The ABri is a 34 residue
peptide that is the major component of
amyloid deposits in familial British
dementia. In the
amyloid deposits, the ABri
peptide adopts aggregated beta-pleated sheet structures, similar to those formed by the Abeta
peptide of
Alzheimer's disease and other
amyloid forming
proteins. As a first step toward elucidating the molecular mechanisms of the beta-
amyloidosis, we explored the ability of the environmental variables (pH and
peptide concentration) to promote beta-sheet fibril structures for synthetic ABri
peptides. The secondary structures and fibril morphology were characterized in parallel using circular dichroism, atomic force microscopy, negative
stain electron microscopy,
Congo red, and
thioflavin-T fluorescence spectroscopic techniques. As seen with other
amyloid proteins, the ABri fibrils had characteristic binding with
Congo red and
thioflavin-T, and the relative amounts of beta-sheet and
amyloid fibril-like structures are influenced strongly by pH. In the acidic pH range 3.1-4.3, the ABri
peptide adopts almost exclusively random structure and a predominantly monomeric aggregation state, on the basis of analytical ultracentrifugation measurements. At neutral pH, 7.1-7.3, the ABri
peptide had limited solubility and produced spherical and amorphous aggregates with predominantly beta-sheet secondary structure, whereas at slightly acidic pH, 4.9, spherical aggregates, intermediate-sized protofibrils, and larger-sized mature
amyloid fibrils were detected by atomic force microscopy. With aging at pH 4.9, the protofibrils underwent further association and eventually formed mature fibrils. The presence of small amounts of aggregated
peptide material or seeds encourage fibril formation at neutral pH, suggesting that generation of such seeds in vivo could promote
amyloid formation. At slightly basic pH, 9.0, scrambling of the Cys5-Cys22
disulfide bond occurred, which could lead to the formation of covalently linked aggregates. The presence of the protofibrils and the enhanced aggregation at slightly acidic pH is consistent with the behavior of other
amyloid-forming
proteins, which supports the premise that a common mechanism may be involved in
protein misfolding and beta-
amyloidosis.