Prion protein

The prion protein (PrP) is a cell surface glycoprotein, which can exist in two alternatively folded conformations: a cellular isoform denoted (PrP^C) and a disease associated isoform termed PrP^Sc.

Prion diseases

The naturally ocuring prion diseases include Creutzfeldt-Jakob disease (CJD) in people, bovine spongiform encephalopathy (BSE) commonly known as "mad cow" disease, scrapie in sheep and goats, and chronic wasting disease in deer. In all cases post mortem analysis of brain tissue is characterized by aggregates of PrP^Sc. The sporadic, genetic and infectious etiologies of prion diseases can be explained by a simple protein-based model in which PrP^C is converted into PrP^Sc that in turn initiates an autocatalytic refolding cascade of PrP^C in a template-dependent manner.

In sporadic prion disease, the spontaneous refolding or misfolding of PrP^C into PrP^Sc initiates the cascade. In genetic prion diseases, point mutations in PrP make this structural transition more likely to occur than in the wild type protein. Infectious etiology is explained by introduction of exogenous PrP^Sc which then initiates refolding of endogenous PrP^C.

Structure of PrP^C

PrP^C has an intrinsically disordered N-terminal region, and a predominantly α-helical C-terminal region from residues ~120-230, containing three α-helices and two short . A connects the middle of helices 2 and 3. The presence of the N-terminal region has little impact on the structure of the C-terminal domain ^[1]. The structure of PrP^C is highly conserved amongst mammals, and only differs slightly in birds, reptiles and amphibians^[2]. The vast majority of structures have been determined by NMR spectroscopy, but two structures have been reported by X-ray crystallography. In sheep PrP, the X-ray structure is similar to those determined by NMR spectroscopy, however in human PrP, the X-ray structure is a dimer in which helix 3 is swapped between monomers, and the disulphide bond is rearranged to be intermolecular between the dimer subunits.

Models of PrP^Sc structure

Fourier transform infrared (FTIR) spectroscopy, and circular dichroism (CD) studies first demonstrated that PrP^Sc had very different proportions of α-helices and β-sheet to PrP^C[3]. There are a number of technical obstacles in determining the atomic resolution structure of PrP^Sc, and the most detailed information to date has been obtained by electron microscopy of 2D crystals^[4]. Analysis of 2D crystals binding specific heavy metal ions, and of redacted constructs of PrP, provide a basis for structural modeling. A model the N-terminal region and part of the C-terminal domain, up to the disulphide bond, refolds into a β-helical structure^[5]. Support for this β-helical model comes from the structure of the fungal prion Het-s (2rnm).

Prion strains

The phenomenon of prion strains (disease subtypes with specific clinical, biochemical and neuropathological features, replicating with high fidelity) was initially difficult to equate with the "protein only" hypothesis of prion diseases. However, there is now evidence from a range if studies suggesting that strains are enciphered in the structure of PrP^Sc. One potential mechanism for this is alternate threading of the β-helix.

Hot Spots in PrP^C for pathogenic conversion

There are several (P102L, P105L, A117V, M129V, G131V, Y145Stop, R148H, Q160Stop, D178N, V180I, T183A, H187R, T188R, E196K, F198S, E200K, D202N, V203I, R208H, V210I, E211Q, Q212P, and Q217R). The pathogenic conversion process from PrP^C to PrP^Sc could be related to the thermal stability of PrP^{C [6]}, since the mutations related to familial forms of the prion diseases are rather concentrated in helices 2 and 3, and the thermodynamical stability profile shows that diverse residues in helices 2 and 3 are less stable ^[7]. Moreover, the conversion might also be related with the global conformational fluctuation of PrP^C, as a Carr–Purcell–Meiboom–Gill relaxation–dispersion study revealed that slow fluctuation on a time scale of microseconds to milliseconds occurs, again, in helices 2 and 3^[8],^[9].

See Also

• Prion
• Journal:JBSD:4

References

1. ↑ Zahn, R et al. (2000) NMR solution structure of the human prion protein Proc. Natl. Acad. Sci. USA 97, 145-150
2. ↑ Calzolai, L et al. (2005) Prion protein NMR structures of chicken, turtle, and frog Proc. Natl. Acad. Sci. USA 102, 651-655
3. ↑ Pan, KM et al. (1993) Conversion of alpha-helices into beta-sheets features in the formation of the scrapie prion proteins Proc. Natl. Acad. Sci. USA 90, 10962-10966
4. ↑ Wille H et al. (2002) Structural studies of the scrapie prion protein by electron crystallography Proc. Natl. Acad. Sci. USA 99, 3563-3568
5. ↑ Govaerts C et al. (2004) Ecidence for assembly of prions with left-handed β-helices into trimers Proc. Natl. Acad. Sci. USA 101, 8342-8347
6. ↑ Kuwata, K. et al. (2007) Hot spots in prion protein for pathogenic conversion Proc. Natl. Acad. Sci. USA 104, 11921–11926
7. ↑ Kuwata, K. et al. (2002) Locally disordered conformer of the hamster prion protein: a crucial intermediate to PrP^Sc Biochemistry 41, 12277–12283
8. ↑ Kuwata, K. et al. (2004) Slow conformational dynamics in the hamster prion protein Biochemistry 43, 4439-4446
9. ↑ Korzhnev, D.M. et al. (2004) Low-populated folding intermediates of Fyn SH3 characterized by relaxation dispersion NMR Nature 430, 586-590