©2012 Landes Bioscience. Do not distribute www.landesbioscience.com Prion 131 Prion 7:2, 131–135; March/April 2013; © 2013 Landes Bioscience EXTRA VIEW EXTRA VIEW Extra View to: Salamat K, Moudjou M, Chapuis J, Herzog L, Jaumain E, Béringue V, et al. Integrity of helix 2-helix 3 domain of the PrP protein is not mandatory for prion replication. J Biol Chem 2012; 287:18953-64; PMID:22511770; http://dx.doi. org/10.1074/jbc.M112.341677 Keywords: prion, PrP protein, insertion, mutagenesis, protein structure Abbreviations: PrP, prion protein; PrP C , normal cellular PrP; PrP Sc , scrapie associated PrP; PK, proteinase K; PrP res , PK-resistant PrP Sc ; GPI, glycophosphatidylinositol Submitted: 10/24/12 Revised: 11/12/12 Accepted: 12/03/12 http://dx.doi.org/10.4161/pri.23110 *Correspondence to: Michel Dron; Email: michel.dron@jouy.inra.fr U pon prion infection, abnormal prion protein (PrP Sc ) self-perpetuate by conformational conversion of α-helix- rich PrP C into β sheet enriched form, leading to formation and deposition of PrP Sc aggregates in affected brains. However the process remains poorly understood at the molecular level and the regions of PrP critical for conversion are still debated. Minimal amino acid substitutions can impair prion replica- tion at many places in PrP. Conversely, we recently showed that bona fide prions could be generated after introduction of eight and up to 16 additional amino acids in the H2-H3 inter-helix loop of PrP. Prion replication also accommo- dated the insertions of an octapeptide at different places in the last turns of H2. This reverse genetic approach reveals an unexpected tolerance of prions to sub- stantial sequence changes in the protease- resistant part which is associated with infectivity. It also demonstrates that con- version does not require the presence of a specific sequence in the middle of the H2-H3 area. We discuss the implications of our findings according to different structural models proposed for PrP Sc and questioned the postulated existence of an N- or C-terminal prion domain in the protease-resistant region. Self-Propagation of a Pathogenic Protein: Conversion of PrP into Different Prion Structures or Assemblies Prions are the etiologic agents of transmissible fatal neurodegenerative Mammalian prions Tolerance to sequence changes—how far? Muhammad Khalid Salamat, Carola Munoz-Montesino, Mohammed Moudjou, Human Rezaei, Hubert Laude, Vincent Béringue and Michel Dron* INRA; UR892 Virologie Immunologie Moléculaires; Jouy-en-Josas, France diseases affecting both man and animals. 1 They are mainly if not solely composed of assemblies of PrP Sc , a conformationally altered isoform of the host-encoded cellular prion protein PrP C . This PrP C is a glycoprotein tethered at the cell surface by a GPI-anchor. The N-terminal region of the protein is unstructured while the C-terminal moiety is a globular domain containing three α helices and two small anti-parallel β strands. 2-4 In contrast, PrP Sc is enriched in β structure, insoluble and tends to aggregate. 5-7 Upon infection, exogenous PrP Sc seeds are thought to self- template host PrP C , leading to further aggregation and deposition mainly in the nervous tissue. PrP C expression is essential for prion replication as shown by the resistance of PrP knockout mice to prion infection and the restoration of their susceptibility after introduction of a PrP transgene. 8,9 Within the same host species, different prion strains can be propagated; those can be differentiated on the basis of the incubation time to disease, the pathology and PrP Sc biochemical signature. Strain properties are assumed to be enciphered within differences in PrP Sc conformation, at the level of the tertiary and/or quaternary structure. 10,11 PrP Sc was initially differentiated from PrP C by its insolubility and resistance to proteases such as proteinase K (PK). 1 While a strain-dependent, variable pro- portion of PrP Sc is now recognized as PK-sensitive, 12-15 its relative degree of infectivity is still debated. For most prion strains, the upstream N-terminal part of PrP Sc is truncated following protein- ase K (PK) treatment while the segment