Probing Residue-Specific Water-Protein Interactions in Oriented Lipid Membranes via Solid-State NMR Spectroscopy

Modulation of Lipid Dynamics in the β-Amyloid Aggregates Induced Membrane Fragmentation

Nonspecific membrane disruption is considered a plausible mechanism for the cytotoxicity induced by β-amyloid (Aβ) aggregates. In scenarios of high local Aβ concentrations, a two-step membrane fragmentation model has been proposed. Initially, membrane-embedded Aβ oligomeric aggregates form, followed by membrane fragmentation. However, the key molecular-level interactions between Aβ oligomeric aggregates and lipids that drive the second-stage membrane fragmentation remain unclear. This study monitors the time-dependent changes in lipid dynamics and water accessibility of model liposomes during Aβ-induced membrane fragmentation. Our results indicate that lipid dynamics on the nanosecond to microsecond time scale undergo rapid acceleration upon initial incubation with membrane-incorporated Aβ oligomeric aggregates, followed by a slow deceleration process. Concurrently, lipid headgroups become less accessible to water. Both observations suggest a carpet-like mechanism of membrane disruption for the Aβ-induced membrane fragmentation process.

Side Chain Hydrogen-Bonding Interactions within Amyloid-like Fibrils Formed by the Low-Complexity Domain of FUS: Evidence from Solid State Nuclear Magnetic Resonance Spectroscopy

In aqueous solutions, the 214-residue low-complexity domain of the FUS protein (FUS-LC) is known to undergo liquid-liquid phase separation and also to self-assemble into amyloid-like fibrils. In previous work based on solid state nuclear magnetic resonance (ssNMR) methods, a structural model for the FUS-LC fibril core was developed, showing that residues 39-95 form the fibril core. Unlike fibrils formed by amyloid-β peptides, α-synuclein, and other amyloid-forming proteins, the FUS-LC core is largely devoid of purely hydrophobic amino acid side chains. Instead, the core-forming segment contains numerous hydroxyl-bearing residues, including 18 serines, six threonines, and eight tyrosines, suggesting that the FUS-LC fibril structure may be stabilized in part by inter-residue hydrogen bonds among side chain hydroxyl groups. Here we describe ssNMR measurements, performed on ²H,¹⁵N,¹³C-labeled FUS-LC fibrils, that provide new information about the interactions of hydroxyl-bearing residues with one another and with water. The ssNMR data support the involvement of specific serine, threonine, and tyrosine residues in hydrogen-bonding interactions. The data also reveal differences in hydrogen exchange rates with water for different side chain hydroxyl groups, providing information about solvent exposure and penetration of water into the FUS-LC fibril core.

Application of paramagnetic relaxation enhancements to accelerate the acquisition of 2D and 3D solid-state NMR spectra of oriented membrane proteins

Oriented sample solid-state NMR (OS-ssNMR) spectroscopy is uniquely suited to determine membrane protein topology at the atomic resolution in liquid crystalline bilayers under physiological temperature. However, the inherent low sensitivity of this technique has hindered the throughput of multidimensional experiments necessary for resonance assignments and structure determination. In this work, we show that doping membrane protein bicelle preparations with paramagnetic ion chelated lipids and exploiting paramagnetic relaxation effects it is possible to accelerate the acquisition of both 2D and 3D multidimensional experiments with significant saving in time. We demonstrate the efficacy of this method for a small membrane protein, sarcolipin, reconstituted in DMPC/POPC/DHPC oriented bicelles. In particular, using Cu²⁺-DMPE-DTPA as a dopant, we observed a decrease of ¹H T₁ of sarcolipin by 2/3, allowing us to reduce the recycle delay up to 3 times. We anticipate that these new developments will enable the routine acquisition of multidimensional OS-ssNMR experiments.