Effect of the disulfide bridge and the C-terminal extension on the oligomerization of the amyloid peptide ABri implicated in familial British dementia

Familial British dementia (FBD) is a rare neurodegenerative disorder and sh ares features with Alzheimer's disease, including amyloid plaque deposits, neurofibrillary tangles, neuronal loss, and progressive dementia. Immunohis tochemical and biochemical analysis of plaques and vascular amyloid of FED brains revealed that a 4 kDa peptide named ABri is the main component of th e highly insoluble amyloid deposits. In FED patients, the ABri peptide is p roduced as a result of a point mutation in the usual stop codon of the BRI gene. This mutation produces a BRI precursor protein Il amino acids longer than the wild-type protein. Mutant and wild-type precursor proteins both un dergo furin cleavage between residues 243 and 244, producing a peptide of 3 4 amino acids in the case of ABri and 23 amino acids in the case of the wil d-type (WT) peptide. Here we demonstrate that the intramolecular disulfide bond in ABri and the C-terminal extension are required to elongate initiall y formed dimers to oligomers and fibrils. In contrast, the shorter WT pepti de did not aggregate under the same conditions. Conformational analyses ind icate that the disulfide bond and the C-terminal extension of ABri are requ ired for the formation of beta -sheet structure. Soluble nonfibrillar ABri oligomers were observed prior to the appearance of mature fibrils. A molecu lar model of ABri containing three beta -strands, and two beta -hairpins an nealed by a disulfide bond, has been constructed, and predicts a hydrophobi c surface which is instrumental in promoting oligomerization.