Site-directed Mutagenesis Demonstrates the Plasticity of the Beta Helix: Implications for the Structure of the Misfolded Prion Protein

The left-handed parallel beta helix (LbetaH) fold has recently received attention as a possible structure for the prion protein (PrP) in its misfolded state. In light of this interest, we have developed an experimental system to examine the structural requirements of the LbetaH fold, using a known LbetaH protein, UDP-N-acetylglucosamine acyltransferase (LpxA), from E. coli. We showed that the beta helix can tolerate nonhydrophobic residues at interior positions and prolines were important, but not critical, in folding of the beta helix. Using our structural studies of the LbetaH, we threaded the sequence of the amyloidogenic fragment of the prion protein (residues 104-143) onto the structure of LpxA. Based on the threading result, we constructed the recombinant PrP-LpxA and tested its functional activity in an E. coli antibiotic sensitivity assay. The results of these experiments suggest that the amyloidogenic PrP fragment may fold into a beta helix in the context of a larger beta-helical structure.