Strategies to Expand the Genetic Code of Mammalian Cells

Genetic code expansion (GCE) in mammalian cells has emerged as a powerful technology for investigating and engineering protein function. This method allows for the precise incorporation of a rapidly growing toolbox of noncanonical amino acids (ncAAs) into predefined sites of target proteins expressed in living cells. Due to the minimal size of these genetically encoded ncAAs, the wide range of functionalities they provide, and the ability to introduce them freely at virtually any site of any protein by simple mutagenesis, this technology holds immense potential for probing the complex biology of mammalian cells and engineering next-generation biotherapeutics. In this review, we provide an overview of the underlying machinery that enables ncAA mutagenesis in mammalian cells and how these are developed. We have also compiled an updated list of ncAAs that have been successfully incorporated into proteins in mammalian cells. Finally, we provide our perspectives on the current challenges that need to be addressed to fully harness the potential of this technology.

Genetic Code Expansion: Recent Developments and Emerging Applications

The concept of genetic code expansion (GCE) has revolutionized the field of chemical and synthetic biology, enabling the site-specific incorporation of noncanonical amino acids (ncAAs) into proteins, thus opening new avenues in research and applications across biology and medicine. In this review, we cover the principles of GCE, including the optimization of the aminoacyl-tRNA synthetase (aaRS)/tRNA system and the advancements in translation system engineering. Notable developments include the refinement of aaRS/tRNA pairs, enhancements in screening methods, and the biosynthesis of noncanonical amino acids. The applications of GCE technology span from synthetic biology, where it facilitates gene expression regulation and protein engineering, to medicine, with promising approaches in drug development, vaccine production, and gene editing. The review concludes with a perspective on the future of GCE, underscoring its potential to further expand the toolkit of biology and medicine. Through this comprehensive review, we aim to provide a detailed overview of the current state of GCE technology, its challenges, opportunities, and the frontier it represents in the expansion of the genetic code for novel biological research and therapeutic applications.

Noncanonical Amino Acid Tools and Their Application to Membrane Protein Studies

Methods rooted in chemical biology have contributed significantly to studies of integral membrane proteins. One recent key approach has been the application of genetic code expansion (GCE), which enables the site-specific incorporation of noncanonical amino acids (ncAAs) with defined chemical properties into proteins. Efficient GCE is challenging, especially for membrane proteins, which have specialized biogenesis and cell trafficking machinery and tend to be expressed at low levels in cell membranes. Many eukaryotic membrane proteins cannot be expressed functionally in E. coli and are most effectively studied in mammalian cell culture systems. Recent advances have facilitated broader applications of GCE for studies of membrane proteins. First, AARS/tRNA pairs have been engineered to function efficiently in mammalian cells. Second, bioorthogonal chemical reactions, including cell-friendly copper-free "click" chemistry, have enabled linkage of small-molecule probes such as fluorophores to membrane proteins in live cells. Finally, in concert with advances in GCE methodology, the variety of available ncAAs has increased dramatically, thus enabling the investigation of protein structure and dynamics by multidisciplinary biochemical and biophysical approaches. These developments are reviewed in the historical framework of the development of GCE technology with a focus on applications to studies of membrane proteins.

Noncanonical Amino Acid Incorporation in Animals and Animal Cells

Noncanonical amino acids (ncAAs) are synthetic building blocks that, when incorporated into proteins, confer novel functions and enable precise control over biological processes. These small yet powerful tools offer unprecedented opportunities to investigate and manipulate various complex life forms. In particular, ncAA incorporation technology has garnered significant attention in the study of animals and their constituent cells, which serve as invaluable model organisms for gaining insights into human physiology, genetics, and diseases. This review will provide a comprehensive discussion on the applications of ncAA incorporation technology in animals and animal cells, covering past achievements, current developments, and future perspectives.

Genetic Code Expansion Approaches to Decipher the Ubiquitin Code

The covalent attachment of Ub (ubiquitin) to target proteins (ubiquitylation) represents one of the most versatile PTMs (post-translational modifications) in eukaryotic cells. Substrate modifications range from a single Ub moiety being attached to a target protein to complex Ub chains that can also contain Ubls (Ub-like proteins). Ubiquitylation plays pivotal roles in most aspects of eukaryotic biology, and cells dedicate an orchestrated arsenal of enzymes to install, translate, and reverse these modifications. The entirety of this complex system is coined the Ub code. Deciphering the Ub code is challenging due to the difficulty in reconstituting enzymatic machineries and generating defined Ub/Ubl-protein conjugates. This Review provides a comprehensive overview of recent advances in using GCE (genetic code expansion) techniques to study the Ub code. We highlight strategies to site-specifically ubiquitylate target proteins and discuss their advantages and disadvantages, as well as their various applications. Additionally, we review the potential of small chemical PTMs targeting Ub/Ubls and present GCE-based approaches to study this additional layer of complexity. Furthermore, we explore methods that rely on GCE to develop tools to probe interactors of the Ub system and offer insights into how future GCE-based tools could help unravel the complexity of the Ub code.

Engineering Pyrrolysine Systems for Genetic Code Expansion and Reprogramming

Over the past 16 years, genetic code expansion and reprogramming in living organisms has been transformed by advances that leverage the unique properties of pyrrolysyl-tRNA synthetase (PylRS)/tRNAPyl pairs. Here we summarize the discovery of the pyrrolysine system and describe the unique properties of PylRS/tRNAPyl pairs that provide a foundation for their transformational role in genetic code expansion and reprogramming. We describe the development of genetic code expansion, from E. coli to all domains of life, using PylRS/tRNAPyl pairs, and the development of systems that biosynthesize and incorporate ncAAs using pyl systems. We review applications that have been uniquely enabled by the development of PylRS/tRNAPyl pairs for incorporating new noncanonical amino acids (ncAAs), and strategies for engineering PylRS/tRNAPyl pairs to add noncanonical monomers, beyond α-L-amino acids, to the genetic code of living organisms. We review rapid progress in the discovery and scalable generation of mutually orthogonal PylRS/tRNAPyl pairs that can be directed to incorporate diverse ncAAs in response to diverse codons, and we review strategies for incorporating multiple distinct ncAAs into proteins using mutually orthogonal PylRS/tRNAPyl pairs. Finally, we review recent advances in the encoded cellular synthesis of noncanonical polymers and macrocycles and discuss future developments for PylRS/tRNAPyl pairs.

Cellular Site-Specific Incorporation of Noncanonical Amino Acids in Synthetic Biology

Over the past two decades, genetic code expansion (GCE)-enabled methods for incorporating noncanonical amino acids (ncAAs) into proteins have significantly advanced the field of synthetic biology while also reaping substantial benefits from it. On one hand, they provide synthetic biologists with a powerful toolkit to enhance and diversify biological designs beyond natural constraints. Conversely, synthetic biology has not only propelled the development of ncAA incorporation through sophisticated tools and innovative strategies but also broadened its potential applications across various fields. This Review delves into the methodological advancements and primary applications of site-specific cellular incorporation of ncAAs in synthetic biology. The topics encompass expanding the genetic code through noncanonical codon addition, creating semiautonomous and autonomous organisms, designing regulatory elements, and manipulating and extending peptide natural product biosynthetic pathways. The Review concludes by examining the ongoing challenges and future prospects of GCE-enabled ncAA incorporation in synthetic biology and highlighting opportunities for further advancements in this rapidly evolving field.

Cracking the Code: Reprogramming the Genetic Script in Prokaryotes and Eukaryotes to Harness the Power of Noncanonical Amino Acids

Over 500 natural and synthetic amino acids have been genetically encoded in the last two decades. Incorporating these noncanonical amino acids into proteins enables many powerful applications, ranging from basic research to biotechnology, materials science, and medicine. However, major challenges remain to unleash the full potential of genetic code expansion across disciplines. Here, we provide an overview of diverse genetic code expansion methodologies and systems and their final applications in prokaryotes and eukaryotes, represented by Escherichia coli and mammalian cells as the main workhorse model systems. We highlight the power of how new technologies can be first established in simple and then transferred to more complex systems. For example, whole-genome engineering provides an excellent platform in bacteria for enabling transcript-specific genetic code expansion without off-targets in the transcriptome. In contrast, the complexity of a eukaryotic cell poses challenges that require entirely new approaches, such as striving toward establishing novel base pairs or generating orthogonally translating organelles within living cells. We connect the milestones in expanding the genetic code of living cells for encoding novel chemical functionalities to the most recent scientific discoveries, from optimizing the physicochemical properties of noncanonical amino acids to the technological advancements for their in vivo incorporation. This journey offers a glimpse into the promising developments in the years to come.

Advances in crosslinking chemistry and proximity-enabled strategies: deciphering protein complexes and interactions

Mass spectrometry, coupled with innovative crosslinking techniques to decode protein conformations and interactions through uninterrupted signal connections, has undergone remarkable progress in recent years. It is crucial to develop selective crosslinking reagents that minimally disrupt protein structure and dynamics, providing insights into protein network regulation and biological functions. Compared to traditional crosslinkers, new bifunctional chemical crosslinkers exhibit high selectivity and specificity in connecting proximal amino acid residues, resulting in stable molecular crosslinked products. The conjugation with specific amino acid residues like lysine, cysteine, arginine and tyrosine expands the XL-MS toolbox, enabling more precise modeling of target substrates and leading to improved data quality and reliability. Another emerging crosslinking method utilizes unnatural amino acids (UAAs) derived from proximity-enabled reactivity with specific amino acids or sulfur-fluoride exchange (SuFEx) reactions with nucleophilic residues. These UAAs are genetically encoded into proteins for the formation of specific covalent bonds. This technique combines the benefits of genetic encoding for live cell compatibility with chemical crosslinking, providing a valuable method for capturing transient and weak protein-protein interactions (PPIs) for mapping PPI coordinates and improving the pharmacological properties of proteins. With continued advancements in technology and applications, crosslinking mass spectrometry is poised to play an increasingly significant role in guiding our understanding of protein dynamics and function in the future.

Hybrid Small-Molecule/Protein Fluorescent Probes

Hybrid small-molecule/protein fluorescent probes are powerful tools for visualizing protein localization and function in living cells. These hybrid probes are constructed by diverse site-specific chemical protein labeling approaches through chemical reactions to exogenous peptide/small protein tags, enzymatic post-translational modifications, bioorthogonal reactions for genetically incorporated unnatural amino acids, and ligand-directed chemical reactions. The hybrid small-molecule/protein fluorescent probes are employed for imaging protein trafficking, conformational changes, and bioanalytes surrounding proteins. In addition, fluorescent hybrid probes facilitate visualization of protein dynamics at the single-molecule level and the defined structure with super-resolution imaging. In this review, we discuss development and the bioimaging applications of fluorescent probes based on small-molecule/protein hybrids.

Applications of genetic code expansion technology in eukaryotes

Unnatural amino acids (UAAs) have gained significant attention in protein engineering and drug development owing to their ability to introduce new chemical functionalities to proteins. In eukaryotes, genetic code expansion (GCE) enables the incorporation of UAAs and facilitates posttranscriptional modification (PTM), which is not feasible in prokaryotic systems. GCE is also a powerful tool for cell or animal imaging, the monitoring of protein interactions in target cells, drug development, and switch regulation. Therefore, there is keen interest in utilizing GCE in eukaryotic systems. This review provides an overview of the application of GCE in eukaryotic systems and discusses current challenges that need to be addressed.

Design principles for engineering light-controlled antibodies

Engineered antibodies are essential tools for research and advanced pharmacy. In the development of therapeutics, antibodies are excellent candidates as they offer both target recognition and modulation. Thanks to the latest advances in biotechnology, light-activated antibody fragments can be constructed to control spontaneous antigen interaction with high spatiotemporal precision. To implement conditional antigen binding, several optogenetic and optochemical engineering concepts have recently been developed. Here, we highlight the various strategies and discuss the features of opto-conditional antibodies. Each concept offers intrinsic advantages beneficial to different applications. In summary, the novel design approaches constitute a complementary toolset to promote current and upcoming antibody technologies with ultimate precision.

Evolution and synthetic biology

Evolutionary observations have often served as an inspiration for biological design. Decoding of the central dogma of life at a molecular level and understanding of the cellular biochemistry have been elegantly used to engineer various synthetic biology applications, including building genetic circuits in vitro and in cells, building synthetic translational systems, and metabolic engineering in cells to biosynthesize and even bioproduce complex high-value molecules. Here, we review three broad areas of synthetic biology that are inspired by evolutionary observations: (i) combinatorial approaches toward cell-based biomolecular evolution, (ii) engineering interdependencies to establish microbial consortia, and (iii) synthetic immunology. In each of the areas, we will highlight the evolutionary premise that was central toward designing these platforms. These are only a subset of the examples where evolution and natural phenomena directly or indirectly serve as a powerful source of inspiration in shaping synthetic biology and biotechnology.

Tumor microenvironment responsive Mn3O4 nanoplatform for in vivo real-time monitoring of drug resistance and photothermal/chemodynamic synergistic therapy of gastric cancer

Background

Postoperative chemotherapy for gastric cancer often causes multidrug resistance (MDR), which has serious consequences for therapeutic effects. Individualized treatment based on accurate monitoring of MDR will greatly improve patient survival.

Results

In this article, a self-enhanced Mn₃O₄ nanoplatform (MPG NPs) was established, which can react with glutathione to produce Mn²⁺ to enhance T1-weighted magnetic resonance imaging (MRI) and mediate in vivo real-time MDR monitoring. In vitro MRI results showed that MRI signals could be enhanced in the presence of hydrogen peroxide and glutathione and at acidic pH. In vivo MRI results indicated that MPG NPs could specifically target MDR cells, thereby realizing real-time monitoring of MDR in gastric cancer. Furthermore, MPG NPs have good chemodynamic activity, which can convert the endogenous hydrogen peroxide of tumor cells into highly toxic hydroxyl radical through Fenton-like reaction at acidic pH to play the role of chemodynamic therapy. In addition, Mn₃O₄ can significantly enhance the chemodynamic therapy effect because of its good photothermal conversion effect. Furthermore, in situ photothermal/chemodynamic synergistic therapy obtained remarkable results, the tumors of the mice in the synergistic therapy group gradually became smaller or even disappeared.

Conclusions

MPG NPs have good biocompatibility, providing a good nanoplatform for real-time monitoring and precise diagnosis and treatment of MDR in gastric cancer.

Graphical Abstract

Supplementary information

The online version contains supplementary material available at 10.1186/s12951-022-01441-6.

Recent Advances in Fluorescence Imaging by Genetically Encoded Non-canonical Amino Acids

Technical innovations in protein labeling with a fluorophore at the specific residue have played a significant role in studying protein dynamics. The genetic code expansion (GCE) strategy enabled the precise installation of fluorophores at the tailored site of proteins in live cells with minimal perturbation of native functions. Considerable advances have been achieved over the past decades in fluorescent imaging using GCE strategies along with bioorthogonal chemistries. In this review, we discuss advances in the GCE-based strategies to site-specifically introduce fluorophore at a defined position of the protein and their bio-imaging applications.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Light-Triggered Click Chemistry

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

Genome recoding strategies to improve cellular properties: mechanisms and advances

The genetic code, once believed to be universal and immutable, is now known to contain many variations and is not quite universal. The basis for genome recoding strategy is genetic code variation that can be harnessed to improve cellular properties. Thus, genome recoding is a promising strategy for the enhancement of genome flexibility, allowing for novel functions that are not commonly documented in the organism in its natural environment. Here, the basic concept of genetic code and associated mechanisms for the generation of genetic codon variants, including biased codon usage, codon reassignment, and ambiguous decoding, are extensively discussed. Knowledge of the concept of natural genetic code expansion is also detailed. The generation of recoded organisms and associated mechanisms with basic targeting components, including aminoacyl-tRNA synthetase-tRNA pairs, elongation factor EF-Tu and ribosomes, are highlighted for a comprehensive understanding of this concept. The research associated with the generation of diverse recoded organisms is also discussed. The success of genome recoding in diverse multicellular organisms offers a platform for expanding protein chemistry at the biochemical level with non-canonical amino acids, genetically isolating the synthetic organisms from the natural ones, and fighting viruses, including SARS-CoV2, through the creation of attenuated viruses. In conclusion, genome recoding can offer diverse applications for improving cellular properties in the genome-recoded organisms.

Reprogramming the genetic code

The encoded biosynthesis of proteins provides the ultimate paradigm for high-fidelity synthesis of long polymers of defined sequence and composition, but it is limited to polymerizing the canonical amino acids. Recent advances have built on genetic code expansion - which commonly permits the cellular incorporation of one type of non-canonical amino acid into a protein - to enable the encoded incorporation of several distinct non-canonical amino acids. Developments include strategies to read quadruplet codons, use non-natural DNA base pairs, synthesize completely recoded genomes and create orthogonal translational components with reprogrammed specificities. These advances may enable the genetically encoded synthesis of non-canonical biopolymers and provide a platform for transforming the discovery and evolution of new materials and therapeutics.

Creation of Bacterial cells with 5-Hydroxytryptophan as a 21st Amino Acid Building Block

While most organisms utilize 20 canonical amino acid building blocks for protein synthesis, adding additional candidates to the amino acid repertoire can greatly facilitate the investigation and manipulation of protein structures and functions. In this study, we report the generation of completely autonomous organisms with a 21^st ncAA, 5-hydroxytryptophan (5HTP). Like 20 canonical amino acids, 5-hydroxytryptophan can be biosynthesized in vivo from simple carbon sources and is subsequently incorporated into proteins in response to the amber stop codon. Using this unnatural organism, we have prepared a single-chain immunoglobulin variable fragment conjugated with a fluorophore and demonstrated the utility of these autonomous cells to monitor oxidative stress. Creation of this and other cells containing the 21^st amino acid will provide an opportunity to generate proteins and organisms with novel activities, as well as to determine the evolutionary consequences of using additional amino acid buildings.

Beyond natural: synthetic expansions of botanical form and function

Powered by developments that enabled genome-scale investigations, systems biology emerged as a field aiming to understand how phenotypes emerge from network functions. These advances fuelled a new engineering discipline focussed on synthetic reconstructions of complex biological systems with the goal of predictable rational design and control. Initially, progress in the nascent field of synthetic biology was slow due to the ad hoc nature of molecular biology methods such as cloning. The application of engineering principles such as standardisation, together with several key technical advances, enabled a revolution in the speed and accuracy of genetic manipulation. Combined with mathematical and statistical modelling, this has improved the predictability of engineering biological systems of which nonlinearity and stochasticity are intrinsic features leading to remarkable achievements in biotechnology as well as novel insights into biological function. In the past decade, there has been slow but steady progress in establishing foundations for synthetic biology in plant systems. Recently, this has enabled model-informed rational design to be successfully applied to the engineering of plant gene regulation and metabolism. Synthetic biology is now poised to transform the potential of plant biotechnology. However, reaching full potential will require conscious adjustments to the skillsets and mind sets of plant scientists.

Near-infrared light controlled fluorogenic labeling of glycoengineered sialic acids in vivo with upconverting photoclick nanoprobe

Sialic acid serves as an important determinant for profiling cell activities in diverse biological and pathological processes. The precise control of sialic acid labeling to visualize its biological pathways under endogenous conditions is significant but still challenging due to the lack of reliable methods. Herein, we developed an effective strategy to spatiotemporally label thesialic acids with a near-infrared (NIR) light activated upconverting nanoprobe (Tz-UCNP). With this photoclickable nanoprobe and a stable N-alkene-d-mannosamine (Ac4ManNIPFA), metabolically synthesized alkene sialic acids on the cell surface were labeled and imaged in real time through fluorogenic cycloaddition. More importantly, we achieved spatially selective visualization of sialic acids in specific tumor tissues of the mice under NIR light activation in a spatially controlled manner. This in situ controllable labeling strategy thus enables the metabolic labeling of specific sialic acids in complex biological systems.

High-throughput screen for sorting cells capable of producing the biofuel feedstock botryococcene

Botryococcene is a branched triterpene produced by the algae Botryococcus braunii. Hydrocracking botryococcene yields a variety of combustible fuels such as gasoline and jet fuel. Engineering host systems and proteins involved in the biosynthesis of botryococcene to optimize production is of interest given these applications. The current study investigates the use of a diaryltetrazole based screen that undergoes a photoclick reaction with terminal alkenes, such as the branched terminal alkene present on botryococcene, to yield a fluorescent product. Host E. coli systems were established to produce botryococcene, squalene, and no triterpene to serve as a control. Cells were incubated with tetrazole and briefly irradiated with UV light to initiate the photoclick reaction. It was found that the botryococcene producing cells yielded observable fluorescence while the squalene and control cells had negligible fluorescence turn-on activity. Fluorescence-activated cell sorting (FACS) was subsequently used to identify and sort botryococcene producing E. coli from a mixture of control and squalene producing cells.

Using genetically incorporated unnatural amino acids to control protein functions in mammalian cells

Genetic code expansion allows unnatural (non-canonical) amino acid incorporation into proteins of interest by repurposing the cellular translation machinery. The development of this technique has enabled site-specific incorporation of many structurally and chemically diverse amino acids, facilitating a plethora of applications, including protein imaging, engineering, mechanistic and structural investigations, and functional regulation. Particularly, genetic code expansion provides great tools to study mammalian proteins, of which dysregulations often have important implications in health. In recent years, a series of methods has been developed to modulate protein function through genetically incorporated unnatural amino acids. In this review, we will first discuss the basic concept of genetic code expansion and give an up-to-date list of amino acids that can be incorporated into proteins in mammalian cells. We then focus on the use of unnatural amino acids to activate, inhibit, or reversibly modulate protein function by translational, optical or chemical control. The features of each approach will also be highlighted.

Site-Specific Labeling of Proteins Using Unnatural Amino Acids

Labeling of a protein with a specific dye or tag at defined positions is a critical step in tracing the subtle behavior of the protein and assessing its cellular function. Over the last decade, many strategies have been developed to achieve selective labeling of proteins in living cells. In particular, the site-specific unnatural amino acid (UAA) incorporation technique has gained increasing attention since it enables attachment of various organic probes to a specific position of a protein in a more precise way. In this review, we describe how the UAA incorporation technique has expanded our ability to achieve site-specific labeling and visualization of target proteins for functional analyses in live cells.

Site-specific ubiquitylation and SUMOylation using genetic-code expansion and sortase

Post-translational modification of proteins with ubiquitin and ubiquitin-like proteins (Ubls) is central to the regulation of eukaryotic cellular processes. Our ability to study the effects of ubiquitylation, however, is limited by the difficulty to prepare homogenously modified proteins in vitro and by the impossibility to selectively trigger specific ubiquitylation events in living cells. Here we combine genetic-code expansion, bioorthogonal Staudinger reduction and sortase-mediated transpeptidation to develop a general tool to ubiquitylate proteins in an inducible fashion. The generated ubiquitin conjugates display a native isopeptide bond and bear two point mutations in the ubiquitin C terminus that confer resistance toward deubiquitinases. Nevertheless, physiological integrity of sortase-generated diubiquitins in decoding cellular functions via recognition by ubiquitin-binding domains is retained. Our approach allows the site-specific attachment of Ubls to nonrefoldable, multidomain proteins and enables inducible and ubiquitin-ligase-independent ubiquitylation of proteins in mammalian cells, providing a powerful tool to dissect the biological functions of ubiquitylation with temporal control.

Fluorogenic "Photoclick" Labeling and Imaging of DNA with Coumarin-Fused Tetrazole in Vivo

Photoclickable fluorogenic probes will enable visualization of specific biomolecules with precise spatiotemporal control in their native environment. However, the fluorogenic tagging of DNA with current photocontrolled clickable probes is still challenging. Herein, we demonstrated the fast (19.5 ± 2.5 M^-1 s^-1) fluorogenic labeling and imaging of DNA in vitro and in vivo with rationally designed coumarin-fused tetrazoles under UV LED photoirradiation. With a water-soluble, nuclear-specific coumarin-fused tetrazole (CTz-SO₃), the metabolically synthesized DNA in cultured cells was effectively labeled and visualized, without fixation, via "photoclick" reaction. Moreover, the photoclickable CTz-SO₃ enabled real-time, spatially controlled imaging of DNA in live zebrafish.

Synthesis and evaluation of photo-activatable β-diarylsydnone-l-alanines for fluorogenic photo-click cyclization of peptides

β-Diarylsydnone-l-alanines were designed and introduced into peptides allowing photo-cyclization only in phosphate containing buffer with concomitant fluorescence generation in live cells.

Proteomic Identification of Protein Tyrosine Phosphatase and Substrate Interactions in Living Mammalian Cells by Genetic Encoding of Irreversible Enzyme Inhibitors

Protein tyrosine phosphatases (PTPs) play critical roles in cell signaling pathways, but identification of unknown PTPs for a given substrate in live cells remain technically challenging. Here, we synthesized a series of tyrosine-based irreversible PTP inhibitors and characterized by site-specific encoding on substrate proteins in cells with an expanded genetic code. By fine-tuning the chemical reactivity, we identified optimal active amino acid probes to covalently cross-link a PTP and its substrate both in vitro and in mammalian cells. Using HER2 as an example, we provide first direct evidence of HER2 Y1023 and SHP2 cross-linking in situ in living human cells. Moreover, proteomic analysis using our approach identified PTP1B as a novel phosphatase for HER2 that specifically dephosphorylated pY1221 position, which may shed light on the puzzle of PTP1B's role in HER2 positive breast cancer. This novel method provides a useful tool for dissecting tyrosine phosphoregulation in living cells.

Upgrading aminoacyl-tRNA synthetases for genetic code expansion

Synthesis of proteins with non-canonical amino acids via genetic code expansion is at the forefront of synthetic biology. Progress in this field has enabled site-specific incorporation of over 200 chemically and structurally diverse amino acids into proteins in an increasing number of organisms. This has been facilitated by our ability to repurpose aminoacyl-tRNA synthetases to attach non-canonical amino acids to engineered tRNAs. Current efforts in the field focus on overcoming existing limitations to the simultaneous incorporation of multiple non-canonical amino acids or amino acids that differ from the l-α-amino acid structure (e.g. d-amino acid or β-amino acid). Here, we summarize the progress and challenges in developing more selective and efficient aminoacyl-tRNA synthetases for genetic code expansion.

Live Cell Imaging of Bioorthogonally Labelled Proteins Generated With a Single Pyrrolysine tRNA Gene

Genetic code expansion enables the incorporation of non-canonical amino acids (ncAAs) into expressed proteins. ncAAs are usually encoded by a stop codon that is decoded by an exogenous orthogonal aminoacyl tRNA synthetase and its cognate suppressor tRNA, such as the pyrrolysine [Formula: see text] pair. In such systems, stop codon suppression is dependent on the intracellular levels of the exogenous tRNA. Therefore, multiple copies of the tRNAPyl gene (PylT) are encoded to improve ncAA incorporation. However, certain applications in mammalian cells, such as live-cell imaging applications, where labelled tRNAs contribute to background fluorescence, can benefit from the use of less invasive minimal expression systems. Accordingly, we studied the effect of tRNAPyl on live-cell fluorescence imaging of bioorthogonally-labelled intracellular proteins. We found that in COS7 cells, a decrease in PylT copy numbers had no measurable effect on protein expression levels. Importantly, reducing PylT copy numbers improved the quality of live-cell images by enhancing the signal-to-noise ratio and reducing an immobile tRNAPyl population. This enabled us to improve live cell imaging of bioorthogonally labelled intracellular proteins, and to simultaneously label two different proteins in a cell. Our results indicate that the number of introduced PylT genes can be minimized according to the transfected cell line, incorporated ncAA, and application.

Therapeutic applications of genetic code expansion

In nature, a limited, conservative set of amino acids are utilized to synthesize proteins. Genetic code expansion technique reassigns codons and incorporates noncanonical amino acids (ncAAs) through orthogonal aminoacyl-tRNA synthetase (aaRS)/tRNA pairs. The past decade has witnessed the rapid growth in diversity and scope for therapeutic applications of this technology. Here, we provided an update on the recent progress using genetic code expansion in the following areas: antibody-drug conjugates (ADCs), bispecific antibodies (BsAb), immunotherapies, long-lasting protein therapeutics, biosynthesized peptides, engineered viruses and cells, as well as other therapeutic related applications, where the technique was used to elucidate the mechanisms of biotherapeutics and drug targets.

Chemically-defined lactose-based autoinduction medium for site-specific incorporation of non-canonical amino acids into proteins

Genetic code expansion technology enables the site-specific incorporation of dozens of non-canonical amino acids (NCAAs) into proteins expressed in live cells. The NCAAs can introduce various chemical functionalities into proteins, ranging from natural post-translational modifications, to spectroscopic probes and chemical handles for bioorthogonal reactions. These chemical groups provide powerful tools for structural, biochemical, and biophysical studies, which may require significant quantities of recombinantly expressed proteins. NCAAs are usually encoded by an in-frame stop codon, such as the TAG (amber) stop codon, which leads to the expression of C-terminally truncated proteins. In addition, the incubation medium should be supplemented with the NCAA at a final concentration of 1-10 mM, which may be challenging when the availability of the NCAA is limited. Hence, bacterial expression of proteins carrying NCAAs can benefit from improvement in protein yield per given amount of added NCAA. Here, we demonstrate the applicability of an optimized chemically-defined lactose-based autoinduction (AI) medium to the expression of proteins carrying a NCAA, using the archaeal pyrrolysyl-tRNA synthetase/tRNA pair from the Methanosarcina genus. Per given amount of added NCAA, the use of AI medium improved protein expression levels by up to 3-fold, compared to IPTG induction, without an increase in misincorporation of canonical amino acids in response to the in-frame stop codon. The suggested medium composition can be used with various Escherichia coli variants transformed with different expression vectors and incubated at different temperatures.

A genetically encoded 19F NMR probe for lysine acetylation

Advances in acetylated protein-protein/DNA interactions depend on the development of a novel NMR (nuclear magnetic resonance) probe to study the conformational changes of acetylated proteins. However, the method for detecting the acetylated protein conformation is underdeveloped. Herein, an acetyllysine mimic has been exploited for detecting the conformational changes of acetylated p53-protein/DNA interactions by genetic code expansion and 19F NMR. This 19F NMR probe shows high structural similarity to acetyllysine and could not be deacetylated by sirtuin deacetylase in vitro/vivo. Moreover, acetylation of p53 K164 is reported to be deacetylated by SIRT2 for the first time.

Playing with the Molecules of Life

Our understanding of the complex molecular processes of living organisms at the molecular level is growing exponentially. This knowledge, together with a powerful arsenal of tools for manipulating the structures of macromolecules, is allowing chemists to to harness and reprogram the cellular machinery in ways previously unimaged. Here we review one example in which the genetic code itself has been expanded with new building blocks that allow us to probe and manipulate the structures and functions of proteins with unprecedented precision.

A molecular engineering toolbox for the structural biologist

Exciting new technological developments have pushed the boundaries of structural biology, and have enabled studies of biological macromolecules and assemblies that would have been unthinkable not long ago. Yet, the enhanced capabilities of structural biologists to pry into the complex molecular world have also placed new demands on the abilities of protein engineers to reproduce this complexity into the test tube. With this challenge in mind, we review the contents of the modern molecular engineering toolbox that allow the manipulation of proteins in a site-specific and chemically well-defined fashion. Thus, we cover concepts related to the modification of cysteines and other natural amino acids, native chemical ligation, intein and sortase-based approaches, amber suppression, as well as chemical and enzymatic bio-conjugation strategies. We also describe how these tools can be used to aid methodology development in X-ray crystallography, nuclear magnetic resonance, cryo-electron microscopy and in the studies of dynamic interactions. It is our hope that this monograph will inspire structural biologists and protein engineers alike to apply these tools to novel systems, and to enhance and broaden their scope to meet the outstanding challenges in understanding the molecular basis of cellular processes and disease.

Surface Functionalization with Carboxylic Acids by Photochemical Microcontact Printing and Tetrazole Chemistry

In this paper, we show that carboxylic acid-functionalized molecules can be patterned by photochemical microcontact printing on tetrazole-terminated self-assembled monolayers. Upon irradiation, tetrazoles eliminate nitrogen to form highly reactive nitrile imines, which can be ligated with several different nucleophiles, carboxylic acids being the most reactive. As a proof of concept, we immobilized trifluoroacetic acid to monitor the reaction with X-ray photoelectron spectroscopy. Moreover, we also immobilized peptides and fabricated carbohydrate-lectin as well as biotin-streptavidin microarrays using this method. Surface-patterning was demonstrated by fluorescence microscopy and time-of-flight secondary ion mass spectrometry.

Engineering the Genetic Code in Cells and Animals: Biological Considerations and Impacts

Expansion of the genetic code allows unnatural amino acids (Uaas) to be site-specifically incorporated into proteins in live biological systems, thus enabling novel properties selectively introduced into target proteins in vivo for basic biological studies and for engineering of novel biological functions. Orthogonal components including tRNA and aminoacyl-tRNA synthetase (aaRS) are expressed in live cells to decode a unique codon (often the amber stop codon UAG) as the desired Uaa. Initially developed in E. coli, this methodology has now been expanded in multiple eukaryotic cells and animals. In this Account, we focus on addressing various biological challenges for rewriting the genetic code, describing impacts of code expansion on cell physiology and discussing implications for fundamental studies of code evolution. Specifically, a general method using the type-3 polymerase III promoter was developed to efficiently express prokaryotic tRNAs as orthogonal tRNAs and a transfer strategy was devised to generate Uaa-specific aaRS for use in eukaryotic cells and animals. The aaRSs have been found to be highly amenable for engineering substrate specificity toward Uaas that are structurally far deviating from the native amino acid, dramatically increasing the stereochemical diversity of Uaas accessible. Preparation of the Uaa in ester or dipeptide format markedly increases the bioavailability of Uaas to cells and animals. Nonsense-mediated mRNA decay (NMD), an mRNA surveillance mechanism of eukaryotic cells, degrades mRNA containing a premature stop codon. Inhibition of NMD increases Uaa incorporation efficiency in yeast and Caenorhabditis elegans. In bacteria, release factor one (RF1) competes with the orthogonal tRNA for the amber stop codon to terminate protein translation, leading to low Uaa incorporation efficiency. Contradictory to the paradigm that RF1 is essential, it is discovered that RF1 is actually nonessential in E. coli. Knockout of RF1 dramatically increases Uaa incorporation efficiency and enables Uaa incorporation at multiple sites, making it feasible to use Uaa for directed evolution. Using these strategies, the genetic code has been effectively expanded in yeast, mammalian cells, stem cells, worms, fruit flies, zebrafish, and mice. It is also intriguing to find out that the legitimate UAG codons terminating endogenous genes are not efficiently suppressed by the orthogonal tRNA/aaRS in E. coli. Moreover, E. coli responds to amber suppression pressure promptly using transposon insertion to inactivate the introduced orthogonal aaRS. Persistent amber suppression evading transposon inactivation leads to global proteomic changes with a notable up-regulation of a previously uncharacterized protein YdiI, for which an unexpected function of expelling plasmids is discovered. Genome integration of the orthogonal tRNA/aaRS in mice results in minor changes in RNA transcripts but no significant physiological impairment. Lastly, the RF1 knockout E. coli strains afford a previously unavailable model organism for studying otherwise intractable questions on code evolution in real time in the laboratory. We expect that genetically encoding Uaas in live systems will continue to unfold new questions and directions for studying biology in vivo, investigating the code itself, and reprograming genomes for synthetic biology.

Improved Photoinduced Fluorogenic Alkene-Tetrazole Reaction for Protein Labeling

The 1,3-dipolar cycloaddition reaction between an alkene and a tetrazole represents one elegant and rare example of fluorophore-forming bioorthogonal chemistry. This is an attractive reaction for imaging applications in live cells that requires less intensive washing steps and/or needs spatiotemporal resolutions. In the present work, as an effort to improve the fluorogenic property of the alkene-tetrazole reaction, an aromatic alkene (styrene) was investigated as the dipolarophile. Over 30-fold improvement in quantum yield of the reaction product was achieved in aqueous solution. According to our mechanistic studies, the observed improvement is likely due to an insufficient protonation of the styrene-tetrazole reaction product. This finding provides useful guidance to the future design of alkene-tetrazole reactions for biological studies. Fluorogenic protein labeling using the styrene-tetrazole reaction was demonstrated both in vitro and in vivo. This was realized by the genetic incorporation of an unnatural amino acid containing the styrene moiety. It is anticipated that the combination of styrene with different tetrazole derivatives can generally improve and broaden the application of alkene-tetrazole chemistry in real-time imaging in live cells.

Protein Crosslinking by Genetically Encoded Noncanonical Amino Acids with Reactive Aryl Carbamate Side Chains

The use of genetically encoded noncanonical amino acids (ncAAs) to construct crosslinks within or between proteins has emerged as a useful method to enhance protein stability, investigate protein-protein interactions, and improve the pharmacological properties of proteins. We report ncAAs with aryl carbamate side chains (PheK and FPheK) that can react with proximal nucleophilic residues to form intra- or intermolecular protein crosslinks. We evolved a pyrrolysyl-tRNA synthetase that incorporates site-specifically PheK and FPheK into proteins in both E. coli and mammalian cells. PheK and FPheK when incorporated into proteins showed good stability during protein expression and purification. FPheK reacted with adjacent Lys, Cys, and Tyr residues in thioredoxin in high yields. In addition, crosslinks could be formed between FPheK and Lys residue of two interacting proteins, including the heavy chain and light chain of an antibody Fab.

Expanding the genetic code of Mus musculus

Here we report the expansion of the genetic code of Mus musculus with various unnatural amino acids including N^ɛ-acetyl-lysine. Stable integration of transgenes encoding an engineered N^ɛ-acetyl-lysyl-tRNA synthetase (AcKRS)/tRNA^Pyl pair into the mouse genome enables site-specific incorporation of unnatural amino acids into a target protein in response to the amber codon. We demonstrate temporal and spatial control of protein acetylation in various organs of the transgenic mouse using a recombinant green fluorescent protein (GFPuv) as a model protein. This strategy will provide a powerful tool for systematic in vivo study of cellular proteins in the most commonly used mammalian model organism for human physiology and disease.

Genetically encoding new bioreactivity

The genetic code can be expanded to include unnatural amino acids (Uaas) by engineering orthogonal components involved in protein translation. To be compatible with live cells, side chains of Uaas have been limited to either chemically inert or bio-orthogonal (i.e., nonreactive toward biomolecules) functionalities. To introduce bioreactivity into live systems, the genetic code has recently been engineered to encode a new class of Uaas, the bioreactive Uaas. These Uaas, after being incorporated into proteins, specifically react with target natural amino acid residues via proximity-enabled bioreactivity, enabling the selective formation of new covalent linkages within and between proteins both in vitro and in live systems. The new covalent bonding ability has been harnessed within proteins to enhance photostability, increase thermostability, staple proteins recombinantly, and build optical nano-switches, and between proteins to pinpoint ligand-receptor interaction, target native receptors irreversibly, and generate covalent macromolecular inhibitors. These diverse bioreactivities, inaccessible to natural proteins, thus open doors to novel protein engineering and provide new avenues for biological studies, biotherapeutics and synthetic biology.

Genetic code expansion in the mouse brain

Site-specific incorporation of non-natural amino acids into proteins, via genetic code expansion with pyrrolysyl tRNA synthetase (PylRS) and tRNA(Pyl)CUA pairs (and their evolved derivatives) from Methanosarcina sp., forms the basis of powerful approaches to probe and control protein function in cells and invertebrate organisms. Here we demonstrate that adeno-associated viral delivery of these pairs enables efficient genetic code expansion in primary neuronal culture, organotypic brain slices and the brains of live mice.

At the Interface of Chemical and Biological Synthesis: An Expanded Genetic Code

The ability to site-specifically incorporate noncanonical amino acids (ncAAs) with novel structures into proteins in living cells affords a powerful tool to investigate and manipulate protein structure and function. More than 200 ncAAs with diverse biological, chemical, and physical properties have been genetically encoded in response to nonsense or frameshift codons in both prokaryotic and eukaryotic organisms with high fidelity and efficiency. In this review, recent advances in the technology and its application to problems in protein biochemistry, cellular biology, and medicine are highlighted.

Incorporation of Unnatural Amino Acids into Proteins Expressed in Mammalian Cells

The site-specific incorporation of unnatural amino acids (Uaas) via genetic code expansion provides a powerful method to introduce synthetic moieties into specific positions of a protein directly in the live cell. The technique, first developed in bacteria, is nowadays widely applicable in mammalian cells. In general, different Uaas are incorporated with different efficiency. By comparing the incorporation efficiency of several Uaas recently designed for bioorthogonal chemistry, we present here a facile dual-fluorescence assay to evaluate relative yields of Uaa incorporation. Several biological questions can be addressed using Uaas tools. In recent years, photo-cross-linking Uaas have been extensively applied to map ligand-binding sites on G protein-coupled receptors (GPCRs). We describe a simple and efficient two-plasmid system to incorporate a photoactivatable Uaa into a class B GPCR, and demonstrate cross-linking to its nonmodified natural ligand.