Inverse correlation of thermal lability and conversion efficiency for five prion protein polymorphic variants.

Transmissible spongiform encephalopathies (TSEs) are associated with the accumulation of deposits of an abnormal form, PrP(Sc), of the host-encoded prion protein, PrP(C). Amino acid substitutions in PrP(C) have long been known to affect TSE disease outcome. In extreme cases in humans, various mutations appear to cause disease. In animals, polymorphisms are associated with variations in disease susceptibility and, in sheep, several polymorphisms have been identified that are known to affect susceptibility of carriers to disease. The mechanisms of polymorphism-mediated modulation of disease susceptibility remain elusive, and we have been studying the effect of various amino acid substitutions at PrP codon 164 (mouse numbering), in the beta2-alpha2 loop region of the prion protein, to attempt to decipher how polymorphisms may affect disease susceptibility. Combined in vitro approaches suggest that there exists a correlation between the ability of protein variants to convert to abnormal isoforms in seeded conversion assays versus the thermal stability of the protein variants, as judged by both thermal denaturation and an unseeded in vitro oligomerization assay. We have performed molecular dynamics simulations to give an indication of backbone conformational changes as a result of amino acid changes and found that alteration of a single residue in PrP can result in local changes in structure that may affect global conformation and stability. Our results are consistent with modulation of disease susceptibility through differential protein stability leading to enhanced generic misfolding of TSE resistance-associated protein variants.