Search

Genetic code expansion (GCE) reprograms the translational machinery to site-specifically incorporate noncanonical amino acids (ncAAs) into a selected protein. The efficiency of GCE in mammalian cells might be compromised by cellular stress responses, among which, the protein kinase R(PKR)-dependent eIF2α phosphorylation pathway can reduce translation rates. Here we test several strategies to engineer the eIF2α pathway and boost the rate of translation and show that such interventions increase GCE efficiency in mammalian cells. In particular, addition of the N-terminal PKR fragment (1-174) provides a substantial enhancement in cytoplasmic GCE and also in GCE realized by OTOs (orthogonally translating designer organelles), which built on the principle of 2D phase separation to enable mRNA-selective ncAA incorporation. Our study demonstrates an approach for improving the efficiency of GCE and provides a means by which the power of designer organelles can be further optimized to tune protein translation., (© 2023. The Author(s).)

Single-molecule Förster-resonance energy transfer (smFRET) experiments allow the study of biomolecular structure and dynamics in vitro and in vivo. We performed an international blind study involving 19 laboratories to assess the uncertainty of FRET experiments for proteins with respect to the measured FRET efficiency histograms, determination of distances, and the detection and quantification of structural dynamics. Using two protein systems with distinct conformational changes and dynamics, we obtained an uncertainty of the FRET efficiency ≤0.06, corresponding to an interdye distance precision of ≤2 Å and accuracy of ≤5 Å. We further discuss the limits for detecting fluctuations in this distance range and how to identify dye perturbations. Our work demonstrates the ability of smFRET experiments to simultaneously measure distances and avoid the averaging of conformational dynamics for realistic protein systems, highlighting its importance in the expanding toolbox of integrative structural biology., (© 2023. The Author(s).)

Protein bioconjugation frequently makes use of aldehydes as reactive handles, with methods for their installation being highly valued. Here a new, powerful strategy to unmask a reactive protein aldehyde is presented. A genetically encoded caged glyoxyl aldehyde, situated in solvent-accessible locations, can be rapidly decaged through treatment with just one equivalent of allylpalladium(ii) chloride dimer at physiological pH. The protein aldehyde can undergo subsequent oxime ligation for site-selective protein modification. Quick yet mild conditions, orthogonality and powerful exposed reactivity make this strategy of great potential in protein modification.

Tetrazine-bearing fluorescent labels enable site-specific tagging of proteins that are genetically manipulated with dienophile modified noncanonical amino acids. The inverse electron demand Diels-Alder reaction between the tetrazine and the dienophile fulfills the criteria of bioorthogonality allowing fluorescent labeling schemes of live cells. Here, we describe the detailed synthetic and labeling protocols of a near infrared emitting siliconrhodamine-tetrazine probe suitable for super-resolution imaging of residue-specifically engineered proteins in mammalian cells.

Small synthetic fluorophores are in many ways superior to fluorescent proteins as labels for imaging. A major challenge is to use them for a protein-specific labeling in living cells. Here, we report on our use of noncanonical amino acids that are genetically encoded via the pyrrolysyl-tRNA/pyrrolysyl-RNA synthetase pair at artificially introduced TAG codons in a recoded E. coli strain. The strain is lacking endogenous TAG codons and the TAG-specific release factor RF1. The amino acids contain bioorthogonal groups that can be clicked to externally supplied dyes, thus enabling protein-specific labeling in live cells. We find that the noncanonical amino acid incorporation into the target protein is robust for diverse amino acids and that the usefulness of the recoded E. coli strain mainly derives from the absence of release factor RF1. However, the membrane permeable dyes display high nonspecific binding in intracellular environment and the electroporation of hydrophilic nonmembrane permeable dyes severely impairs growth of the recoded strain. In contrast, proteins exposed on the outer membrane of E. coli can be labeled with hydrophilic dyes with a high specificity as demonstrated by labeling of the osmoporin OmpC. Here, labeling can be made sufficiently specific to enable single molecule studies as exemplified by OmpC single particle tracking.

Introduction of bioorthogonal functionalities (e.g., trans-cyclooctene-TCO) into a protein of interest by site-specific genetic encoding of non-canonical amino acids (ncAAs) creates uniquely targetable platforms for fluorescent labeling schemes in combination with tetrazine-functionalized dyes. However, fluorescent labeling of an intracellular protein is usually compromised by high background, arising from the hydrophobicity of ncAAs; this is typically compensated for by hours-long washout to remove excess ncAAs from the cellular interior. To overcome these problems, we designed, synthesized, and tested new, hydrophilic TCO-ncAAs. One derivative, DOTCO-lysine was genetically incorporated into proteins with good yield. The increased hydrophilicity shortened the excess ncAA washout time from hours to minutes, thus permitting rapid labeling and subsequent fluorescence microscopy., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Genetic code expansion is emerging as an important tool for manipulation and labeling of proteins in vitro and in vivo. In combination with click-chemistry it allows site-specific labeling of target proteins with small organic fluorophores. This is achieved by cotranslational incorporation of noncanonical amino acids (ncAAs) in target proteins via orthogonal tRNA/amino-acyl tRNA synthetase pairs. In a subsequent step, ncAAs are labeled with small dyes via click-chemistry. Small labeling tags and free choice of which fluorophore to use and where to put it into the protein are of particular importance for single molecule science and superresolution microscopy. Such genetically encoded click chemistry tools facilitate high resolution studies of protein function in live cells, viral imaging, as well as the improved design of antibody-drug conjugates., (Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Contemporary structural biology research promises more than just static snap-shots of molecular machineries. This goal is not just facilitated by combining different structural biology techniques, but also by new tools from the field of protein and genetic engineering, as well as from chemistry. Genetic encoding of noncanonical amino acids (ncAAs) through codon-suppression technology provides an excellent opportunity to probe biomolecules using different structural biology methods. In this article, we review the applications of ncAA incorporation into proteins for determining structural information through various techniques with the main focus on crosslinking mass spectrometry and single-molecule FRET-based techniques. Furthermore, advances and limitations of the incorporation of multiple ncAAs are discussed, with respect to design of an ideal host organism for modern and integrative structural biology research., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

We describe a protocol for the rapid labeling of cell-surface proteins in living mammalian cells using click chemistry. The labeling method is based on strain-promoted alkyne-azide cycloaddition (SPAAC) and strain-promoted inverse-electron-demand Diels-Alder cycloaddition (SPIEDAC) reactions, in which noncanonical amino acids (ncAAs) bearing ring-strained alkynes or alkenes react, respectively, with dyes containing azide or tetrazine groups. To introduce ncAAs site specifically into a protein of interest (POI), we use genetic code expansion technology. The protocol can be described as comprising two steps. In the first step, an Amber stop codon is introduced--by site-directed mutagenesis--at the desired site on the gene encoding the POI. This plasmid is then transfected into mammalian cells, along with another plasmid that encodes an aminoacyl-tRNA synthetase/tRNA (RS/tRNA) pair that is orthogonal to the host's translational machinery. In the presence of the ncAA, the orthogonal RS/tRNA pair specifically suppresses the Amber codon by incorporating the ncAA into the polypeptide chain of the POI. In the second step, the expressed POI is labeled with a suitably reactive dye derivative that is directly supplied to the growth medium. We provide a detailed protocol for using commercially available ncAAs and dyes for labeling the insulin receptor, and we discuss the optimal surface-labeling conditions and the limitations of labeling living mammalian cells. The protocol involves an initial cloning step that can take 4-7 d, followed by the described transfections and labeling reaction steps, which can take 3-4 d.

Blowing up: Rapid expansion of the genetic code, beyond what nature initially intended, has given chemical biologists innumerable tools for protein engineering and biological studies. This special issue highlights some of the most recent applications of these techniques in in vivo systems., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The ideal fluorescent probe for bioimaging is bright, absorbs at long wavelengths and can be implemented flexibly in living cells and in vivo. However, the design of synthetic fluorophores that combine all of these properties has proved to be extremely difficult. Here, we introduce a biocompatible near-infrared silicon-rhodamine probe that can be coupled specifically to proteins using different labelling techniques. Importantly, its high permeability and fluorogenic character permit the imaging of proteins in living cells and tissues, and its brightness and photostability make it ideally suited for live-cell super-resolution microscopy. The excellent spectroscopic properties of the probe combined with its ease of use in live-cell applications make it a powerful new tool for bioimaging.

Single molecule Fluorescence Resonance Energy Transfer (FRET) has been widely applied to study structure, function and dynamics of complex biological systems. Labeling of proteins at specific positions with fluorescent dyes is a challenging and key step for any single molecule FRET measurement. Genetic code expansion has facilitated site specific incorporation of unnatural amino acids into proteins. These unnatural amino acid bears bioorthognal functional groups that provide opportunity to install a unique chemical handle into proteins. Propargyllysine is an unnatural amino acid which, when incorporated into a protein, can be exploited to attach commercially available fluorescent azide dyes through copper-catalyzed alkyne-azide cycloaddition click reaction (also known as click reaction). We describe here an optimized strategy to combine synthesis of propargyllysine, its genetic incorporation in the protein and click reaction to site-specifically label the protein with azide derivative of Alexa® 488. Later the protein is labeled at unique cysteine residue via maleimide coupling chemistry with acceptor Alexa® 594 dye to yield double labeled protein as required for any single molecule FRET experiments., (Copyright © 2013 Elsevier Inc. All rights reserved.)

While innovations in modern microscopy, spectroscopy, and nanoscopy techniques have made single molecule observation a standard in many laboratories, the actual design of meaningful fluorescence reporter systems now hinders major scientific breakthroughs. Even though the field of chemical biology is supercharging the fluorescence toolbox, surprisingly few strategies exist that make the transition from model systems to biologically relevant applications. At the same time, the number of microscopy techniques is growing dramatically. We explain our view on how the impact of modern technologies is influenced not only by further hard- and software developments, but also by the availability and suitability of protein-engineering tools. We identify how the largely independent research fields of chemical biology and fluorescence nanoscopy can influence each other to synergistically drive future technology that can visualize the localization, structure, and dynamics of molecular function without constraints., (Copyright © 2013 WILEY Periodicals, Inc.)

Visualizing biomolecules by fluorescent tagging is a powerful method for studying their behaviour and function inside cells. We prepared and genetically encoded an unnatural amino acid (UAA) that features a bicyclononyne moiety. This UAA offered exceptional reactivity in strain-promoted azide-alkyne cycloadditions. Kinetic measurements revealed that the UAA reacted also remarkably fast in the inverse-electron-demand Diels-Alder cycloaddition with tetrazine-conjugated dyes. Genetic encoding of the new UAA inside mammalian cells and its subsequent selective labeling at low dye concentrations demonstrate the usefulness of the new amino acid for future imaging studies., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Single-molecule methods have matured into central tools for studies in biology. Foerster resonance energy transfer (FRET) techniques, in particular, have been widely applied to study biomolecular structure and dynamics. The major bottleneck for a facile and general application of these studies arises from the need to label biological samples site-specifically with suitable fluorescent dyes. In this work, we present an optimized strategy combining click chemistry and the genetic encoding of unnatural amino acids (UAAs) to overcome this limitation for proteins. We performed a systematic study with a variety of clickable UAAs and explored their potential for high-resolution single-molecule FRET (smFRET). We determined all parameters that are essential for successful single-molecule studies, such as accessibility of the probes, expression yield of proteins, and quantitative labeling. Our multiparameter fluorescence analysis allowed us to gain new insights into the effects and photophysical properties of fluorescent dyes linked to various UAAs for smFRET measurements. This led us to determine that, from the extended tool set that we now present, genetically encoding propargyllysine has major advantages for state-of-the-art measurements compared to other UAAs. Using this optimized system, we present a biocompatible one-step dual-labeling strategy of the regulatory protein RanBP3 with full labeling position freedom. Our technique allowed us then to determine that the region encompassing two FxFG repeat sequences adopts a disordered but collapsed state. RanBP3 serves here as a prototypical protein that, due to its multiple cysteines, size, and partially disordered structure, is not readily accessible to any of the typical structure determination techniques such as smFRET, NMR, and X-ray crystallography.

A general strategy for the site-specific dual-labeling of proteins for single-molecule fluorescence resonance energy transfer is presented. A genetically encoded unnatural ketone amino acid was labeled with a hydroxylamine-containing fluorophore with high yield (>95%) and specificity. This methodology was used to construct dual-labeled T4 lysozyme variants, allowing the study of T4 lysozyme folding at single-molecule resolution. The presented strategy is anticipated to expand the scope of single-molecule protein structure and function studies.

In his commentary in this issue of Molecular Cell , 1 Struhl reasons that the term "intrinsically disordered regions" represents a vague and confusing concept for protein function. However, the term "intrinsically disordered" highlights the important physicochemical characteristic of conformational heterogeneity. Thus, "intrinsically disordered" is the counterpart to the term "folded, " with neither term having specific functional implications. In his commentary in this issue of Molecular Cell ,1 Struhl reasons that the term "intrinsically disordered regions" represents a vague and confusing concept for protein function. However, the term "intrinsically disordered" highlights the important physicochemical characteristic of conformational heterogeneity. Thus, "intrinsically disordered" is the counterpart to the term "folded, " with neither term having specific functional implications. [ABSTRACT FROM AUTHOR]

The synthesis of a set of tetrazine-bearing fluorogenic dyes suitable for intracellular labeling of proteins in live cells is presented. The red excitability and emission properties ensure minimal autofluorescence, while through-bond energy-transfer-based fluorogenicity reduces nonspecific background fluorescence of unreacted dyes. The tetrazine motif efficiently quenches fluorescence of the phenoxazine core, which can be selectively turned on chemically upon bioorthogonal inverse-electron-demand Diels-Alder reaction with proteins modified genetically with strained trans-cyclooctenes. [ABSTRACT FROM AUTHOR]

trans-Cyclooctene groups incorporated into proteins via non-canonical amino acids (ncAAs) are emerging as specific handles for bioorthogonal chemistry. Here, we present a highly improved synthetic access to the axially and the equatorially linked trans-cyclooct-2-ene isomers ( 1 a, b). We further show that the axially connected isomer has a half-life about 10 times higher than the equatorial isomer and reacts with tetrazines much faster, as determined by stopped-flow experiments. The improved properties resulted in different labeling performance of the insulin receptor on the surface of intact cells. [ABSTRACT FROM AUTHOR]

The bacteriophage ΦCD27 is capable of lysing Clostridium difficile, a pathogenic bacterium that is a major cause for nosocomial infection. A recombinant CD27L endolysin lyses C. difficile in vitro, and represents a promising alternative as a bactericide. To better understand the lysis mechanism, we have determined the crystal structure of an autoproteolytic fragment of the CD27L endolysin. The structure covers the C-terminal domain of the endolysin, and represents a novel fold that is identified in a number of lysins that target Clostridia bacteria. The structure indicates endolysin cleavage occurs at the stem of the linker connecting the catalytic domain with the C-terminal domain. We also solved the crystal structure of the C-terminal domain of a slow cleaving mutant of the CTP1L endolysin that targets C. tyrobutyricum. Two distinct dimerization modes are observed in the crystal structures for both endolysins, despite a sequence identity of only 22% between the domains. The dimers are validated to be present for the full length protein in solution by right angle light scattering, small angle X-ray scattering and cross-linking experiments using the cross-linking amino acid p-benzoyl-L-phenylalanine (pBpa). Mutagenesis on residues contributing to the dimer interfaces indicates that there is a link between the dimerization modes and the autocleavage mechanism. We show that for the CTP1L endolysin, there is a reduction in lysis efficiency that is proportional to the cleavage efficiency. We propose a model for endolysin triggering, where the extended dimer presents the inactive state, and a switch to the side-by-side dimer triggers the cleavage of the C-terminal domain. This leads to the release of the catalytic portion of the endolysin, enabling the efficient digestion of the bacterial cell wall. [ABSTRACT FROM AUTHOR]

The ideal fluorescent probe for bioimaging is bright, absorbs at long wavelengths and can be implemented flexibly in living cells and in vivo. However, the design of synthetic fluorophores that combine all of these properties has proved to be extremely difficult. Here, we introduce a biocompatible near-infrared silicon-rhodamine probe that can be coupled specifically to proteins using different labelling techniques. Importantly, its high permeability and fluorogenic character permit the imaging of proteins in living cells and tissues, and its brightness and photostability make it ideally suited for live-cell super-resolution microscopy. The excellent spectroscopic properties of the probe combined with its ease of use in live-cell applications make it a powerful new tool for bioimaging. [ABSTRACT FROM AUTHOR]

The author discuss options for designing fluorescent sensors and reporters and strategies to develop their properties. They define sensor and reporter molecules as related to cell biology and note that bacterial and plant proteins and protein domains that lack enzyme activity can be useful in building sensors in mammalian cells or intramolecular sensors. Enzyme activity, memory sensors, and sensors in animal models are also discussed.

We genetically encoded the photocaged amino acid 4,5-dimethoxy-2-nitrobenzylserine (DMNB-Ser, 1) in Saccharomyces cerevisiae in response to the amber nonsense codon TAG. This amino acid was converted to serine in living cells by irradiation with relatively low-energy blue light and was used to noninvasively photoactivate phosphorylation of the transcription factor Pho4, which controls the cellular response to inorganic phosphate. When substituted at phosphoserine sites that control nuclear export of Pho4, 1 blocks phosphorylation and subsequent export by the receptor Msn5 (ref. 2). We triggered phosphorylation of individual serine residues with a visible laser pulse and monitored nuclear export of Pho4-GFP fusion constructs in real time. We observed distinct export kinetics for differentially phosphorylated Pho4 mutants, which demonstrates dynamic regulation of Pho4 function. This methodology should also facilitate the analysis of other cellular processes involving free serine residues, including catalysis, biomolecular recognition and ion transport. [ABSTRACT FROM AUTHOR]

Amino Acids for Live ‐ Cell Labeling. The non ‐ canonical amino acid trans ‐ cyclooctene ‐ lysine (TCO*) allows the rapid and specific labeling of proteins by click chemistry. In their Communication on page 12266 ff., C. Schultz et al. present a novel synthesis route for TCO* and investigate the stability and reactivity of the isomers, as well as their utility for live ‐ cell labeling. The reactivity of TCO* conformers with tetrazines in comparison to other common click reactions is systematically quantified by using density functional calculations. The axial isomer of TCO* incorporated in a green fluorescent protein reacts with a tetrazine attached to Cy5. The relevant orbitals involved in this extremely fast click reaction are shown on a background of mammalian cells expressing insulin receptors selectively labeled by this method. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the site-specific genetic incorporation of an environmentally sensitive, fluorescent amino acid into proteins in Saccharomyces cerevisiae, an orthogonal Escherichia coli amber suppressor tRNA. The study utilized a method that serves as a local probe of conformational changes in the structure of protein caused by ligand binding. The study has also incorporated fluorescent amino acid into proteins in yeast.