Membrane protein structure determination by solid state NMR

In the last 3–5 years, solid state NMR has matured as a method for defining structural details of membrane-embedded proteins and peptides. It is still not, and may never be, a method of choice for complete structural resolution, but the power of the approach for certain systems, and in resolving details for binding sites of ligands and specific parts of proteins, is being proved. As with solution state NMR, stable NMR isotope (2H, 13C, 15N, 19F) incorporation into such large and complex biomolecules has made a tremendous impact in the area, aiding spectral assignments, increasing the amount and value of the information gained, and permitting identification of specific functional intermediates. The practical details of handling membrane proteins, which has for so long dogged every structural and biochemical study of membrane proteins, including crystallography, are not absent in the solid state NMR approach. It is simply that the protein can be studied in situ, or at least in reconstituted systems when the purified or isolated proteins can be reintroduced into a membrane for study. In addition, the molecular weight limit (Mr < 30 000) of solution state NMR (due to the slow molecular tumbling rate with respect to the applied field), is not limiting with solid state methods. Solid state NMR methods can now be used to study membrane proteins and peptides, but support from chemical, molecular biological and biochemical approaches are required to produce systems suitable for giving the desired molecular information. Specialized instrumentation is also required (high power NMR probes with spinning or static capability, specific pulse methods, good temperature stability and wide bore magnets) and some applications can present a challenge experimentally in setting up the NMR instrument. However, the method is emerging as a useful addition for the structural biologist.