[Conformational regulation of amyloid beta-peptide by lipid membranes and metal ions].

Conformational transition of monomeric amyloid beta-peptide (Abeta) to a self-associated beta-sheet structure is considered to be an initial step in the development of Alzheimer's disease. Several lines of evidence suggest that physiologically abundant lipid membranes and metal ions are involved in this step. We have demonstrated that Abeta binds to the phosphatidylcholine membrane in the lamellar gel phase but not in the liquid crystalline phase by using fluorescence and circular dichroism spectroscopy. The membrane-bound Abeta molecule takes alpha-helical or beta-sheet structure depending on the temperature. Tightly packed phosphatidylcholine membranes appear to serve as a platform for non-electrostatic binding and self-association of Abeta. We have also examined Zn(II) and Cu(II) binding modes of Abeta by Raman spectroscopy. The Raman spectra demonstrate that three histidine residues in the N-terminal region of Abeta provide primary metal binding sites. Zn(II) binds to the N(tau) atom of histidine and the peptide aggregates through intermolecular His-Zn-His bridges. In contrast, Cu(II) ion is chelated by the N(pi) atom of histidine and deprotonated main-chain amide nitrogens to form a soluble complex. Our findings on the conformational regulation of Abeta may help in better understanding the molecular basis for the development of Alzheimer's disease.

Non-electrostatic binding and self-association of amyloid beta-peptide on the surface of tightly packed phosphatidylcholine membranes.

Self-association of amyloid beta-peptide (Abeta) is considered to be an initial step in the development of Alzheimer's disease and is known to be promoted by negatively charged lipid membranes. We have examined the possibility of non-electrostatic Abeta-membrane interaction by using neutral phosphatidylcholine lipids. Fluorescence and circular dichroism spectra have clearly shown that Abeta binds to the phosphatidylcholine membrane in the lamellar gel phase but not in the ripple gel or liquid crystalline phase, indicating the importance of the tight lipid packing characteristic of the lamellar gel phase. The Abeta-membrane binding occurs at both low and high salt concentrations, ensuring the non-electrostatic nature of the interaction. The membrane-bound Abeta molecule takes a monomeric alpha-helical or self-associated beta-sheet structure depending on the temperature, peptide/lipid ratio, and salt concentration. The flat surface of tightly packed phosphatidylcholine membranes appears to serve as a platform for non-electrostatic binding and self-association of Abeta.

Clustered negative charges on the lipid membrane surface induce beta-sheet formation of prion protein fragment 106-126.

The conformational conversion of prion protein (PrP) from an alpha-helix-rich normal cellular isoform (PrPC) to a beta-sheet-rich pathogenic isoform (PrPSc) is a key event in the development of prion diseases, and it takes place in caveolae, cavelike invaginations of the plasma membrane. A peptide homologous to residues 106-126 of human PrP (PrP106-126) is known to share several properties with PrPSc, e.g., the capability to form a beta-sheet and toxicity against PrPC-expressing cells. PrP106-126 is thus expected to represent a segment of PrP that is involved in the formation of PrPSc. We have examined the effect of lipid membranes containing negatively charged ganglioside, an important component of caveolae, on the secondary structure of PrP106-126 by circular dichroism. The peptide forms an alpha-helical or a beta-sheet structure on the ganglioside-containing membranes. The beta-sheet content increases with an increase of the peptide:lipid ratio, indicating that the beta-sheet formation is linked with self-association of the positively charged peptide on the negatively charged membrane surface. Analogous beta-sheet formation is also induced by membranes composed of negatively charged and neutral glycerophospholipids with high and low melting temperatures, respectively, in which lateral phase separation and clustering of negatively charged lipids occur as shown by Raman spectroscopy. Since ganglioside-containing membranes also exhibit lateral phase separation, clustered negative charges are concluded to be responsible for the beta-sheet formation of PrP106-126. In caveolae, clustered ganglioside molecules are likely to interact with the residue 106-126 region of PrPC to promote the PrPC-to-PrPSc conversion.