Multistep Conformation Selection in Amyloid Assembly

Abstract

Defining pathways for amyloid assembly could impact therapeutic strategies for as many as 50 disease states. Here we show that amyloid assembly is subject to different forces regulating nucleation and propagation steps and provide evidence that the more global β-sheet/β-sheet facial complementarity is a critical determinant for amyloid nucleation and structural selection.

Figure 1

(a, b) Sequence alignment of (a) antiparallel in-register β-sheet and (b) antiparallel out-of-register β-sheet of Aβ(16–22) assemblies. (c, d) Projections down the H-bond axis, with the side chains for the front peptide drawn in black and those for the H-bonded second peptide in gray, highlighting cross-strand side-chain interactions of (c) antiparallel in-register β-sheets and (d) antiparallel out-of-register β-sheets. Positively charged side chains are indicated with blue and negatively charged side chains with green. V and A side chains are highlighted in orange, indicating the preferred packing of the valine side chain with the less bulky alanine side chain.

Figure 2

TEM images of 1.0 mM [1-¹³C]F19 Aβ(16–22) in 40% acetonitrile in water at neutral pH for (a) 1 h, (b) 1 day, (c) 3 days, and (d) 9 days and at acidic pH for (e) 1 h, (f) 2 days, (g) 7 days, and (h) 14 days. Scale bars = 200 nm.

Figure 3

Molar ellipticities at 217 nm measured by CD spectroscopy for 1 mM [1-¹³C]F19 Aβ(16–22) solution in 40% acetonitrile at (blue, right y axis) acidic pH (pH = 2) and (black, left y axis) neutral pH (pH = 6) over time.

Figure 4

(a–d) Representative fits of the IR spectra for [1-¹³C]F19 Aβ(16–22) incubated at (a–c) neutral pH for (a) 1 h, (b) 5 days, and (c) 14 days and (d) at acidic pH for 7 days. Linear combinations of spectra corresponding to mature assemblies with parallel (black), in-register antiparallel (red), and out-of-register antiparallel (green) β-sheets and unassembled peptide (blue) are presented as well as the resulting sums (orange lines), which are compared to the experimental spectra (orange squares). (e, f) IR spectral coefficients derived from fits to the IR spectra of assembled [1-¹³C]F19 Aβ(16–22) under (e) acidic and (f) neutral pH assembly conditions.