FIGURE 2.

Structural models of elk and human side chain packing within the β2-α2 loop may explain CWD transmission barriers. Atomic space-filling models of amino acid side chains within the β2-α2 loop of PrP were modeled as a parallel β-sheet.⁵³ In this model, the CWD PrP^Sc and cervid PrP^C (top pair) interdigitate in a steric zipper. In contrast, the CWD PrP^Sc and human PrP^C (bottom pair) interaction generates a steric clash (blue rectangle) and a cavity (arrow) that would be incompatible with zipper formation and may explain why CWD does not convert human PrP^C. Amino acids common to both cervids and humans are yellow; human-specific residues are green.