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 r egion 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 di fferent environments. To investigate the molecular determinants of the phys icochemical characteristics of PrP106-126, we synthesized a series of analo gues 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 ana lysis of the secondary structure and aggregation properties of PrP106-126 a nd its analogues showed the following. (1) His(111) is central to the confo rmational changes of PrP peptides. (2) Amidation of the C-terminal Gly(126) yields a predominantly random coil structure, abolishes the molecular poly morphism and decreases the propensity of PrP 106-126 to generate amyloid fi brils, (3) PrP106-126 V, carrying an Ala --> Val substitution at position 1 17, does not demonstrate a fibrillogenic ability superior to that of PrP 10 6-126, However, the presence of Val at position 117 increases the aggregati on properties of the amidated peptide. (4) Amyloid fibrils are not required for neurotoxicity because the effects of PrP106-126 NH2 on primary neurona l cultures were similar to those of the wild-type sequence. Conversely, ast roglial proliferation is related to the presence of amyloid fibrils, sugges ting that astrogliosis in prion encephalopathies without amyloid deposits i s a mediated effect rather than a direct effect of disease-specific PrP iso forms.