Polymerization of the amyloid beta (A␤) peptide into protease-resistant fibrils is a significant step in the pathogenesis of Alzheimer's disease. It has not been possible to obtain detailed structural information about this process with conventional techniques because the peptide has limited solubility and does not form crystals. In this work, we present experimental results leading to a molecular level model for fibril formation. Systematically selected A␤-fragments containing the A␤ 16 -20 sequence, previously shown essential for A␤-A␤ binding, were incubated in a physiological buffer. Electron microscopy revealed that the shortest fibril-forming sequence was A␤ 14 -23 . Substitutions in this decapeptide impaired fibril formation and deletion of the decapeptide from A␤ 1-42 inhibited fibril formation completely. All studied peptides that formed fibrils also formed stable dimers and/or tetramers. Molecular modeling of A␤ 14 -23 oligomers in an antiparallel ␤-sheet conformation displayed favorable hydrophobic interactions stabilized by salt bridges between all charged residues. We propose that this decapeptide sequence forms the core of A␤-fibrils, with the hydrophobic C terminus folding over this core. The identification of this fundamental sequence and the implied molecular model could facilitate the design of potential inhibitors of amyloidogenesis.

We have previously shown that short peptides incorporating the sequence KLVFF can bind to the ϳ40-amino acid residue Alzheimer amyloid ␤-peptide (

Pathogenic mutations in presenilin 1 (PS1) are associated with Ϸ50% of early-onset familial Alzheimer disease. PS1 is endoproteolytically cleaved to yield a 30-kDa N-terminal fragment (NTF) and an 18-kDa C-terminal fragment (CTF). Using COS7 cells transfected with human PS1, we have found that phorbol 12,13-dibutyrate and forskolin increase the state of phosphorylation of serine residues of the human CTF. Phosphorylation of the human CTF resulted in a shift in electrophoretic mobility from a single major species of 18 kDa to a doublet of 20-23 kDa. This mobility shift was also observed with human PS1 that had been transfected into mouse neuroblastoma (N2a) cells. Treatment of the phosphorylated CTF doublet with phage protein phosphatase eliminated the 20-to 23-kDa doublet while enhancing the 18-kDa species, consistent with the interpretation that the electrophoretic mobility shift was due to the addition of phosphate to the 18-kDa species. The NTF and CTF eluted from a gel filtration column at an estimated mass of over 100 kDa, suggesting that these fragments exist as an oligomerized species. Upon phosphorylation of the PS1 CTF, the apparent mass of the NTF-or CTF-containing oligomers was unchanged. Thus, the association of PS1 fragments may be maintained during cycles of phosphorylation͞dephosphory-lation of the PS1 CTF.