Amino Acid Selective 13C Labeling and 13C Scrambling Profile Analysis of Protein α and Side-Chain Carbons in Escherichia coli Utilized for Protein Nuclear Magnetic Resonance

Tracing of Amino Acids Dynamics in Cell Lines Based on 18O Stable Isotope Labeling

Metabolite levels and turnover rates are necessary to understand metabolomic dynamics in a living organism fully. Amino acids can play distinct roles in various cellular processes, and their abnormal levels are associated with pathological conditions, including cancer. Therefore, their levels, especially turnover rates, may provide enormous information about a phenotype. 13C- or 13C,15N-labeled amino acids have also been commonly used to trace amino acid metabolism. This study presented a new methodology based on 18O labeling for amino acids that relied on monitoring mass isotopologues to calculate the turnover rates of amino acids. The method optimization studies were carried over for selective amino acid monitoring. This methodology provides a rapid, robust, and simple GC-MS method for analyzing the fluxes of amino acid metabolism. The developed method was applied to fetal human colon (FHC) and human colon carcinoma (Caco-2) cell lines to determine cancer-induced shifts in the turnover rates of amino acids. These results defined metabolic reprogramming in Caco-2 cells through increased glutamate and serine turnovers and sharply decreased turnovers of aspartate, threonine, and methionine, therefore pointing to some metabolic vulnerabilities in the metabolism of cancerous cells. The simple mechanism of the developed methodology, the availability of affordable 18O-enriched water, and the ease of application can open a new arena in fluxomics analysis.

Isotope Reverse-Labeled Infrared Spectroscopy as a Probe of In-Cell Protein Structure

While recent years have seen great progress in determining the three-dimensional structure of isolated proteins, monitoring protein structure inside live cells remains extremely difficult. Here, we examine the utility of Fourier transform infrared (FTIR) spectroscopy as a probe of protein structure in live bacterial cells. Selective isotope enrichment is used both to distinguish recombinantly expressed NuG2b protein from the cellular background and to examine the conformation of specific residues in the protein. To maximize labeling flexibility and to improve spectral resolution between label and main-band peaks, we carry out isotope-labeling experiments in "reverse-labeling" mode: cells are initially grown in 13C-enriched media, with specific 12C-labeled amino acids added when protein expression is induced. 1 Because FTIR measurements require only around 20 μL of sample and each measurement takes only a few minutes to complete, isotope-labeling costs are minimal, allowing us to label multiple different residues in parallel in simultaneously grown cultures. For the stable NuG2b protein, isotope difference spectra from live bacterial cultures are nearly identical to spectra from isolated proteins, confirming that the structure of the protein is unperturbed by the cellular environment. By combining such measurements with site-directed mutagenesis, we further demonstrate that the local conformation of individual amino acids can be monitored, allowing us to determine, for example, whether a specific site in the protein contributes to α-helix or β-sheet structures.

Segmental and site-specific isotope labelling strategies for structural analysis of posttranslationally modified proteins

Posttranslational modifications can alter protein structures, functions and locations, and are important cellular regulatory and signalling mechanisms. Spectroscopic techniques such as nuclear magnetic resonance, infrared and Raman spectroscopy, as well as small-angle scattering, can provide insights into the structural and dynamic effects of protein posttranslational modifications and their impact on interactions with binding partners. However, heterogeneity of modified proteins from natural sources and spectral complexity often hinder analyses, especially for large proteins and macromolecular assemblies. Selective labelling of proteins with stable isotopes can greatly simplify spectra, as one can focus on labelled residues or segments of interest. Employing chemical biology tools for modifying and isotopically labelling proteins with atomic precision provides access to unique protein samples for structural biology and spectroscopy. Here, we review site-specific and segmental isotope labelling methods that are employed in combination with chemical and enzymatic tools to access posttranslationally modified proteins. We discuss illustrative examples in which these methods have been used to facilitate spectroscopic studies of posttranslationally modified proteins, providing new insights into biology.

Efficient production of a functional G protein-coupled receptor in E. coli for structural studies

G protein-coupled receptors (GPCRs) are transmembrane signal transducers which regulate many key physiological process. Since their discovery, their analysis has been limited by difficulties in obtaining sufficient amounts of the receptors in high-quality, functional form from heterologous expression hosts. Albeit highly attractive because of its simplicity and the ease of isotope labeling for NMR studies, heterologous expression of functional GPCRs in E. coli has proven particularly challenging due to the absence of the more evolved protein expression and folding machinery of higher eukaryotic hosts. Here we first give an overview on the previous strategies for GPCR E. coli expression and then describe the development of an optimized robust protocol for the E. coli expression and purification of two mutants of the turkey β₁-adrenergic receptor (β₁AR) uniformly or selectively labeled in ¹⁵N or ²H,¹⁵N. These mutants had been previously optimized for thermal stability using insect cell expression and used successfully in crystallographic and NMR studies. The same sequences were then used for E. coli expression. Optimization of E. coli expression was achieved by a quantitative analysis of losses of receptor material at each step of the solubilization and purification procedure. Final yields are 0.2-0.3 mg receptor per liter culture. Whereas both expressed mutants are well folded and competent for orthosteric ligand binding, the less stable YY-β₁AR mutant also comprises the two native tyrosines Y^5.58 and Y^7.53, which enable G protein binding. High-quality ¹H-¹⁵N TROSY spectra were obtained for E. coli-expressed YY-β₁AR in three different functional states (antagonist, agonist, and agonist + G protein-mimicking nanobody-bound), which are identical to spectra obtained of the same forms of the receptor expressed in insect cells. NdeI and AgeI restriction sites introduced into the expression plasmid allow for the easy replacement of the receptor gene by other GPCR genes of interest, and the provided quantitative workflow analysis may guide the respective adaptation of the purification protocol.

Membrane Protein Structure Determination and Characterisation by Solution and Solid-State NMR

Biological membranes define the interface of life and its basic unit, the cell. Membrane proteins play key roles in membrane functions, yet their structure and mechanisms remain poorly understood. Breakthroughs in crystallography and electron microscopy have invigorated structural analysis while failing to characterise key functional interactions with lipids, small molecules and membrane modulators, as well as their conformational polymorphism and dynamics. NMR is uniquely suited to resolving atomic environments within complex molecular assemblies and reporting on membrane organisation, protein structure, lipid and polysaccharide composition, conformational variations and molecular interactions. The main challenge in membrane protein studies at the atomic level remains the need for a membrane environment to support their fold. NMR studies in membrane mimetics and membranes of increasing complexity offer close to native environments for structural and molecular studies of membrane proteins. Solution NMR inherits high resolution from small molecule analysis, providing insights from detergent solubilised proteins and small molecular assemblies. Solid-state NMR achieves high resolution in membrane samples through fast sample spinning or sample alignment. Recent developments in dynamic nuclear polarisation NMR allow signal enhancement by orders of magnitude opening new opportunities for expanding the applications of NMR to studies of native membranes and whole cells.