Fast Motions of Key Methyl Groups in Amyloid-β Fibrils

Abstract

Amyloid-β (Aβ) peptide is the major component of plaques found in Alzheimer's disease patients. Using solid-state H NMR relaxation performed on selectively deuterated methyl groups, we probed the dynamics in the threefold symmetric and twofold symmetric polymorphs of native Aβ as well as the protofibrils of the D23N mutant. Specifically, we investigated the methyl groups of two leucine residues that belong to the hydrophobic core (L17 and L34) as well as M35 residues belonging to the hydrophobic interface between the cross-β subunits, which has been previously found to be water-accessible. Relaxation measurements performed over 310-140 K and two magnetic field strengths provide insights into conformational variability within and between polymorphs. Core packing variations within a single polymorph are similar to what is observed for globular proteins for the core residues, whereas M35 exhibits a larger degree of variability. M35 site is also shown to undergo a solvent-dependent dynamical transition in which slower amplitude motions of methyl axes are activated at high temperature. The motions, modeled as a diffusion of methyl axis, have activation energy by a factor of 2.7 larger in the twofold compared with the threefold polymorph, whereas D23N protofibrils display a value similar to the threefold polymorph. This suggests enhanced flexibility of the hydrophobic interface in the threefold polymorph. This difference is only observed in the hydrated state and is absent in the dry fibrils, highlighting the role of solvent at the cavity. In contrast, the dynamic behavior of the core is hydration-independent.

Solvent-Driven Dynamical Crossover in the Phenylalanine Side-Chain from the Hydrophobic Core of Amyloid Fibrils Detected by 2H NMR Relaxation

“…The presence of the dynamical transition in F19 in the core of amyloid fibrils combined with the data for methyl groups of the M35 side-chain, 29 which does not point toward the core (Figure 1), supports the notion that fibrils, in general, are capable of undergoing dynamical transitions. M35 undergoes a cross-over from the dominance of fast methyl rotations at low temperatures to slower diffusive motions of the methyl axis at higher temperatures.…”

“…3) was the same, around 221 K, for methyl groups in M35 in both the 3fwet and 2fwet fibrils. 29 In contrast, there are clear differences in the cross-over temperatures for the two native polymorphs at the F19 site, with the 3-fold morphology at 219 K, but the 2-fold one at 263 K. Thus, the additional slower, larger-amplitude modes induced by the solvent do not necessarily have the same temperature of onset at all sites and are sensitive to the polymorphic state of the fibrils. For the M35 site, slow diffusive motion of the methyl axis was shown to have higher activation energy (i.e., more restricted) in the 2fwet fibrils compared to the 3fwet fibrils, while D23N protofibrils display a value similar to the 3-fold polymorph.…”

“…29, 30 However, the dynamics of methyl groups in the core remained invariant between the dry and hydrated fibrils, unlike the dynamics at the methyl groups of M35 which defines the contacts between the cross-β subunits. 29, 30 The M35 side-chain has been shown to undergo a solvent-driven transition. 29 …”

Static solid-state 2H NMR methods in studies of protein side-chain dynamics

“…31 and 19). (66) The dynamical transition is manifested at this site as an onset of dominance of the slower diffusive motions of the methyl axis at around 250–240 K in addition to methyl rotations, which dominate at lower temperatures. The diffusive motions are clearly hydration-dependent and are suppressed in the dry samples, in which methyl rotations remain dominant across the entire range.…”

“…18 and 19) is a comparison between two polymorphs in the amyloid fibril samples at the methionine side-chain pointing into a water-exposed hydrophobic cavity: the difference between two different polymorphs is much more clearly seen with the T 1Q measurements compared with the T 1Z ones. (66)…”