Molecular determinants of the physicochemical properties of a critical prion protein region comprising residues 106-126

Prion diseases are marked by the cerebral accumulation of conformationally modified forms of the cellular prion protein (PrPC), known as PrPres. The region comprising the residues 106-126 of human PrP seems to have a key role in this conformational conversion, because a synthetic peptide homologous with this sequence (PrP106-126) adopts different secondary structures in different environments. To investigate the molecular determinants of the physicochemical characteristics of PrP106-126, we synthesized a series of analogues including PrP106-126 H-D, PrP106-126 A and PrP106-126 K, with L-His --> D-His, His --> Ala and His --> Lys substitutions respectively at position ill, PrP106-126 NH2 with amidation of the C-terminus, PrP106-126 V with an Ala --> Val substition at position 117, and PrP106-126 VNH2 with an Ala --> Val substitution at position 117 and amidation of the C-terminus, The analysis of the secondary structure and aggregation properties of PrP106-126 and its analogues showed the following. (1) His(111) is central to the conformational changes of PrP peptides. (2) Amidation of the C-terminal Gly(126) yields a predominantly random coil structure, abolishes the molecular polymorphism and decreases the propensity of PrP 106-126 to generate amyloid fibrils, (3) PrP106-126 V, carrying an Ala --> Val substitution at position 117, does not demonstrate a fibrillogenic ability superior to that of PrP 106-126, However, the presence of Val at position 117 increases the aggregation properties of the amidated peptide. (4) Amyloid fibrils are not required for neurotoxicity because the effects of PrP106-126 NH2 on primary neuronal cultures were similar to those of the wild-type sequence. Conversely, astroglial proliferation is related to the presence of amyloid fibrils, suggesting that astrogliosis in prion encephalopathies without amyloid deposits is a mediated effect rather than a direct effect of disease-specific PrP isoforms.