NMR as a tool to investigate the structure, dynamics and function of membrane proteins

Abstract

Membrane-protein NMR occupies a unique niche for determining structures, assessing dynamics, examining folding, and studying the binding of lipids, ligands and drugs to membrane proteins. However, NMR analyses of membrane proteins also face special challenges that are not encountered with soluble proteins, including sample preparation, size limitation, spectral crowding and sparse data accumulation. This Perspective provides a snapshot of current achievements, future opportunities and possible limitations in this rapidly developing field.

Figure 1

A selection of recent de novo membrane protein structures determined by solution or solid-state NMR. Protein abbreviations (PDB accession codes) and their organisms of origin are: OprG (2N6L): outer membrane protein G from Pseudomonas aeruginosa, a transporter for small amino acids; YadA (2LME): the transmembrane domain of the adhesin A from Yersinia enterocolitica, an outer membrane autotransporter; pSRII (2KSY): phototaxis receptor sensory rhodopsin II from Natronomonas pharaonis; TSPO (2MGY): mitochondrial translocator protein from mouse, a cholesterol and porphyrin importer into mitochondria; ASR (2M3G): sensory rhodopsin from cyanobacterium Anabaena. β-sheet (top row) or α-helical (bottom row) transmembrane segments are rainbow-color coded from red to pink in the order of their amino acid sequences (N- to C-termini), with additional α-helical (top row) or β-sheet (bottom row) structures colored in cyan. β-barrel proteins (top row) are positioned with the extracellular sides on top and α-helical proteins (bottom row) are positioned with cytosolic or intra-mitochondrial space on the bottom.

Figure 2

Membrane protein function and dynamics as elucidated by NMR. (a) Dynamics of a transporter as modulated by the addition of a ligand. Conformational exchange between the global outward-facing and inward-facing states may be faster in the un-liganded apo-form than in the ligand-bound form. Conformational exchange dynamics may be further modified by the chemical nature of different ligands. (b) G-protein coupled receptors undergo conformational exchange in the un-liganded apo-state. The motions of different domains may be connected, but not strictly coupled. Different agonists and antagonists bound to one domain may differently affect the equilibrium of activated and inactivated states of another domain. This simplified cartoon shows only a two-state exchange of the two transmembrane helices of the seven-helix receptor that participate in ligand binding, but each state may in reality consist of multiple local conformational sub-states and, in addition, also involves coupled conformational changes of adjacent helices.