Mixing apparatus for preparing NMR samples under pressure

Isolation, folding and structural investigations of the amino acid transporter OEP16

Membrane proteins compose more than 30% of all proteins in the living cell. However, many membrane proteins have low abundance in the cell and cannot be isolated from natural sources in concentrations suitable for structure analysis. The overexpression, reconstitution, and stabilization of membrane proteins are complex and remain a formidable challenge in membrane protein characterization. Here we describe a novel, in vitro folding procedure for a cation-selective channel protein, the outer envelope membrane protein 16 (OEP16) of pea chloroplast, overexpressed in Escherichia coli in the form of inclusion bodies. The protein is purified and then folded with detergent on a Ni-NTA affinity column. Final concentrations of reconstituted OEP16 of up to 24 mg/ml have been achieved, which provides samples that are sufficient for structural studies by NMR and crystallography. Reconstitution of OEP16 in detergent micelles was monitored by circular dichroism, fluorescence, and NMR spectroscopy. Tryptophan fluorescence spectra of heterologous expressed OEP16 in micelles are similar to spectra of functionally active OEP16 in liposomes, which indicates folding of the membrane protein in detergent micelles. CD spectroscopy studies demonstrate a folded protein consisting primarily of α-helices. ¹⁵N-HSQC NMR spectra also provide evidence for a folded protein. We present here a convenient, effective and quantitative method to screen large numbers of conditions for optimal protein stability by using microdialysis chambers in combination with fluorescence spectroscopy. Recent collection of multidimensional NMR data at 500, 600 and 800 MHz demonstrated that the protein is suitable for structure determination by NMR and stable for weeks during data collection.

Performance of cryogenic probes as a function of ionic strength and sample tube geometry

The pursuit for more sensitive NMR probes culminated with development of the cryogenic cooled NMR probe. A key factor for the sensitivity is the overall resistance of RF circuitry and sample. Lowering the coil temperature to approximately 25 K and the use of superconducting coil material has greatly reduced the resistance contribution of the hardware. However, the resistance of a salty sample remains the same and evolves as the major factor determining the signal-to-noise ratio. Several approaches have been proposed to reduce the resistance contribution of the sample. These range from encapsulating proteins in a water cavity formed by reverse micelles in low viscosity fluids to the optimal selection of low mobility, low conductivity buffer ions. Here we demonstrate that changing the sample diameter has a pronounced effect on the sample resistance and this results in dramatic improvements of the signal-to-noise ratio and shorter pi/2 pulses. We determined these parameters for common 5 mm NMR tubes under different experimental conditions and compared them to the 2, 3 and 4 mm tubes, in addition, 5mm Shigemi tubes were included since these are widely used. We demonstrate benefits and applicability of studying NMR samples with up to 4M salt concentrations in cryogenic probes. Under high salt conditions, best results in terms of short pi/2 pulses and high signal-to-noise ratios are obtained using 2 or 3mm NMR tubes, especially when limited sample is available. The 4 mm tube is preferred when sample amounts are abundant at intermediate salt conditions.

Self contained high pressure cell, apparatus and procedure for the preparation of encapsulated proteins dissolved in low viscosity fluids for NMR spectroscopy

The design of a sample cell for high performance nuclear magnetic resonance (NMR) at elevated pressure is described. The cell has been optimized for the study of encapsulated proteins dissolved in low viscosity fluids but is suitable for more general NMR spectroscopy of biomolecules at elevated pressure. The NMR cell is comprised of an alumina toughened zirconia tube mounted on a self-sealing non-magnetic metallic valve. The cell has several advantages including relatively low cost, excellent NMR performance, high pressure tolerance, chemical inertness and a relatively large active volume. Also described is a low volume sample preparation device which allows for the preparation of samples under high hydrostatic pressure and their subsequent transfer to the NMR cell.

High-resolution NMR studies of encapsulated proteins in liquid ethane

Many of the difficulties presented by large, aggregation-prone, and membrane proteins to modern solution NMR spectroscopy can be alleviated by actively seeking to increase the effective rate of molecular reorientation. An emerging approach involves encapsulating the protein of interest within the protective shell of a reverse micelle and dissolving the resulting particle in a low viscosity fluid, such as the short chain alkanes. Here we present the encapsulation of proteins with high structural fidelity within reverse micelles dissolved in liquid ethane. The addition of appropriate cosurfactants can significantly reduce the pressure required for successful encapsulation. At these reduced pressures, the viscosity of the ethane solution is low enough to provide sufficiently rapid molecular reorientation to significantly lengthen the spin-spin NMR relaxation times of the encapsulated protein.

NMR spectroscopy of proteins encapsulated in a positively charged surfactant

Traditionally, large proteins, aggregation-prone proteins, and membrane proteins have been difficult to examine by modern multinuclear and multidimensional solution NMR spectroscopy. A major limitation presented by these protein systems is that their slow molecular reorientation compromises many aspects of the more powerful solution NMR methods. Several approaches have emerged to deal with the various spectroscopic difficulties arising from slow molecular reorientation. One of these takes the approach of actively seeking to increase the effective rate of molecular reorientation by encapsulating the protein of interest within the protective shell of a reverse micelle and dissolving the resulting particle in a low viscosity fluid. Since the encapsulation is largely driven by electrostatic interactions, the preparation of samples of acidic proteins suitable for NMR spectroscopy has been problematic owing to the paucity of suitable cationic surfactants. Here, it is shown that the cationic surfactant CTAB may be used to prepare samples of encapsulated anionic proteins dissolved in low viscosity solvents. In a more subtle application, it is further shown that this surfactant can be employed to encapsulate a highly basic protein, which is completely denatured upon encapsulation using an anionic surfactant.