Point substitution in the central hydrophobic cluster of a human beta-amyloid congener disrupts peptide folding and abolishes plaque competence

Alzheimer's disease (AD) is pathologically characterized by the presence of numerous insoluble amyloid plaques in the brain composed primarily of a 40-43 amino acid peptide, the human beta-amyloid peptide (A beta). The process of A beta deposition can be modeled in vitro by deposition of physiological concentrations of radiolabeled A beta onto preexisting amyloid in preparations of unfixed AD cerebral cortex. Using this model system, it has been shown that A beta deposition is biochemically distinct from A beta aggregation and occurs readily at physiological A beta concentrations, but which regions and conformations of A beta are essential to A beta deposition is poorly understood. We report here that an active congener, A beta(10-35)-NH2, displays time dependence, pH-activity profile, and kinetic order of deposition similar to A beta(1-40), and is sufficiently soluble for NMR spectroscopy in water under conditions where it actively deposits. To examine the importance of the central hydrophobic cluster of A beta (LVFFA, residues 17-21) for in vitro A beta deposition, an A beta(10-35)-NH2 analog with a single point substitution (F19T) in this region was synthesized and examined. Unlike A beta(10-35)-NH2, the F19T analog was plaque growth incompetent, and NMR analysis indicated that the mutant peptide was significantly less folded than wildtype A beta. These results support previous studies suggesting that the plaque competence of A beta correlates with peptide folding. Since compounds that alter A beta folding may reduce amyloid deposition, the central hydrophobic cluster of A beta will be a tempting target for structure-based drug design when high-resolution structural information becomes available.