Structural changes in a hydrophobic domain of the prion protein induced byhydration and by Ala -> Val and Pro -> Leu substitutions

X-ray diffraction was used to study the structure of assemblies formed by s ynthetic peptide fragments of the prion protein (PrP) that include the hydr ophobic domain implicated in the Gerstmann-Straussler-Scheinker (GSS) mutat ion (P102L). The effects of hydration on polypeptide assembly and of Ala -- > Val substitutions in the hydrophobic domain were characterized. Synthetic peptides included: (i) Syrian hamster (SHa) hydrophobic core, SHa106-122 ( KTNMKHMAGAAAAGAVV); (ii) SHa104-122(3A-V), with A --> V mutations at 113, 1 15 and 118 (KPKTNMKHMVGVAAVGAVV); (iii) mouse (Mo) wild-type sequence of th e N-terminal hydrophobic domain, Mo89-143WT; and (iv) the same mouse sequen ce with leucine substitution for proline at residue number 101, Mo89-143(P1 01L). Samples of SHa106-122 that formed assemblies while drying under ambie nt conditions showed X-ray patterns indicative of 33 Angstrom thick slab-li ke structures having extensive H-bonding and intersheet stacking. By contra st, lyophilized peptide that was equilibrated against 100% relative humidit y showed assemblies with only a few layers of beta-sheets. The Ala --> Val substitutions in SHa104-122 and Mo89-143(P101L) resulted in the formation o f 40 Angstrom wide, cross-beta fibrils. Observation of similar size beta-sh eet fibrils formed by peptides SHa104-122(3A-V) and the longer Mo89-143(P10 1L) supports the notion that the hydrophobic sequence forms a template or c ore that promotes the beta-folding of the longer peptide. The substitution of amino acids in the mutants, e.g. 3A --> V and P101L, enhances the foldin g of the peptide into compact structural units, significantly enhancing the formation of the extensive beta-sheet fibrils. (C) 2000 Academic Press.