Prion infection is characterized by the conversion of host cellular prion protein (PrP C ) into disease-related conformers (PrP Sc ) and can be arrested in vivo by passive immunization with anti-PrP monoclonal antibodies. Here, we show that the ability of an antibody to cure prion-infected cells correlates with its binding affinity for PrP C rather than PrP Sc . We have visualized this interaction at the molecular level by determining the crystal structure of human PrP bound to the Fab fragment of monoclonal antibody ICSM 18, which has the highest affinity for PrP C and the highest therapeutic potency in vitro and in vivo. In this crystal structure, human PrP is observed in its native PrP C conformation. Interactions between neighboring PrP molecules in the crystal structure are mediated by close homotypic contacts between residues at position 129 that lead to the formation of a 4-strand intermolecular ␤-sheet. The importance of this residue in mediating protein-protein contact could explain the genetic susceptibility and prion strain selection determined by polymorphic residue 129 in human prion disease, one of the strongest common susceptibility polymorphisms known in any human disease.Creutzfeldt-Jakob disease ͉ PrP-Fab complex ͉ monoclonal antibody ͉ prion therapeutics

There are two common forms of prion protein (PrP) in humans, with either methionine or valine at position 129. This polymorphism is a powerful determinant of the genetic susceptibility of humans toward both sporadic and acquired forms of prion disease and restricts propagation of particular prion strains. Despite its key role, we have no information on the effect of this mutation on the structure, stability, folding, and dynamics of the cellular form of PrP (PrP C ). Here, we show that the mutation has no measurable effect on the folding, dynamics, and stability of PrP C . Our data indicate that the 129M/V polymorphism does not affect prion propagation through its effect on PrP C ; rather, its influence is likely to be downstream in the disease mechanism. We infer that the M/V effect is mediated through the conformation or stability of disease-related PrP (PrP Sc ) or intermediates or on the kinetics of their formation.The prion diseases are a group of fatal neurodegenerative diseases that include scrapie in sheep and goats; bovine spongiform encephalopathy (BSE) 1 in cattle; and Creutzfeldt-Jakob disease (CJD), Gerstmann-Strä ussler-Scheinker disease, fatal familial insomnia (FFI), and kuru in humans. The human diseases may be inherited, arise sporadically, or be acquired through exposure to infectious prions (1, 2). Although rare in humans, intense interest has focused on these diseases both because of their unique biology and because of the occurrence of variant CJD, a new form of human prion disease, and the experimental evidence that it is caused by a BSE-like prion strain (3-5).According to the "protein-only" hypothesis (6), prions are composed principally or entirely of abnormal isoforms of hostencoded prion protein (PrP) (7). The disease-related isoform, PrP Sc , is derived from its normal cellular precursor, PrP C , by a post-translational process that involves conformational change. PrP Sc can be distinguished biochemically from PrP C by its partial protease resistance and detergent insolubility.Although the precise molecular events involved in this conversion remain ill defined, molecular genetic and in vitro studies support the hypothesis that some sort of direct interaction between PrP Sc and either PrP C or some less organized state occurs. This interaction results in the PrP Sc conformation being imposed upon the substrate protein, and the process of conversion is favored by sequence complementarity (8 -13). A key piece of evidence supporting this and the protein-only hypothesis in general is the finding that the large majority of cases of sporadic CJD are homozygous with respect to a common polymorphism at position 129 in the human prion protein, in which either methionine or valine can be encoded (only ϳ49% of the UK population are homozygous with respect to this polymorphism) (9).Elderly survivors of the kuru epidemic (an acquired prion disease largely restricted to the Fore linguistic group of the Papua New Guinea Highlands, which was transmitted during endocannibalistic feasts) who had mult...

The role of conformational intermediates in the conversion of prion protein from its normal cellular form (PrP(C)) to the disease-associated "scrapie" form (PrP(Sc)) remains unknown. To look for such intermediates in equilibrium conditions, we have examined the unfolding transitions of PrP(C), primarily using the chemical denaturant guanidine hydrochloride (GuHCl). When the protein conformation is assessed by NMR, there is a gradual shift of NMR signals in the regions between residues 125-146 and 186-196. The denaturant dependence of these shifts shows that in aqueous solution the native and locally unfolded conformations are both significantly populated. Following this shift, there is the major unfolding transition to generate a substantially unfolded population. However, analysis of NMR chemical shift and intensity changes shows that there is persistent structure in the molecule well beyond this major cooperative unfolding transition. Residual structure within this state is extensive and encompasses the majority of the secondary structure elements found in the native state of the protein.

A considerable body of evidence now shows that PrP (prion protein) binds metal ions with high affinity and it has been claimed that the binding of copper (II) ions to PrP confers SOD (superoxide dismutase) activity. In turn, it has been suggested that PrP is a synaptic dismutase and that loss of this function, as a result of the conversion of PrP(C) into PrP(Sc), results in pathology and hence morbidity associated with prion disease. However, contrary to previous reports, in the present study we have found that PrP exhibits no detectable dismutase activity above baseline levels measured for copper (II) ions in water when assayed using a reliable procedure with a detection limit of at least 2 units of activity/mg of protein. This was true when the assay was performed with either PrP refolded from a denatured state in the presence of copper, as in previous studies, or native PrP loaded with copper. Thus if PrP has any role in oxidative stress, it must be indirect as a regulator of protective cellular responses.

According to the protein-only hypothesis, infectious mammalian prions, which exist as distinct strains with discrete biological properties, consist of multichain assemblies of misfolded cellular prion protein (PrP). A critical test would be to produce prion strains synthetically from defined components. Crucially, high-titre ‘synthetic' prions could then be used to determine the structural basis of infectivity and strain diversity at the atomic level. While there have been multiple reports of production of prions from bacterially expressed recombinant PrP using various methods, systematic production of high-titre material in a form suitable for structural analysis remains a key goal. Here, we report a novel high-throughput strategy for exploring a matrix of conditions, additives and potential cofactors that might generate high-titre prions from recombinant mouse PrP, with screening for infectivity using a sensitive automated cell-based bioassay. Overall, approximately 20 000 unique conditions were examined. While some resulted in apparently infected cell cultures, this was transient and not reproducible. We also adapted published methods that reported production of synthetic prions from recombinant hamster PrP, but again did not find evidence of significant infectious titre when using recombinant mouse PrP as substrate. Collectively, our findings are consistent with the formation of prion infectivity from recombinant mouse PrP being a rare stochastic event and we conclude that systematic generation of prions from recombinant PrP may only become possible once the detailed structure of authentic ex vivo prions is solved.

Prion propagation involves a conformational transition of the cellular form of prion protein (PrP C ) to a disease-specific isomer (PrP Sc ), shifting from a predominantly ␣-helical conformation to one dominated by ␤-sheet structure. This conformational transition is of critical importance in understanding the molecular basis for prion disease. Here, we elucidate the conformational properties of a disulfide-reduced fragment of human PrP spanning residues 91-231 under acidic conditions, using a combination of heteronuclear NMR, analytical ultracentrifugation, and circular dichroism. We find that this form of the protein, which similarly to PrP Sc , is a potent inhibitor of the 26 S proteasome, assembles into soluble oligomers that have significant ␤-sheet content. The monomeric precursor to these oligomers exhibits many of the characteristics of a molten globule intermediate with some helical character in regions that form helices I and III in the PrP C conformation, whereas helix II exhibits little evidence for adopting a helical conformation, suggesting that this region is a likely source of interaction within the initial phases of the transformation to a ␤-rich conformation. This precursor state is almost as compact as the folded PrP C structure and, as it assembles, only residues 126 -227 are immobilized within the oligomeric structure, leaving the remainder in a mobile, random-coil state.

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