) plus two cofactors, synthetic POPG and total RNA isolated from normal mouse liver[38,43]
These data suggested that, similar to the barrier observed in animal study[20], there is a barrier for propagating recPrPa-adapted recPrion to recPrPb and that both polymorphic residues 108 and 189 contribute to the barrier

Summary

Introduction

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are a group of fatal neurodegenerative diseases that includes Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle, and chronic wasting disease (CWD) in cervids[1]. Because threonine tends to form β-sheet structures[23], such a sequence of β-sheet-prone threonines in a helix is highly unusual This local instability has been proposed as the underlying structural basis for initiating the PrPC-to-PrPSc conversion[24]. Munoz-Montestino reported that deleting 4 or 5 amino acids in this region (including all four threonines) does not affect the conversion of ovine PrP to infectious prions in RK13 cells[34]. These data apparently contradict the observations from prion transmission studies in Prnpb knock-in mice, which showed that the threonine 189 (mouse numbering) significantly affects disease incubation times[20]

Methods

Results

Conclusion