A-Helical PrP

Like any other protein, the molecular structure of the prion is subject to conformational flexibility and to various thermal-induced fluctuations between varying conformational states. However, if these fluctuations permit the PrP conformation to be attained, then this abnormal conformer promotes the widespread conversion of PrP to PrP , leading to the precipitous deposition of the abnormal protein throughout the brain (mirrored by the rapid and relentlessly downhill clinical course). This pathological self-propagating shape conversion of a-helical PrP to P-sheet PrP may in principle be initiated by a seed PrP molecule in the neurotoxic conformation. This explains the transmissibility of prion diseases and accounts for how susceptible humans exposed to beef from an animal with mad cow disease develop variant Creutzfeldt-Jakob disease. 

PrPc has a predominantly a-helical conformation 796 Reverse genetics approaches to studying PrPc 797 The function of PrPc remains unknown 797 PrP knockout mice have subtle abnormalities of synaptic transmission 797... 

Fig. 2. Primary structure of hamster PrP (Stahl et al., 1993). The first 22 residues at the N-terminus are the signal sequence. PrPc is completely digested by proteinase K, whereas the N-terminal sequence of PrPSc to residue 89 (arrow, closed head) is digested. — CHO indicates the glycosylation sites at residues 181 and 197 Gpi the glycosylpho-sphatidylinositol anchor at 231 and the N-terminal octarepeats. In one case of human prion disease, a stop codon was found at 145 (arrow, open head) (Kitamoto et al., 1993). HI, H2, H3, and H4 denote the predicted a-helices (Huang et al, 1994, 1996), and A-C denote the a-helices and SI, S2 the /(-strands determined by solution NMR (James et al, 1997).

Fig. 5. Physical-chemical parameters as a function of residue number for hamster PrP (Inouye and Kirschner, 1998). The parameters (arbitrary scale) are charge at pH 7 hydrophobicity a-helix (solid), /8-strand (dashed) turn (solid), coil (dashed) a-helical (solid) and /8-strand amphiphilicity (dashed). The predicted helices (Huang et al., 1994) are labeled HI, H2, H3, and H4, and the NMR-observed helices and /8-strands are A-C and SI, S2, respectively (James et al., 1997).

Govaerts et al. (2004) proposed a parallel /1-helix model for prion rods that is consistent in overall dimensions with their low-resolution EM studies of two-dimensional PrP 27-30 crystals (Wille et al, 2002). In this model, residues 89-174 form 4 coils, or complete helical turns (Jenkins and Pickersgill, 2001), of a left-handed, parallel /Hielix (Fig. 5B). The coils of one monomer are proposed to stack on the coils of another to form an extended triangular -structure. Three of these triangular units pack together to form the fibril (Fig. 5G and D). The G-terminal a-helices (a2 and a3) of monomeric PrP are proposed to retain their native structure in the fibril and pack around the outside of the trimer (Fig. 5G and D). The presence of these helices in the prion rods is consistent with antibody binding studies (Peretz et al, 1997), the presence of a disulfide bond (Turk et al, 1988), and FTIR measurements (Wille et al, 1996). 

The abnormal deposits found in the brains of CJD victims consist of an abnormal isoform of PrP. Prion protein is normally found in cells. Detailed structural studies show that normal cellular PrP (PrP ) is a soluble protein whose conformation is rich in a-helices with very little P-sheet. The PrP protein extracted from the brains of CJD victims (i.e., PrP ) is identical in primary amino acid sequence to the normal PrP (PrP ). However, PrP has a much greater content of P-sheet conformation with little a-helical structure. Thus PrP is neurotoxic because of its three-dimensional structure. When the prion protein is predominantly in an a-helical conformation it is nontoxic when the prion protein is predominantly in a P-sheet conformation, it kills neurons. The prion protein is thus made neurotoxic not by its amino acid composition but by its conformation. This concept is both fascinating and terrifying. Prion diseases are transmissible thus prions are infectious agents. However, prions are not like bacteria or viruses, or other infectious microbes—they are simply protein molecules. Prions are not microbes with cell membranes and nucleic acids they are not living things. Indeed, prions are not even infectious molecules, they are infectious molecular shapes. 

Tycko R, Savtchenko R, Ostapchenko VG et al (2010) The a-helical C-terminal domain of full-length recombinant PrP converts to an in-register parallel [1-sheet structure in PrP fibrils evidence from solid state nuclear magnetic resonance. Biochemistry 49 9488-9497... 

Before we discuss approaches for the synthesis of the PrP the overall structure should be explained. Mature PrP, a protein of about 240 amino acids, exhibits three domains (see [30] for primary sequence and posttranslational modifications of hamster PrP). The N-terminal domain is an intrinsically unstructured fragment with around 100 amino acids. It harbors five octarepeats whose function is unknown. The central core of PrP is structured with mainly a-helical elements. It is followed by the C-terminal fragment which is equipped with the GPI anchor. Further posttranslational modifications are two carbohydrate moieties which are connected to Asn residues at the second helix (a2) of the structured domain. Because infectivity has been associated with the central core (for a review see [31]), most chemical work has been focused on this region. 

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