Spatial separation of beta-sheet domains of beta-amyloid: disruption of each beta-sheet by N-methyl amino acids.
In a recent model of beta-amyloid (Abeta) fibrils, based mainly on solid-state NMR data, a molecular layer consists of two beta-sheets (residues 12-23 and 31-40 of Abeta1-40), folded onto one another by a connecting "bend" structure (residues 25-29) in the side-chain dimension. In this paper, we use two N-methyl amino acids to disrupt each of the two beta-sheets individually (2NMe(NTerm), residues 17 and 19; and 2NMe(CTerm), residues 37 and 39), or both of them at the same time (4NMe, with the above four N-methylated residues). Our data indicate that incorporation of two N-methyl amino acids into one beta-sheet is sufficient to disrupt that sheet while leaving the other, unmodified beta-sheet intact and able to form fibrils. We show, however, that disruption of each of the two beta-sheets has strikingly different effects on fibrillogenesis kinetics and fibril morphology. Both 2NMe(NTerm) and 2NMe(CTerm) form fibrils at similar rates, but more slowly than that of unmodified Abeta1-40. Electron microscopy shows that 2NMe(NTerm) forms straight fibrils with fuzzy amorphous material coating the edges, while 2NMe(CTerm) forms very regular, highly twisted fibrils-in both cases, distinct from the morphology of Abeta1-40 fibrils. Both 2NMe peptides show a "CMC" approximately four times greater than that of Abeta1-40. CD spectra of these peptides also evolve differently in time: whereas the CD spectra of 2NMe(NTerm) evolve little over 10 days, those of 2NMe(CTerm) show a transition to high beta-sheet content at about day 4-5. We also show that disruption of both beta-sheet domains, as in 4NMe, prevents fibril formation altogether, and renders Abeta1-40 highly water soluble and monomeric, and with solvent-exposed side chains. In summary, our data show (1) that the two beta-sheet domains fold in a semiautonomous manner, since disrupting each one still allows the other to fold; (2) that disruption of the N-terminal beta-sheet has a more profound effect on fibrillogenesis than disruption of the C-terminal beta-sheet, suggesting that the former is the more critical for the overall structure of the fibril; and (3) that disruption of both beta-sheet domains renders the peptide monomeric and unable to form fibrils.
Sciarretta
Kimberly L
KL
Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, Illinois 60637, USA.
Boire
Adrienne
A
Gordon
David J
DJ
Meredith
Stephen C
SC
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry*; D000682:Amyloid / Q000737:chemistry / Q000378:metabolism*; D016229:Amyloid beta-Peptides / Q000737:chemistry* / Q000378:metabolism; D002942:Circular Dichroism; D006801:Humans; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism; D017433:Protein Structure, Secondary
D000596:Amino Acids; D000682:Amyloid; D016229:Amyloid beta-Peptides; D010446:Peptide Fragments; C078592:amyloid beta-protein (1-40)