Efficient DNP NMR of membrane proteins: sample preparation protocols, sensitivity, and radical location

Multidimensional solid-state NMR spectroscopy of plant cell walls

Plant biomass has become an important source of bio-renewable energy in modern society. The molecular structure of plant cell walls is difficult to characterize by most atomic-resolution techniques due to the insoluble and disordered nature of the cell wall. Solid-state NMR (SSNMR) spectroscopy is uniquely suited for studying native hydrated plant cell walls at the molecular level with chemical resolution. Significant progress has been made in the last five years to elucidate the molecular structures and interactions of cellulose and matrix polysaccharides in plant cell walls. These studies have focused on primary cell walls of growing plants in both the dicotyledonous and grass families, as represented by the model plants Arabidopsis thaliana, Brachypodium distachyon, and Zea mays. To date, these SSNMR results have shown that 1) cellulose, hemicellulose, and pectins form a single network in the primary cell wall; 2) in dicot cell walls, the protein expansin targets the hemicellulose-enriched region of the cellulose microfibril for its wall-loosening function; and 3) primary wall cellulose has polymorphic structures that are distinct from the microbial cellulose structures. This article summarizes these key findings, and points out future directions of investigation to advance our fundamental understanding of plant cell wall structure and function.

High resolution observed in 800 MHz DNP spectra of extremely rigid type III secretion needles

The cryogenic temperatures at which dynamic nuclear polarization (DNP) solid-state NMR experiments need to be carried out cause line-broadening, an effect that is especially detrimental for crowded protein spectra. By increasing the magnetic field strength from 600 to 800 MHz, the resolution of DNP spectra of type III secretion needles (T3SS) could be improved by 22 %, indicating that inhomogeneous broadening is not the dominant effect that limits the resolution of T3SS needles under DNP conditions. The outstanding spectral resolution of this system under DNP conditions can be attributed to its low overall flexibility.

Solid-State NMR Investigation of the Conformation, Proton Conduction, and Hydration of the Influenza B Virus M2 Transmembrane Proton Channel

Together with the influenza A virus, influenza B virus causes seasonal flu epidemics. The M2 protein of influenza B (BM2) forms a tetrameric proton-conducting channel that is important for the virus lifecycle. BM2 shares little sequence homology with AM2, except for a conserved HxxxW motif in the transmembrane (TM) domain. Unlike AM2, no antiviral drugs have been developed to block the BM2 channel. To elucidate the proton-conduction mechanism of BM2 and to facilitate the development of BM2 inhibitors, we have employed solid-state NMR spectroscopy to investigate the conformation, dynamics, and hydration of the BM2 TM domain in lipid bilayers. BM2 adopts an α-helical conformation in lipid membranes. At physiological temperature and low pH, the proton-selective residue, His19, shows relatively narrow (15)N chemical exchange peaks for the imidazole nitrogens, indicating fast proton shuttling that interconverts cationic and neutral histidines. Importantly, pH-dependent (15)N chemical shifts indicate that His19 retains the neutral population to much lower pH than His37 in AM2, indicating larger acid-dissociation constants or lower pKa's. We attribute these dynamical and equilibrium differences to the presence of a second titratable histidine, His27, which may increase the proton-dissociation rate of His19. Two-dimensional (1)H-(13)C correlation spectra probing water (1)H polarization transfer to the peptide indicates that the BM2 channel becomes much more hydrated at low pH than at high pH, particularly at Ser12, indicating that the pore-facing serine residues in BM2 mediate proton relay to the proton-selective histidine.