Interactions of unmodified and pyroglutamylated amyloid beta peptides with lipid membranes.

Abstract

The amyloid β (Aβ) peptide forms toxic assemblies that play an essential role in Alzheimer's disease. Aβ occurs in different forms; the most abundant and cytotoxic species are the 42- and 40-residue peptides (Aβ1-42, Aβ1-40) and the N-terminally truncated/pyroglutamylated AβpE3-42 and AβpE3-40. One of the mechanisms of toxicity is membrane binding, destabilization, and formation of ion-conducting pores. Here, the structural features of the above-mentioned Aβ peptides free in buffer and in the presence of lipid vesicles (60% POPC, 30% POPG, 10% cholesterol) are studied by circular dichroism (CD) and fluorescence spectroscopy. It is shown that the tyrosine can be effectively excited in the 210-240 nm range. Splitting of tyrosine emission band into two components (∼307 nm and ∼340 nm) is detected and attributed to solvent-protected and exposed fractions, respectively; the 340 nm component results from strong H-bonding between the tyrosine hydroxyl and HPO42- of the buffer. Moreover, the two populations can be selectively excited at ∼220 and ∼240 nm, allowing determination of the degree of solvent accessibility and hence the compactness of the tertiary fold of the peptides. Combined with CD, the data indicate that Aβ1-42 forms tightly packed, twisted β-sheet structure and in membranes acquires fraction of α-helix. Aβ1-40 forms less stable and more solvent-exposed β-sheet structure, and the vesicles induce partially unordered structure. AβpE3-42 forms relatively loose β-sheet; weaker overall intensity and red-shifted nπ∗ CD band indicate less twisted β-sheet. Membranes protect from solvent and induce partial α-helical structure. AβpE3-40 forms loose, solvent accessible β-sheet in buffer and solvent-protected α/β structure in membranes. These data suggest that Aβ1-42 can form β-barrel-like ion channels. AβpE3-40 can do so less effectively. Aβ1-40 and AβpE3-42 may be less efficient in forming channels because of insufficient β-sheet fraction or narrow β-barrels.