Rachel Munro

Solid-state NMR spectroscopy structure determination of a lipid-embedded heptahelical membrane protein

Determination of structure of integral membrane proteins, especially in their native environment, is a formidable challenge in structural biology. Here we demonstrate that magic angle spinning solid-state NMR spectroscopy can be used to determine structures of membrane proteins reconstituted in synthetic lipids, an environment similar to the natural membrane. We combined a large number of experimentally determined interatomic distances and local torsional restraints to solve the structure of an oligomeric membrane protein of common seven-helical fold, Anabaena sensory rhodopsin (ASR). We determined the atomic resolution detail of the oligomerization interface of the ASR trimer, and the arrangement of helices, side chains and the retinal cofactor in the monomer.

Paramagnetic relaxation enhancement reveals oligomerization interface of a membrane protein.

Protein-protein interactions play critical roles in cellular function and oligomerization of membrane proteins is a commonly observed phenomenon. Determining the oligomerization state and defining the intermolecular interface in the bilayer is generally a difficult task. Here, we use site-specific spin labeling to demonstrate that relaxation enhancements induced by covalently attached paramagnetic tag can provide distance restraints defining the intermonomer interface in oligomers formed by a seven-helical transmembrane protein Anabaena Sensory Rhodopsin (ASR). We combine these measurements with visible CD spectroscopy and cross-linking experiments to demonstrate that ASR forms tight trimers in both detergents and lipids.

Yeast-expressed human membrane protein aquaporin-1 yields excellent resolution of solid-state MAS NMR spectra

One of the biggest challenges in solid-state NMR studies of membrane proteins is to obtain a homogeneous natively folded sample giving high spectral resolution sufficient for structural studies. Eukaryotic membrane proteins are especially difficult and expensive targets in this respect. Methylotrophic yeast Pichia pastoris is a reliable producer of eukaryotic membrane proteins for crystallography and a promising economical source of isotopically labeled proteins for NMR. We show that eukaryotic membrane protein human aquaporin 1 can be doubly (13C/15N) isotopically labeled in this system and functionally reconstituted into phospholipids, giving excellent resolution of solid-state magic angle spinning NMR spectra.

Cysteine-Specific Labeling of Proteins with a Nitroxide Biradical for Dynamic Nuclear Polarization NMR

Dynamic nuclear polarization (DNP) enhances the signal in solid-state NMR of proteins by transferring polarization from electronic spins to the nuclear spins of interest. Typically, both the protein and an exogenous source of electronic spins, such as a biradical, are either codissolved or suspended and then frozen in a glycerol/water glassy matrix to achieve a homogeneous distribution. While the use of such a matrix protects the protein upon freezing, it also reduces the available sample volume (by ca. a factor of 4 in our experiments) and causes proportional NMR signal loss. Here we demonstrate an alternative approach that does not rely on dispersing the DNP agent in a glassy matrix. We synthesize a new biradical, ToSMTSL, which is based on the known DNP agent TOTAPOL, but also contains a thiol-specific methanethiosulfonate group to allow for incorporating this biradical into a protein in a site-directed manner. ToSMTSL was characterized by EPR and tested for DNP of a heptahelical transmembrane protein, Anabaena sensory rhodopsin (ASR), by covalent modification of solvent-exposed cysteine residues in two (15)N-labeled ASR mutants. DNP enhancements were measured at 400 MHz/263 GHz NMR/EPR frequencies for a series of samples prepared in deuterated and protonated buffers and with varied biradical/protein ratios. While the maximum DNP enhancement of 15 obtained in these samples is comparable to that observed for an ASR sample cosuspended with ~17 mM TOTAPOL in a glycerol-d8/D2O/H2O matrix, the achievable sensitivity would be 4-fold greater due to the gain in the filling factor. We anticipate that the DNP enhancements could be further improved by optimizing the biradical structure. The use of covalently attached biradicals would broaden the applicability of DNP NMR to structural studies of proteins.

Oligomeric Structure of Anabaena Sensory Rhodopsin in a Lipid Bilayer Environment by Combining Solid-State NMR and Long-range DEER Constraints

Oligomerization of membrane proteins is common in nature. Here, we combine spin-labeling double electron-electron resonance (DEER) and solid-state NMR (ssNMR) spectroscopy to refine the structure of an oligomeric integral membrane protein, Anabaena sensory rhodopsin (ASR), reconstituted in a lipid environment. An essential feature of such a combined approach is that it provides structural distance restraints spanning a range of ca 3-60Å while using the same sample preparation (i.e., mutations, paramagnetic labeling, and reconstitution in lipid bilayers) for both ssNMR and DEER. Direct modeling of the multispin effects on DEER signal allowed for the determination of the oligomeric order and for obtaining long-range DEER distance restraints between the ASR trimer subunits that were used to refine the ssNMR structure of ASR. The improved structure of the ASR trimer revealed a more compact packing of helices and side chains at the intermonomer interface, compared to the structure determined using the ssNMR data alone. The extent of the refinement is significant when compared with typical helix movements observed for the active states of homologous proteins. Our combined approach of using complementary DEER and NMR measurements for the determination of oligomeric structures would be widely applicable to membrane proteins where paramagnetic tags can be introduced. Such a method could be used to study the effects of the lipid membrane composition on protein oligomerization and to observe structural changes in protein oligomers upon drug, substrate, and co-factor binding.

Sparse 13 C labelling for solid-state NMR studies of P. pastoris expressed eukaryotic seven-transmembrane proteins

We demonstrate a novel sparse 13C labelling approach for methylotrophic yeast P. pastoris expression system, towards solid-state NMR studies of eukaryotic membrane proteins. The labelling scheme was achieved by co-utilizing natural abundance methanol and specifically 13C labelled glycerol as carbon sources in the expression medium. This strategy improves the spectral resolution by 1.5 fold, displays site-specific labelling patterns, and has advantages for collecting long-range distance restraints for structure determination of large eukaryotic membrane proteins by solid-state NMR.

Isotope labeling of eukaryotic membrane proteins in yeast for solid-state NMR.

Solid-state NMR (ssNMR) is a rapidly developing technique for exploring structure and dynamics of membrane proteins, but its progress is hampered by its low sensitivity. Despite the latest technological advances, routine ssNMR experiments still require several milligrams of isotopically labeled protein. While production of bacterial membrane proteins on this scale is usually feasible, obtaining such quantities of eukaryotic membrane proteins is often impossible or extremely costly. We have demonstrated that, by using isotopic labeling in yeast Pichia pastoris, one can inexpensively produce milligram quantities of doubly labeled functional samples, which yield multidimensional ssNMR spectra of high resolution suitable for detailed structural investigation. This was achieved by combining protocols of economical isotope labeling of soluble proteins previously used for solution NMR with protocols of expression of eukaryotic membrane proteins successfully employed for other methods. We review two cases of such isotope labeling, of fungal rhodopsin from Leptosphaeria maculans and human aquaporin-1.