Prions: Protein Aggregation and Infectious Diseases - Physiological ...

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