Journal of Biomolecular NMR最新文献

N-linked glycosylation is an essential and highly conserved co- and post-translational protein modification in all domains of life. In humans, genetic defects in N-linked glycosylation pathways result in metabolic diseases collectively called Congenital Disorders of Glycosylation. In this modification reaction, a mannose rich oligosaccharide is transferred from a lipid-linked donor substrate to a specific asparagine side-chain within the -N-X-T/S- sequence (where X ≠ Proline) of the nascent protein. Oligosaccharyltransferase (OST), a multi-subunit membrane embedded enzyme catalyzes this glycosylation reaction in eukaryotes. In yeast, Ost4 is the smallest of nine subunits and bridges the interaction of the catalytic subunit, Stt3, with Ost3 (or its homolog, Ost6). Mutations of any C-terminal hydrophobic residues in Ost4 to a charged residue destabilizes the enzyme and negatively impacts its function. Specifically, the V23D mutation results in a temperature-sensitive phenotype in yeast. Here, we report the reconstitution of both purified recombinant Ost4 and Ost4V23D each in a POPC/POPE lipid bilayer and their resonance assignments using heteronuclear 2D and 3D solid-state NMR with magic-angle spinning. The chemical shifts of Ost4 changed significantly upon the V23D mutation, suggesting a dramatic change in its chemical environment.

A streamlined one-day protocol is described to produce isotopically methyl-labeled protein with high levels of deuterium for NMR studies. Using this protocol, the D₂O and ²H-glucose content of the media and protonation level of ILV labeling precursors (ketobutyrate and ketovalerate) were varied. The relaxation rate of the multiple-quantum (MQ) state that is present during the HMQC-TROSY pulse sequence was measured for different labeling schemes and this rate was used to predict upper limits of molecular weights for various labeling schemes. The use of deuterated solvents (D₂O) or deuterated glucose is not required to obtain ¹H-¹³C correlated NMR spectra of a 50 kDa homodimeric protein that are suitable for assignment by mutagenesis. High quality spectra of 100-150 kDa proteins, suitable for most applications, can be obtained without the use of deuterated glucose. The proton on the β-position of ketovalerate appears to undergo partial exchange with deuterium under the growth conditions used in this study.

In NMR spectroscopy of biomolecular systems, the use of fluorine-19 probes benefits from a clean background and high sensitivity. Therefore, ¹⁹F-labeling procedures are of wide-spread interest. Here, we use 5-fluoroindole as a precursor for cost-effective residue-specific introduction of 5-fluorotryptophan (5F-Trp) into G protein-coupled receptors (GPCRs) expressed in Pichia pastoris. The method was successfully implemented with the neurokinin 1 receptor (NK1R). The ¹⁹F-NMR spectra of 5F-Trp-labeled NK1R showed one well-separated high field-shifted resonance, which was assigned by mutational studies to the "toggle switch tryptophan". Residue-selective labeling thus enables site-specific investigations of this functionally important residue. The method described here is inexpensive, requires minimal genetic manipulation and can be expected to be applicable for yeast expression of GPCRs at large.

Monoclonal antibodies (mAbs) are biotherapeutics that have achieved outstanding success in treating many life-threatening and chronic diseases. The recognition of an antigen is mediated by the fragment antigen binding (Fab) regions composed by four different disulfide bridge-linked immunoglobulin domains. NMR is a powerful method to assess the integrity, the structure and interaction of Fabs, but site specific analysis has been so far hampered by the size of the Fabs and the lack of approaches to produce isotopically labeled samples. We proposed here an efficient in vitro method to produce [¹⁵N, ¹³C, ²H]-labeled Fabs enabling high resolution NMR investigations of these powerful therapeutics. As an open system, the cell-free expression mode enables fine-tuned control of the redox potential in presence of disulfide bond isomerase to enhance the formation of native disulfide bonds. Moreover, inhibition of transaminases in the S30 cell-free extract offers the opportunity to produce perdeuterated Fab samples directly in ¹H₂O medium, without the need for a time-consuming and inefficient refolding process. This specific protocol was applied to produce an optimally labeled sample of a therapeutic Fab, enabling the sequential assignment of ¹H_N, ¹⁵N, ¹³C′, ¹³C_α, ¹³C_β resonances of a full-length Fab. 90% of the backbone resonances of a Fab domain directed against the human LAMP1 glycoprotein were assigned successfully, opening new opportunities to study, at atomic resolution, Fabs’ higher order structures, dynamics and interactions, using solution-state NMR.

We present an economic and straightforward method to introduce ¹³C-¹⁹F spin systems into the deuterated aromatic side chains of phenylalanine as reporters for various protein NMR applications. The method is based on the synthesis of [4-¹³C, 2,3,5,6-²H₄] 4-fluorophenylalanine from the commercially available isotope sources [2-¹³C] acetone and deuterium oxide. This compound is readily metabolized by standard Escherichia coli overexpression in a glyphosate-containing minimal medium, which results in high incorporation rates in the corresponding target proteins.

Despite the prevalence and importance of glycoproteins in human biology, methods for isotope labeling suffer significant limitations. Common prokaryotic platforms do not produce mammalian post-translation modifications that are essential to the function of many human glycoproteins, including immunoglobulin G1 (IgG1). Mammalian expression systems require complex media and thus introduce significant costs to achieve uniform labeling. Expression with Pichia is available, though expertise and equipment requirements surpass E. coli culture. We developed a system utilizing Saccharomyces cerevisiae, [¹³C]-glucose, and [¹⁵N]-ammonium chloride with complexity comparable to E. coli. Here we report two vectors for expressing the crystallizable fragment (Fc) of IgG1 for secretion into the culture medium, utilizing the ADH2 or DDI2 promoters. We also report a strategy to optimize the expression yield using orthogonal Taguchi arrays. Lastly, we developed two different media formulations, a standard medium which provides 86-92% ¹⁵N and 30% ¹³C incorporation into the polypeptide, or a rich medium which provides 98% ¹⁵N and 95% ¹³C incorporation as determined by mass spectrometry. This advance represents an expression and optimization strategy accessible to experimenters with the capability to grow and produce proteins for NMR-based experiments using E. coli.

Although NMR spectroscopy is routinely used to study the conformational dynamics of biomolecules, robust analyses of the data are challenged in cases where exchange is more complex than two-state, such as when a 'visible' major conformer exchanges with two 'invisible' minor states on the millisecond timescale. It is becoming increasingly clear that chemical exchange saturation transfer (CEST) NMR experiments that were initially developed to study systems undergoing slow interconversion are also sensitive to intermediate-fast timescale biomolecular conformational exchange. Here we investigate the utility of the amide ¹⁵N CEST experiment to characterise protein three-state exchange occurring on the millisecond timescale by studying the interconversion between the folded (F) state of the FF domain from human HYPA/FBP11 (WT FF) and two of its folding intermediates I1 and I2. Although ¹⁵N CPMG experiments are consistent with the F state interconverting with a single minor state on the millisecond timescale, ¹⁵N CEST data clearly establish an exchange process between F and a pair of minor states. A unique three-state exchange model cannot be obtained by analysis of ¹⁵N CEST data recorded at a single temperature. However, including the relative sign of the difference in the chemical shifts of the two minor states based on a simple two-state analysis of CEST data recorded at multiple temperatures, results in a robust three-state model in which the F, I1 and I2 states interconvert with each other on the millisecond timescale ( $k_{ex,FI1}$ ~ 550 s^-1, $k_{ex,FI2}$ ~ 1200 s^-1, $k_{ex,I1I2}$ ~ 5000 s^-1), with I1 and I2 sparsely populated at ~ 0.15% and ~ 0.35%, respectively, at 15 °C. A computationally demanding grid-search of exchange parameter space is not required to extract the best-fit exchange parameters from the CEST data. The utility of the CEST experiment, thus, extends well beyond studies of conformers in slow exchange on the NMR chemical shift timescale, to include systems with interconversion rates on the order of thousands/second.

In this study, we present the synthesis and incorporation of a metabolic isoleucine precursor compound for selective methylene labeling. The utility of this novel α-ketoacid isotopologue is shown by incorporation into the protein Brd4-BD1, which regulates gene expression by binding to acetylated histones. High quality single quantum ¹³C-¹ H-HSQC were obtained, as well as triple quantum HTQC spectra, which are superior in terms of significantly increased ¹³C-T₂ times. Additionally, large chemical shift perturbations upon ligand binding were observed. Our study thus proves the great sensitivity of this precursor as a reporter for side-chain dynamic studies and for investigations of CH-π interactions in protein-ligand complexes.

Reducing sugars can spontaneously react with free amines in protein side chains leading to posttranslational modifications (PTMs) called glycation. In contrast to glycosylation, glycation is a non-enzymatic modification with consequences on the overall charge, solubility, aggregation susceptibility and functionality of a protein. Glycation is a critical quality attribute of therapeutic monoclonal antibodies. In addition to glucose, also disaccharides like maltose can form glycation products. We present here a detailed NMR analysis of the Amadori product formed between proteins and maltose. For better comparison, data collection was done under denaturing conditions using 7 M urea-d₄ in D₂O. The here presented correlation patterns serve as a signature and can be used to identify maltose-based glycation in any protein that can be denatured. In addition to the model protein BSA, which can be readily glycated, we present data of the biotherapeutic abatacept containing maltose in its formulation buffer. With this contribution, we demonstrate that NMR spectroscopy is an independent method for detecting maltose-based glycation, that is suited for cross-validation with other methods.

The focus of this project is to take advantage of the large NMR chemical shift anisotropy of ¹⁹F to determine the orientation of fluorine labeled biomolecules in situ in oriented biological systems such as muscle. The difficulty with a single fluorine atom is that the orientation determined from a chemical shift is not singlevalued in the case of a fully anisotropic chemical shift tensor. The utility of a labeling approach with two fluorine labels in a fixed molecular framework where one of the labels has an axially symmetric chemical shift anisotropy such as a CF₃ group and the other has a fully asymmetric chemical shift anisotropy such as 5-fluorotryptophan is evaluated. The result is that the orientation of the label can be determined straightforwardly from a single one-dimensional ¹⁹F NMR spectrum. The potential applications are widespread and not limited to biological applications.