Crystal structure of human prion protein bound to a therapeutic antibody

Abstract

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 beta-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.

Fig. 1.

The relationship between antibody affinity and ability to inhibit prion propagation for the ICSM antibodies. (A) IC₅₀s (M) for 3-day treatment of chronically infected N2a cells with antibody are plotted against K_d (M) for recognition of recombinant PrP by antibody as measured by ELISA (Fig. S1 and Table S1). Antibodies raised to α-PrP are shown in red and to β-PrP in blue. Equivalence between IC₅₀ and K_d is shown by the dashed diagonal line. (B) Median count of FACS measurements (n = 3) for the antibodies indicated, detected with FITC-conjugated anti-mouse secondary antibody on NS0 mouse cells.

Fig. 2.

The complex between recombinant PrP^119-231 and the ICSM 18-Fab as determined by X-ray crystallography. (A) PrP^119-231 is shown in green with the heavy and light chains of the Fab in cyan and magenta, respectively. (B) Expanded view of the PrP/Fab interface. The participating PrP residues are labeled in black and those from the Fab heavy and light chains in blue and magenta, respectively. Potential hydrogen bonds are shown as dashed lines.

Fig. 3.

The interaction of PrP chains in the crystal. (A) Illustration of the intermolecular 4-stranded antiparallel β-sheet formed between neighboring PrP chains (in cyan and green) emphasizing residue 129 at the molecular interface (see Inset). (B) Superimposition of the ovine [red (25)] and human (green) PrP dimers from the respective crystal structures. Note the common occurrence of the 4-stranded intermolecular β-sheet.