Contemporary Methods in Structure Determination of Membrane Proteins by Solution NMR - NMR of Proteins and Small Biomolecules - page 123

NOE Nuclear Overhauser enhancement

NOESY Nuclear Overhauser spectroscopy

NUS Non-uniform sampling

Omp Outer membrane protein

PRE Paramagnetic relaxation enhancement

pSRII Photosensitive rhodopsin II

RCSA Residual chemical shift anisotropy

RDC Residual dipolar coupling

SDS Sodium dodecyl sulfate

TM Transmembrane

TROSY Transverse relaxation spectroscopy

UNC2 Uncoupling protein 2

VDAC Voltage dependent anion channel

1 Introduction

Membrane proteins confer a remarkable array of functionalities to the membranes
that define cellular boundaries [ 1 , 2 ]. They are responsible for the controlled
transport of nutrients, electrolytes, signaling agents, and toxins across an otherwise
inert lipid bilayer, and also make it possible for a cell to sense and communicate
with its environment, a vital process for a wide range of biological events. The fact
that alterations in membrane protein function are linked to a number of disease
states; e.g., cystic fibrosis, Alzheimer's disease, and long QT syndrome [ 3 - 6 ], and
that 50% of known drug targets are membrane proteins [ 7 , 8 ], has made this class of
proteins an attractive target in drug discovery efforts. Consequently there is a high
level of interest in understanding how membrane proteins function at the atomic
level, and in finding ways in which these functions can be disrupted or enhanced.
High-resolution structures greatly facilitate efforts to address these issues, yet at
present there are only ~300 unique membrane proteins for which structures have
been determined ( http://blanco.biomol.uci.edu/mpstruc/listAll/list ). Although a
large number of these have been provided by X-ray crystallography, relatively
recent developments in the study of large protein complexes by solution NMR have
greatly increased the ability of this approach to provide important insights into
membrane protein structure and function.
Solution NMR has unique capabilities to provide structural insights for proteins
that are refractory to crystallization [ 9 - 11 ], and to characterize functionally rele-
vant dynamic processes at atomic resolution [ 12 - 15 ]. However, the hydrophobic
nature of membrane proteins greatly complicates handling and biophysical analyses
in general. This gives rise to significant challenges, particularly for solution NMR
of membrane proteins, in (1) development of cost-effective strategies to produce
isotopically labeled membrane protein samples, (2) identification of detergent or
lipid solutions that can maintain the protein in a folded, soluble state with a complex
size that would be compatible with solution NMR, and (3) acquisition of the NMR