Prions: Protein Aggregation and Infectious Diseases - Physiological ...
Prions: Protein Aggregation and Infectious Diseases - Physiological ...
Prions: Protein Aggregation and Infectious Diseases - Physiological ...
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1110 ADRIANO AGUZZI AND ANNA MARIA CALELLA<br />
tions were minimal. In contrast, combined expression of<br />
anchorless <strong>and</strong> wild-type PrP resulted in accelerated clinical<br />
scrapie (108).<br />
1. Structures of purified PrP C <strong>and</strong> PrP Sc<br />
Early low-resolution structural studies indicated that<br />
PrP C had a high -helix content (40% of the protein) <strong>and</strong><br />
relatively little -sheet (3% of the protein) (379). These<br />
findings were further refined by Wüthrich <strong>and</strong> colleagues<br />
who determined the fine structure of PrP C by nuclear<br />
magnetic resonance spectroscopy (Fig. 1A) (229, 421),<br />
<strong>and</strong> later also by crystallographic studies (271). The NH2 proximal half of the molecule is not structured at all,<br />
whereas the COOH-proximal half is arranged in three<br />
-helices corresponding, for the human PrP C , to the residues<br />
144–154, 173–194, <strong>and</strong> 200–228, interspersed with<br />
an antiparallel -pleated sheet formed by -str<strong>and</strong>s at<br />
residues 128–131 <strong>and</strong> 161–164. A single disulfide bond is<br />
found between cysteine residues 179 <strong>and</strong> 214 (421, 422,<br />
531). It is unlikely that the NH2 terminus is r<strong>and</strong>omly<br />
coiled in vivo, since functional studies in transgenic mice<br />
imply that the domain comprising amino acids 32–121<br />
carries out important physiological functions (453). It is<br />
possible that the flexible tail of PrP C acquires a defined<br />
structure when PrP C is present within membrane rafts<br />
(362).<br />
In contrast to PrP C , the -sheet content of PrP Sc<br />
comprises 40% of the protein, whereas -helices comprise<br />
30% of the protein as measured by Fourier-transform<br />
infrared (379) <strong>and</strong> CD spectroscopy (Fig. 1B) (436). No<br />
high-resolution structure is available for PrP Sc , although<br />
interesting models have been conjectured on the basis of<br />
electron crystallography studies (524). Prion rods of the<br />
NH2-terminally truncated form of PrP Sc derived by limited<br />
proteolysis, PrP 27–30 , exhibit green-gold birefringence after<br />
staining with Congo red, indicating that this isoform<br />
has a high -sheet content (410). Indeed, PrP 27–30 poly-<br />
FIG. 1. Structural features of PrP C <strong>and</strong> PrP Sc . A: NMR structure of<br />
the mouse prion protein domain (121–231). The ribbon diagram indicates<br />
the positions of the three helices (yellow) <strong>and</strong> the antiparallel<br />
two-str<strong>and</strong>ed -sheet (cyan). The connecting loops are displayed in<br />
green if their structure is well defined <strong>and</strong> in magenta otherwise. The<br />
disulfide bond between Cys-179 <strong>and</strong> Cys-214 is shown in white. The<br />
NH 2-terminal segment of residues 121–124 <strong>and</strong> the COOH-terminal segment<br />
220–231 are disordered <strong>and</strong> not displayed. (Figure kindly provided<br />
by S. Hornemann.) B: Fourier transform infrared spectroscopy of prion<br />
proteins. The amide I b<strong>and</strong> (1,700–1,600 cm 1 ) of transmission FTIR<br />
spectra of PrP C (black line), PrP Sc (gray line), <strong>and</strong> PrP 27–30 (dotted line).<br />
These proteins were suspended in a buffer in D 2O containing 0.15 M<br />
sodium chloride/10 mM sodium phosphate, pD 7.5 (uncorrected)/0.12%<br />
ZW. The spectra are scaled independently to be full scale on the ordinate<br />
axis (absorbance). [From Pan et al. (379).] C: electron micrographs of<br />
negatively stained <strong>and</strong> immunogold-labeled prion proteins. Each panel<br />
shows in following order PrP C , PrP Sc , <strong>and</strong> Prion rods, composed of<br />
PrP 27–30 , that were negatively stained with uranyl acetate. Bar, 100 nm.<br />
[From Pan et al. (379).]<br />
Physiol Rev VOL 89 OCTOBER 2009 www.prv.org