NMR structures of membrane proteins in phospholipid bilayers

Abstract

Membrane proteins have always presented technical challenges for structural studies because of their requirement for a lipid environment. Multiple approaches exist including X-ray crystallography and electron microscopy that can give significant insights into their structure and function. However, nuclear magnetic resonance (NMR) is unique in that it offers the possibility of determining the structures of unmodified membrane proteins in their native environment of phospholipid bilayers under physiological conditions. Furthermore, NMR enables the characterization of the structure and dynamics of backbone and side chain sites of the proteins alone and in complexes with both small molecules and other biopolymers. The learning curve has been steep for the field as most initial studies were performed under non-native environments using modified proteins until ultimately progress in both techniques and instrumentation led to the possibility of examining unmodified membrane proteins in phospholipid bilayers under physiological conditions. This review aims to provide an overview of the development and application of NMR to membrane proteins. It highlights some of the most significant structural milestones that have been reached by NMR spectroscopy of membrane proteins; especially those accomplished with the proteins in phospholipid bilayer environments where they function.

Effects of naturally occurring arginine 14 deletion on phospholamban conformational dynamics and membrane interactions

“…Challenges associated with studying membrane proteins called for a combination of solution and solid-state NMR experiments [25]. We found that the deletion of Arg14 slightly alters the transmembrane domain of the protein, as well as induces a pronounced shift of the conformational equilibrium of the cytoplasmic domain toward the membrane-detached R state.…”

Solid-state NMR and membrane proteins

“…The three basic approaches to obtaining high resolution – motional averaging, spin dilution, and spin manipulation - are used in solid-state NMR studies of membrane proteins. It is worth noting that membrane proteins require the full range of solid-state NMR methods and instrumentation, and this accounts for the relatively long development period from the first double-resonance spectra of a liposome sample to the complete structures of membrane proteins that are now being determined [21]. …”

Paramagnetic relaxation enhancement of membrane proteins by incorporation of the metal-chelating unnatural amino acid 2-amino-3-(8-hydroxyquinolin-3-yl)propanoic acid (HQA)

“…Our primary research interest is in structure determination of membrane proteins in their native environment of phospholipid bilayers (Opella 2013) (Radoicic et al 2014). However, on the path towards this goal, aspects of membrane protein sample preparation and, to some extent, experimental methods are first worked out with micelle (Mesleh et al 2003), bicelle (Son et al 2012), or nanodisc (Park et al 2011a) samples that are tractable for solution NMR.…”