Effect of introducing a short amyloidogenic sequence from the Aβ peptide at the N-terminus of 18-residue amphipathic helical peptides.

Fibril formation is the hallmark of pathogenesis in Alzheimer's disease and other amyloid disorders caused by conformational alterations leading to the aggregation of soluble monomers. Aβ40 self-associates to form amyloid fibrils. Its central seven-residue segment KLVFFAE (Aβ16-22), which is thought to be crucial for fibril formation of the full-length peptide, forms fibrils even in isolation. Context-dependent induction of amyloid formation by such sequences in peptides, which otherwise do not have that propensity, is of considerable interest. We have examined the effect of introducing the Aβ16-22 sequence at the N-terminus of two amphipathic helical 18-residue peptides Ac-WYSEMKRNVQRLERAIEE-am and Ac-KQLIRFLKRLDRNLWGLA-am, which have high average hydrophobic moment <μH> values but have net charges of 0 and +4, respectively, at neutral pH. Upon incubation in aqueous buffer, fibril-like aggregates were discernible by transmission electron microscopy for the peptide with only 0 net charge, which also displayed ThT binding and β-structure. Although both the sequences have been derived from amphipathic helical segments in globular proteins and possess high average hydrophobic moments, the +4 charge peptide lacks the ability to form fibrils, while the peptide with 0 charge has the tendency to form fibrillar structures. Variation in the net charge and the presence of several glutamic acids in the sequence of the peptide with net charge 0 appear to favor the formation of fibrils when the Aβ16-22 sequence is attached at the N-terminus.