Prion protein: Difference between revisions

=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 cervids. In all cases ''post mortem'' analysis of brain tissue is characterterized 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 which 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 happen 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 a natively unstructured N-terminal region, and a predominantly α-helical C-terminal region from residues ~120-230. containing three α-helices and two short β-strands. A single disulfide bond 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 <ref>Zahn, R. ''et al.'' (2000) NMR solution structure of the human prion protein ''Proc. Natl. Acad. Sci. USA''  '''97''', 145-150 </ref>. The structure of PrP^C is highly conserved amongst mammals, and only differs slightly in birds, reptiles and amphibians<ref>Calzolai, L ''et al.'' (2005) Prion protein NMR structures of chicken, turtle, and frog 'Proc. Natl. Acad. Sci. USA'' '''102''', 651-655</ref>.

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 other PrPs determined by NMR spectroscopy, however in human PrP, the X-ray structure is a dimer in which helix 3 is swapped with respect to the monomer, 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<ref>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</ref>. There are a number of technical obstacles in determining the atomic resolution structure of PrP(sup)Sc</sup>, and the most detailed information to date has been obtained by electron microscopy of 2D crystals. Differential binding of metal ions to these 2D crystals, and redacted constructs of PrP, provide a basis for structural modeling.

A model the N-terminal region and much of the C-terminal domain, up to the disulphide bond, refolds into a β-helical structure

Support for this β-helical model comes from the structure of the fungal prion Het-s ([[2rnm]]).

=Prion strains=

The strain phenomenon of prion strains (disease subtype replicating with high fidelity and producing specific clinical, biochemical and neuropathological features) 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.

=Selected PrP structures=

All structures determined by NMR unless otherwise specified

==Human PrP==

* [[1qlx]] HuPrP residues 23-230  

* [[1qm0]] HuPrP residues 90-230  

* [[1qm2]] HuPrP residues 121-230  

* [[1i4m]] HuPrP residues 119-226 (determined by X-ray crystallography)   

* [[1fkc]] HuPrP,E200K residues  90-231 (genetic prion disease)

* [[1h0l]] HuPrP residues 121-230, with an additional disulphide bond analogous to the homolog [[Doppel]]

==Other species PrPs==