Solution NMR Spectroscopy for the Determination of Structures of Membrane Proteins in a Lipid Environment

NMR spectroscopy has substantially benefited from the recent technical advances to maintain its position as an eminent technique for the characterization of the solution structures and dynamics of membrane proteins at the level of atomic resolution. NMR offers a combination of several versatile strategies, for example, choice of molecular assemblies, like micelles, bicelles, and nanodiscs, with appropriate deuterated or non-deuterated detergents and phospholipids; temperature, and ionic strength; isotope labeling with ²H, ¹³C, ¹⁵N, with or without protonation of Ala, Met, Ile (δ1), Leu, and Val methyl protons; combinatorial labeling or unlabeling of specific amino acids; TROSY based-, non-uniform sampling (NUS) based-, BEST, and other NMR experiments; measurement of residual dipolar couplings using stretched polyacrylamide gels or DNA nanotubes; and spin-labeling and paramagnetic relaxation enhancements (PRE). The right combinations of these strategies together with availability of high-field NMR spectrometers (upto 1.2 GHz ¹H frequency) equipped with highly-sensitive cryogenically cooled-probes have allowed the perseverant investigator to successfully overcome several of the conventional pitfalls associated with the NMR technique and membrane proteins viz. low sensitivity, poor sample stability, spectral crowding, and a limited number of NOEs and other constraints for structure calculations. This has resulted in a steady growth in the number of successfully determined NMR structures of large and complex membrane proteins over the last two decades, and NMR spectroscopy in solution state is well-entrenched to hold its place amongst the various techniques used for the structure determination of an ever-larger number of membrane proteins.