Solid-state NMR (ssNMR) is a versatile technique that can be used for the characterization of various materials, ranging from small molecules to biological samples, including membrane proteins. ssNMR can probe...

Recent advances in the structural biology of disease-relevant α-synuclein fibrils have revealed a variety of structures, yet little is known about the process of fibril aggregate formation. Characterization of intermediate species that form during aggregation is crucial; however, this has proven very challenging because of their transient nature, heterogeneity, and low population. Here, we investigate the aggregation of α-synuclein bound to negatively charged phospholipid small unilamellar vesicles. Through a combination of kinetic and structural studies, we identify key time points in the aggregation process that enable targeted isolation of prefibrillar and early fibrillar intermediates. By using solid-state nuclear magnetic resonance, we show the gradual buildup of structural features in an α-synuclein fibril filament, revealing a segmental folding process. We identify distinct membrane-binding domains in α-synuclein aggregates, and the combined data are used to present a comprehensive mechanism of the folding of α-synuclein on lipid membranes.

The protein AlkL is known to increase permeability of the outer membrane of bacteria for hydrophobic molecules, yet the mechanism of transport has not been determined. Differing crystal and NMR structures of homologous proteins resulted in a controversy regarding the degree of structure and the role of long extracellular loops. Here we solve this controversy by determining the de novo NMR structure in near-native lipid bilayers, and by accessing structural dynamics relevant to hydrophobic substrate permeation through molecular-dynamics simulations and by characteristic NMR relaxation parameters. Dynamic lateral exit sites large enough to accommodate substrates such as carvone or octane occur through restructuring of a barrel extension formed by the extracellular loops.

The arrangement of histidine side chains in influenza A M2 tetramer determines their pK a values, which define pH-controlled proton conduction critical to the virus lifecycle. Both water-associated and hydrogen-bonded imidazole–imidazolium histidine quaternary structures have been proposed, based on crystal structures and NMR chemical shifts, respectively. Here we show, using the conduction domain construct of M2 in lipid bilayers, that the imidazole rings are hydrogen bonded even at a pH of 7.8 in the neutral charge state. An intermolecular 8.9 ± 0.3 Hz 2h J NN hydrogen bond is observed between H37 Nε and Nδ recorded in a fully protonated sample with 100 kHz magic-angle spinning. This interaction could not be detected in the drug-bound sample.

Determination of the environment surrounding a protein is often key to understanding its function and can also be used to infer the structural properties of the protein. By using proton‐detected solid‐state NMR, we show that reduced spin diffusion within the protein under conditions of fast magic‐angle spinning, high magnetic field, and sample deuteration allows the efficient measurement of site‐specific exposure to mobile water and lipids. We demonstrate this site specificity on two membrane proteins, the human voltage dependent anion channel, and the alkane transporter AlkL from Pseudomonas putida. Transfer from lipids is observed selectively in the membrane spanning region, and an average lipid‐protein transfer rate of 6 s−1 was determined for residues protected from exchange. Transfer within the protein, as tracked in the 15N‐1H 2D plane, was estimated from initial rates and found to be in a similar range of about 8 to 15 s−1 for several resolved residues, explaining the site specificity.