Expanding the Genetic Code to Study Protein-Protein Interactions

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.

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.

Deciphering functional roles of protein succinylation and glutarylation using genetic code expansion

Post-translational modifications (PTMs) dynamically regulate cellular processes. Lysine undergoes a range of acylations, including malonylation, succinylation (SucK) and glutarylation (GluK). These PTMs increase the size of the lysine side chain and reverse its charge from +1 to -1 under physiological conditions, probably impacting protein structure and function. To understand the functional roles of these PTMs, homogeneously modified proteins are required for biochemical studies. While the site-specific encoding of PTMs and their mimics via genetic code expansion has facilitated the characterization of the functional roles of many PTMs, negatively charged lysine acylations have defied this approach. Here we describe site-specific incorporation of SucK and GluK into proteins via temporarily masking their negative charge through thioester derivatives. We prepare succinylated and glutarylated bacterial and mammalian target proteins, including non-refoldable multidomain proteins. This allows us to study how succinylation and glutarylation impact enzymatic activity of metabolic enzymes and regulate protein-DNA and protein-protein interactions in biological processes from replication to ubiquitin signalling.

Friedel-Crafts reactions for biomolecular chemistry

Chemical tools and principles have become central to biological and medical research/applications by leveraging a range of classical organic chemistry reactions. Friedel-Crafts alkylation and acylation are arguably some of the most well-known and used synthetic methods for the preparation of small molecules but their use in biological and medical fields is relatively less frequent than the other reactions, possibly owing to the notion of their plausible incompatibility with biological systems. This review demonstrates advances in Friedel-Crafts alkylation and acylation reactions in a variety of biomolecular chemistry fields. With the discoveries and applications of numerous biomolecule-catalyzed or -assisted processes, these reactions have garnered considerable interest in biochemistry, enzymology, and biocatalysis. Despite the challenges of reactivity and selectivity of biomolecular reactions, the alkylation and acylation reactions demonstrated their utility for the construction and functionalization of all the four major biomolecules (i.e., nucleosides, carbohydrates/saccharides, lipids/fatty acids, and amino acids/peptides/proteins), and their diverse applications in biological, medical, and material fields are discussed. As the alkylation and acylation reactions are often fundamental educational components of organic chemistry courses, this review is intended for both experts and nonexperts by discussing their basic reaction patterns (with the depiction of each reaction mechanism in the ESI) and relevant real-world impacts in order to enrich chemical research and education. The significant growth of biomolecular Friedel-Crafts reactions described here is a testament to their broad importance and utility, and further development and investigations of the reactions will surely be the focus in the organic biomolecular chemistry fields.

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.

Insights into the dynamics of the Ca2+ release-activated Ca2+ channel pore-forming complex Orai1

An important calcium (Ca2+) entry pathway into the cell is the Ca2+ release-activated Ca2+ (CRAC) channel, which controls a series of downstream signaling events such as gene transcription, secretion and proliferation. It is composed of a Ca2+ sensor in the endoplasmic reticulum (ER), the stromal interaction molecule (STIM), and the Ca2+ ion channel Orai in the plasma membrane (PM). Their activation is initiated by receptor-ligand binding at the PM, which triggers a signaling cascade within the cell that ultimately causes store depletion. The decrease in ER-luminal Ca2+ is sensed by STIM1, which undergoes structural rearrangements that lead to coupling with Orai1 and its activation. In this review, we highlight the current understanding of the Orai1 pore opening mechanism. In this context, we also point out the questions that remain unanswered and how these can be addressed by the currently emerging genetic code expansion (GCE) technology. GCE enables the incorporation of non-canonical amino acids with novel properties, such as light-sensitivity, and has the potential to provide novel insights into the structure/function relationship of CRAC channels at a single amino acid level in the living cell.

Spatiotemporal and direct capturing global substrates of lysine-modifying enzymes in living cells

Protein-modifying enzymes regulate the dynamics of myriad post-translational modification (PTM) substrates. Precise characterization of enzyme-substrate associations is essential for the molecular basis of cellular function and phenotype. Methods for direct capturing global substrates of protein-modifying enzymes in living cells are with many challenges, and yet largely unexplored. Here, we report a strategy to directly capture substrates of lysine-modifying enzymes via PTM-acceptor residue crosslinking in living cells, enabling global profiling of substrates of PTM-enzymes and validation of PTM-sites in a straightforward manner. By integrating enzymatic PTM-mechanisms, and genetically encoding residue-selective photo-crosslinker into PTM-enzymes, our strategy expands the substrate profiles of both bacterial and mammalian lysine acylation enzymes, including bacterial lysine acylases PatZ, YiaC, LplA, TmcA, and YjaB, as well as mammalian acyltransferases GCN5 and Tip60, leading to discovery of distinct yet functionally important substrates and acylation sites. The concept of direct capturing substrates of PTM-enzymes via residue crosslinking may extend to the other types of amino acid residues beyond lysine, which has the potential to facilitate the investigation of diverse types of PTMs and substrate-enzyme interactive proteomics.

Affinity-Directed Site-Specific Protein Labeling and Its Application to Antibody-Drug Conjugates

Chemically modified proteins have diverse applications; however, conventional chemo-selective methods often yield heterogeneously labeled products. To address this limitation, site-specific protein labeling holds significant potential, driving extensive research in this area. Nevertheless, site-specific modification of native proteins remains challenging owing to the complexity of their functional groups. Therefore, a method for site-selective labeling of intact proteins is aimed to design. In this study, a novel approach to traceless affinity-directed intact protein labeling is established, which leverages small binding proteins and genetic code expansion technology. By applying this method, a site-specific antibody labeling with a drug, which leads to the production of highly effective antibody-drug conjugates specifically targeting breast cancer cell lines is achieved. This approach enables traceless conjugation of intact target proteins, which is a critical advantage in pharmaceutical applications. Furthermore, small helical binding proteins can be easily engineered for various target proteins, thereby expanding their potential applications in diverse fields. This innovative approach represents a significant advancement in site-specific modification of native proteins, including antibodies. It also bears immense potential for facilitating the development of therapeutic agents for various diseases.

Techniques for Developing Reliable Machine Learning Classifiers Applied to Understanding and Predicting Protein:Protein Interaction Hot Spots

With machine learning now transforming the sciences, successful prediction of biological structure or activity is mainly limited by the extent and quality of data available for training, the astute choice of features for prediction, and thorough assessment of the robustness of prediction on a variety of new cases. In this chapter, we address these issues while developing and sharing protocols to build a robust dataset and rigorously compare several predictive classifiers using the open-source Python machine learning library, scikit-learn. We show how to evaluate whether enough data has been used for training and whether the classifier has been overfit to training data. The most telling experiment is 500-fold repartitioning of the training and test sets, followed by prediction, which gives a good indication of whether a classifier performs consistently well on different datasets. An intuitive method is used to quantify which features are most important for correct prediction.The resulting well-trained classifier, hotspotter, can robustly predict the small subset of amino acid residues on the surface of a protein that are energetically most important for binding a protein partner: the interaction hot spots. Hotspotter has been trained and tested here on a curated dataset assembled from 1046 non-redundant alanine scanning mutation sites with experimentally measured change in binding free energy values from 97 different protein complexes; this dataset is available to download. The accessible surface area of the wild-type residue at a given site and its degree of evolutionary conservation proved the most important features to identify hot spots. A variant classifier was trained and validated for proteins where only the amino acid sequence is available, augmented by secondary structure assignment. This version of hotspotter requiring fewer features is almost as robust as the structure-based classifier. Application to the ACE2 (angiotensin converting enzyme 2) receptor, which mediates COVID-19 virus entry into human cells, identified the critical hot spot triad of ACE2 residues at the center of the small interface with the CoV-2 spike protein. Hotspotter results can be used to guide the strategic design of protein interfaces and ligands and also to identify likely interfacial residues for protein:protein docking.

Biosynthesis, Engineering, and Delivery of Selenoproteins

Selenocysteine (Sec) was discovered as the 21st genetically encoded amino acid. In nature, site-directed incorporation of Sec into proteins requires specialized biosynthesis and recoding machinery that evolved distinctly in bacteria compared to archaea and eukaryotes. Many organisms, including higher plants and most fungi, lack the Sec-decoding trait. We review the discovery of Sec and its role in redox enzymes that are essential to human health and important targets in disease. We highlight recent genetic code expansion efforts to engineer site-directed incorporation of Sec in bacteria and yeast. We also review methods to produce selenoproteins with 21 or more amino acids and approaches to delivering recombinant selenoproteins to mammalian cells as new applications for selenoproteins in synthetic biology.

Large-scale, in-cell photocrosslinking at single-residue resolution reveals the molecular basis for glucocorticoid receptor regulation by immunophilins

The Hsp90 co-chaperones FKBP51 and FKBP52 play key roles in steroid-hormone-receptor regulation, stress-related disorders, and sexual embryonic development. As a prominent target, glucocorticoid receptor (GR) signaling is repressed by FKBP51 and potentiated by FKBP52, but the underlying molecular mechanisms remain poorly understood. Here we present the architecture and functional annotation of FKBP51-, FKBP52-, and p23-containing Hsp90-apo-GR pre-activation complexes, trapped by systematic incorporation of photoreactive amino acids inside human cells. The identified crosslinking sites clustered in characteristic patterns, depended on Hsp90, and were disrupted by GR activation. GR binding to the FKBPFK1, but not the FKBPFK2, domain was modulated by FKBP ligands, explaining the lack of GR derepression by certain classes of FKBP ligands. Our findings show how FKBPs differentially interact with apo-GR, help to explain the differentiated pharmacology of FKBP51 ligands, and provide a structural basis for the development of improved FKBP ligands.

A genetically encodable and fluorogenic photo-crosslinker via photo-induced defluorination acyl fluoride exchange

We report a genetically encodable m-trifluoromethylaniline modified L-lysine (m-TFMAK) which defluorinates upon light activation and covalently conjugates to native residues via acyl fluoride exchange. The encoded m-TFMAK photo-crosslinks with temporal controllability, residue selectivity, and fluorogenic tracking features, making it suitable for identifying protein interactions in living systems.

Photocrosslinking-induced CRAC channel-like Orai1 activation independent of STIM1

Ca2+ release-activated Ca2+ (CRAC) channels, indispensable for the immune system and various other human body functions, consist of two transmembrane (TM) proteins, the Ca2+-sensor STIM1 in the ER membrane and the Ca2+ ion channel Orai1 in the plasma membrane. Here we employ genetic code expansion in mammalian cell lines to incorporate the photocrosslinking unnatural amino acids (UAA), p-benzoyl-L-phenylalanine (Bpa) and p-azido-L-phenylalanine (Azi), into the Orai1 TM domains at different sites. Characterization of the respective UAA-containing Orai1 mutants using Ca2+ imaging and electrophysiology reveal that exposure to UV light triggers a range of effects depending on the UAA and its site of incorporation. In particular, photoactivation at A137 using Bpa in Orai1 activates Ca2+ currents that best match the biophysical properties of CRAC channels and are capable of triggering downstream signaling pathways such as nuclear factor of activated T-cells (NFAT) translocation into the nucleus without the need for the physiological activator STIM1.

Discovery of lipid-mediated protein-protein interactions in living cells using metabolic labeling with photoactivatable clickable probes

Protein-protein interactions (PPIs) are essential and pervasive regulatory elements in biology. Despite the development of a range of techniques to probe PPIs in living systems, there is a dearth of approaches to capture interactions driven by specific post-translational modifications (PTMs). Myristoylation is a lipid PTM added to more than 200 human proteins, where it may regulate membrane localization, stability or activity. Here we report the design and synthesis of a panel of novel photocrosslinkable and clickable myristic acid analog probes, and their characterization as efficient substrates for human N-myristoyltransferases NMT1 and NMT2, both biochemically and through X-ray crystallography. We demonstrate metabolic incorporation of probes to label NMT substrates in cell culture and in situ intracellular photoactivation to form a covalent crosslink between modified proteins and their interactors, capturing a snapshot of interactions in the presence of the lipid PTM. Proteomic analyses revealed both known and multiple novel interactors of a series of myristoylated proteins, including ferroptosis suppressor protein 1 (FSP1) and spliceosome-associated RNA helicase DDX46. The concept exemplified by these probes offers an efficient approach for exploring the PTM-specific interactome without the requirement for genetic modification, which may prove broadly applicable to other PTMs.

Uncovering the Domain-Specific Interactome of the TAF1 Tandem Reader Using Site-Specific Azide-Acetyllysine Photochemistry

Combinatorial readout of histone post-translational modifications by tandem reader modules mediates crosstalk among different histone modifications. To identify the domain-specific interactome of the tandem reader, we engineered the dual bromodomain of TATA-binding protein-associated factor-1 (TAF1) to carry a photoactivatable unnatural amino acid, 4-azido-l-phenylalanine (AzF), via amber suppressor mutagenesis. Using computational approaches, we modeled the targeted residues of TAF1 with AzF to predict the cross-linking distance between the reactive arylazide and its interacting partner. We developed three photoactivatable TAF1 tandem-bromodomain analogues, viz., Y1403AzF in bromodomain 1 (BD1), W1526AzF in bromodomain 2 (BD2), and Y1403AzF/W1526AzF in both BD1 and BD2. Circular dichroism and a thermal shift assay were used to confirm the structural integrity of the engineered readers. Using the TAF1 tandem-bromodomain analogues, we characterized their histone ligand binding properties by isothermal titration calorimetry and photo-cross-linking experiments. We found that the dual bromodomain of TAF1 independently binds and cross-links to different acetylated histone ligands. We further used the engineered BD1 and BD2 analogues of the TAF1 tandem readers to identify their domain-specific interacting partners at the cellular level. Both BD1 and BD2 independently cross-link to a unique interactome, and importantly, the dual cross-linker carrying TAF1 analogue could capture both BD1- and BD2-specific interactomes. Our work concludes that BD1 and BD2 of the TAF1 tandem reader independently recognize their interacting partners to regulate downstream cellular functions.

Rational design, production and in vitro analysis of photoxenoproteins

In synthetic biology, the artificial control of proteins by light is of growing interest since it enables the spatio-temporal regulation of downstream molecular processes. This precise photocontrol can be established by the site-directed incorporation of photo-sensitive non-canonical amino acids (ncAAs) into proteins, which generates so-called photoxenoproteins. Photoxenoproteins can be engineered using ncAAs that facilitate the irreversible activation or reversible regulation of their activity upon irradiation. In this chapter, we provide a general outline of the engineering process based on the current methodological state-of-the-art to obtain artificial photocontrol in proteins using the ncAAs o-nitrobenzyl-O-tyrosine as example for photocaged ncAAs (irreversible), and phenylalanine-4'-azobenzene as example for photoswitchable ncAAs (reversible). We thereby focus on the initial design as well as the production and characterization of photoxenoproteins in vitro. Finally, we outline the analysis of photocontrol under steady-state and non-steady-state conditions using the allosteric enzyme complexes imidazole glycerol phosphate synthase and tryptophan synthase as examples.

Yeast Display Enables Identification of Covalent Single-Domain Antibodies against Botulinum Neurotoxin Light Chain A

While covalent drug discovery is reemerging as an important route to small-molecule therapeutic leads, strategies for the discovery and engineering of protein-based irreversible binding agents remain limited. Here, we describe the use of yeast display in combination with noncanonical amino acids (ncAAs) to identify irreversible variants of single-domain antibodies (sdAbs), also called VHHs and nanobodies, targeting botulinum neurotoxin light chain A (LC/A). Starting from a series of previously described, structurally characterized sdAbs, we evaluated the properties of antibodies substituted with reactive ncAAs capable of forming covalent bonds with nearby groups after UV irradiation (when using 4-azido-l-phenylalanine) or spontaneously (when using O-(2-bromoethyl)-l-tyrosine). Systematic evaluations in yeast display format of more than 40 ncAA-substituted variants revealed numerous clones that retain binding function while gaining either UV-mediated or spontaneous crosslinking capabilities. Solution-based analyses indicate that ncAA-substituted clones exhibit site-dependent target specificity and crosslinking capabilities uniquely conferred by ncAAs. Interestingly, not all ncAA substitution sites resulted in crosslinking events, and our data showed no apparent correlation between detected crosslinking levels and distances between sdAbs and LC/A residues. Our findings highlight the power of yeast display in combination with genetic code expansion in the discovery of binding agents that covalently engage their targets. This platform streamlines the discovery and characterization of antibodies with therapeutically relevant properties that cannot be accessed in the conventional genetic code.

Expanding the chemical repertoire of protein-based polymers for drug-delivery applications

Expanding the chemical repertoire of natural and artificial protein-based polymers (PBPs) can enable the production of sequence-defined, yet chemically diverse, biopolymers with customized or new properties that cannot be accessed in PBPs composed of only natural amino acids. Various approaches can enable the expansion of the chemical repertoire of PBPs, including chemical and enzymatic treatments or the incorporation of unnatural amino acids. These techniques are employed to install a wide variety of chemical groups-such as bio-orthogonally reactive, cross-linkable, post-translation modifications, and environmentally responsive groups-which, in turn, can facilitate the design of customized PBP-based drug-delivery systems with modified, fine-tuned, or entirely new properties and functions. Here, we detail the existing and emerging technologies for expanding the chemical repertoire of PBPs and review several chemical groups that either demonstrate or are anticipated to show potential in the design of PBP-based drug delivery systems. Finally, we provide our perspective on the remaining challenges and future directions in this field.

DFT-Guided Discovery of Ethynyl-Triazolyl-Phosphinates as Modular Electrophiles for Chemoselective Cysteine Bioconjugation and Profiling

We report the density functional theory (DFT) guided discovery of ethynyl-triazolyl-phosphinates (ETPs) as a new class of electrophilic warheads for cysteine selective bioconjugation. By using CuI -catalysed azide alkyne cycloaddition (CuAAC) in aqueous buffer, we were able to access a variety of functional electrophilic building blocks, including proteins, from diethynyl-phosphinate. ETP-reagents were used to obtain fluorescent peptide-conjugates for receptor labelling on live cells and a stable and a biologically active antibody-drug-conjugate. Moreover, we were able to incorporate ETP-electrophiles into an azide-containing ubiquitin under native conditions and demonstrate their potential in protein-protein conjugation. Finally, we showcase the excellent cysteine-selectivity of this new class of electrophile in mass spectrometry based, proteome-wide cysteine profiling, underscoring the applicability in homogeneous bioconjugation strategies to connect two complex biomolecules.

Hydrophobic cavity-directed azide-acetyllysine photochemistry for profiling non-histone interacting partners of bromodomain protein 1

Bromodomain containing protein 1 (BRD1) plays critical roles in chromatin acetylation, gene transcription, erythropoiesis, and brain development. BRD1 is also implicated in several human conditions and is a therapeutic target for cancer. Although, the bromodomain is known to bind acetylated histones, how the function of BRD1 is regulated via non-histone acetylation is unexplored. To identify the non-histone acetylome of BRD1, we develop an R585AzF variant carrying photo responsive 4-azido phenylalanine (AzF) via amber suppressor mutagenesis. We demonstrate biochemical integrity of the AzF-containing analogue and its ability to crosslink non-histone interacting partners present in human cells. Subsequent proteomic experiments led to the identification of the novel BRD1 interactome representing diverse signaling pathways. As a proof-of-concept demonstration, we validated acetylated PDIA1 protein as a bona fide binding partner of BRD1. Our work suggests that BRD1 interacts with additional acetyllysine motifs, beyond those characterized in histone proteins.

Chemical editing of proteoglycan architecture

Proteoglycans are heterogeneous macromolecular glycoconjugates that orchestrate many important cellular processes. While much attention has focused on the poly-sulfated glycosaminoglycan chains that decorate proteoglycans, other important elements of their architecture, such as core proteins and membrane localization, have garnered less emphasis. Hence, comprehensive structure-function relationships that consider the replete proteoglycan architecture as glycoconjugates are limited. Here we present an extensive approach to study proteoglycan structure and biology by fabricating defined semisynthetic modular proteoglycans that can be tailored for cell surface display. The expression of proteoglycan core proteins with unnatural amino acids permits bioorthogonal click chemistry with functionalized glycosaminoglycans for methodical dissection of the parameters required for optimal binding and function of various proteoglycan-binding proteins. We demonstrate that these sophisticated materials can recapitulate the functions of native proteoglycan ectodomains in mouse embryonic stem cell differentiation and cancer cell spreading while permitting the analysis of the contributing architectural elements toward function.

Tuning the Properties of Protein-Based Polymers Using High-Performance Orthogonal Translation Systems for the Incorporation of Aromatic Non-Canonical Amino Acids

The incorporation of non-canonical amino acids (ncAAs) using engineered aminoacyl-tRNA synthetases (aaRSs) has emerged as a powerful methodology to expand the chemical repertoire of proteins. However, the low efficiencies of typical aaRS variants limit the incorporation of ncAAs to only one or a few sites within a protein chain, hindering the design of protein-based polymers (PBPs) in which multi-site ncAA incorporation can be used to impart new properties and functions. Here, we determined the substrate specificities of 11 recently developed high-performance aaRS variants and identified those that enable an efficient multi-site incorporation of 15 different aromatic ncAAs. We used these aaRS variants to produce libraries of two temperature-responsive PBPs-elastin- and resilin-like polypeptides (ELPs and RLPs, respectively)-that bear multiple instances of each ncAA. We show that incorporating such aromatic ncAAs into the protein structure of ELPs and RLPs can affect their temperature responsiveness, secondary structure, and self-assembly propensity, yielding new and diverse families of ELPs and RLPs, each from a single DNA template. Finally, using a molecular model, we demonstrate that the temperature-responsive behavior of RLPs is strongly affected by both the hydrophobicity and the size of the unnatural aromatic side-chain. The ability to efficiently incorporate multiple instances of diverse ncAAs alongside the 20 natural amino acids can help to elucidate the effect of ncAA incorporation on these and many other PBPs, with the aim of designing additional precise and chemically diverse polymers with new or improved properties.

A guide to designing photocontrol in proteins: methods, strategies and applications

Light is essential for various biochemical processes in all domains of life. In its presence certain proteins inside a cell are excited, which either stimulates or inhibits subsequent cellular processes. The artificial photocontrol of specifically proteins is of growing interest for the investigation of scientific questions on the organismal, cellular and molecular level as well as for the development of medicinal drugs or biocatalytic tools. For the targeted design of photocontrol in proteins, three major methods have been developed over the last decades, which employ either chemical engineering of small-molecule photosensitive effectors (photopharmacology), incorporation of photoactive non-canonical amino acids by genetic code expansion (photoxenoprotein engineering), or fusion with photoreactive biological modules (hybrid protein optogenetics). This review compares the different methods as well as their strategies and current applications for the light-regulation of proteins and provides background information useful for the implementation of each technique.

Expanding the functionality of proteins with genetically encoded dibenzo[b,f][1,4,5]thiadiazepine: a photo-transducer for photo-click decoration

Genetic incorporation of novel noncanonical amino acids (ncAAs) that are specialized for the photo-click reaction allows the precisely orthogonal and site-specific functionalization of proteins in living cells under photo-control. However, the development of a r̲ing-strain i̲n situ l̲oadable d̲ipolarophile (RILD) as a genetically encodable reporter for photo-click bioconjugation with spatiotemporal controllability is quite rare. Herein, we report the design and synthesis of a photo-switchable d̲ib̲enzo[b,f][1,4,5]t̲hiad̲iazepine-based a̲lanine (DBTDA) ncAA, together with the directed evolution of a pyrrolysyl-tRNA synthetase/tRNACUA pair (PylRS/tRNACUA), to encode the DBTDA into recombinant proteins as a RILD in living E. coli cells. The fast-responsive photo-isomerization of the DBTDA residue can be utilized as a converter of photon energy into ring-strain energy to oscillate the conformational changes of the parent proteins. Due to the photo-activation of RILD, the photo-switching of the DBTDA residue on sfGFP and OmpC is capable of promoting the photo-click ligation with diarylsydnone (DASyd) derived probes with high efficiency and selectivity. We demonstrate that the genetic code expansion (GCE) with DBTDA benefits the studies on the distribution of decorated OmpC-DBTD on specific E. coli cells under a spatiotemporal resolved photo-stimulation. The GCE for encoding DBTDA enables further functional diversity of artificial proteins in living systems.

Substrate Profiling of Mitochondrial Caseinolytic Protease P via a Site-Specific Photocrosslinking Approach

Approaches for profiling protease substrates are critical for defining protease functions, but remain challenging tasks. We combine genetic code expansion, photocrosslinking and proteomics to identify substrates of the mitochondrial (mt) human caseinolytic protease P (hClpP). Site-specific incorporation of the diazirine-bearing amino acid DiazK into the inner proteolytic chamber of hClpP, followed by UV-irradiation of cells, allows to covalently trap substrate proteins of hClpP and to substantiate hClpP's major involvement in maintaining overall mt homeostasis. In addition to confirming many of the previously annotated hClpP substrates, our approach adds a diverse set of new proteins to the hClpP interactome. Importantly, our workflow allows identifying substrate dynamics upon application of external cues in an unbiased manner. Identification of unique hClpP-substrate proteins upon induction of mt oxidative stress, suggests that hClpP counteracts oxidative stress by processing of proteins that are involved in respiratory chain complex synthesis and maturation as well as in catabolic pathways.

Uncover New Reactivity of Genetically Encoded Alkyl Bromide Non-Canonical Amino Acids

Genetically encoded non-canonical amino acids (ncAAs) with electrophilic moieties are excellent tools to investigate protein-protein interactions (PPIs) both in vitro and in vivo. These ncAAs, including a series of alkyl bromide-based ncAAs, mainly target cysteine residues to form protein-protein cross-links. Although some reactivities towards lysine and tyrosine residues have been reported, a comprehensive understanding of their reactivity towards a broad range of nucleophilic amino acids is lacking. Here we used a recently developed OpenUaa search engine to perform an in-depth analysis of mass spec data generated for Thioredoxin and its direct binding proteins cross-linked with an alkyl bromide-based ncAA, BprY. The analysis showed that, besides cysteine residues, BprY also targeted a broad range of nucleophilic amino acids. We validated this broad reactivity of BprY with Affibody/Z protein complex. We then successfully applied BprY to map a binding interface between SUMO2 and SUMO-interacting motifs (SIMs). BprY was further applied to probe SUMO2 interaction partners. We identified 264 SUMO2 binders, including several validated SUMO2 binders and many new binders. Our data demonstrated that BprY can be effectively used to probe protein-protein interaction interfaces even without cysteine residues, which will greatly expand the power of BprY in studying PPIs.

Therapeutic peptides: current applications and future directions

Peptide drug development has made great progress in the last decade thanks to new production, modification, and analytic technologies. Peptides have been produced and modified using both chemical and biological methods, together with novel design and delivery strategies, which have helped to overcome the inherent drawbacks of peptides and have allowed the continued advancement of this field. A wide variety of natural and modified peptides have been obtained and studied, covering multiple therapeutic areas. This review summarizes the efforts and achievements in peptide drug discovery, production, and modification, and their current applications. We also discuss the value and challenges associated with future developments in therapeutic peptides.

Film-like organelles equip cells with multiple genetic codes

The simultaneous incorporation of distinct noncanonical amino acids into different proteins within eukaryotic cells remains challenging. This new study by Reinkemeier and Lemke demonstrates how 2D phase separation can be used to engineer spatially separated organelles. These film-like organelles translate proteins independently from each other and the canonical genetic code.

Genetic Code Expansion System for Tight Control of Gene Expression in Bombyx mori Cell Lines

Simple Summary

Bombyx mori is a lepidopteran insect with economic value. Its genetic background is clear, and genome sequence is relatively complete, but the function of many genes has not been determined. The genetic code expansion system has become an important means of gene function research. In this study, a genetic code expansion system suitable for B. mori cells was established. This system included a modified tRNA^Pyl/Pyrrolysyl-tRNA synthetase (PylRS) pair from Methanosarcina mazei, the reporter gene D[TAG]G formed by DsRed and EGFP through amber stop codon TAG connection and the unnatural amino acid H-Lys(Boc)-OH. In silkworm BmE and BmNs cell lines, the reporter gene expression was strictly controlled by H-Lys(Boc)-OH in the presence of both PylRS and tRNA^Pyl. The silkworm genetic code expansion system established here is another useful controllable gene expression system besides tetracycline induced expression system.

Abstract

Inducible gene expression systems are important tools for studying gene function and to control protein synthesis. With the completion of the detailed map of the silkworm (Bombyx mori) genome, the study of Bombyx mori has entered the post-genome era. While the functions of many genes have been described in detail, many coding genes remain unidentified. Except for the available tetracycline induction system, there is currently a dearth of other effective induction systems for B. mori. A genetic code expansion system can be used for protein labeling and to regulate gene expression. Here, we have established a genetic code expansion system for B. mori based on the well-researched tRNA^Pyl/PylRS pair from Methanosarcina mazei. We used H-Lys(Boc)-OH, which is a lysine derivative to efficiently and tightly control the expression of the reporter gene DsRed[TAG]EGFP (D[TAG]G), which encoded a H-Lys(Boc)-OH-bearing protein fused with DsRed and EGFP (here regarded as D[Boc]G) in B. mori cell lines BmE and BmNs. In D[TAG]G, the amber stop codon is recognized as the orthogonal tRNA^Pyl. Successful application of genetic code expansion system in silkworm cell lines will support the research into the function of silkworm genes and paves the way for the identification of new genes and protein markers in silkworm.

An efficient system for bioconjugation based on a widely applicable engineered O-glycosylation tag

Bioconjugates are an important class of therapeutic molecules. To date, O-glycan-based metabolic glycoengineering has had limited use in this field, due to the complexities of the endogenous O-glycosylation pathway and the lack of an O-glycosylation consensus sequence. Here, we describe the development of a versatile on-demand O-glycosylation system that uses a novel, widely applicable 5 amino acid O-glycosylation tag, and a metabolically engineered UDP-galactose-4-eperimase (GALE) knock-out cell line. Optimization of the primary sequence of the tag enables the production of Fc-based proteins with either single or multiple O-glycans with complexity fully controlled by media supplementation. We demonstrate how the uniformly labeled proteins containing exclusively N-azido-acetylgalactosamine are used for CLICK chemistry-based bioconjugation to generate site-specifically fluorochrome-labeled antibodies, dual-payload molecules, and bioactive Fc-peptides for applications in basic research and drug discovery. To our knowledge, this is the first description of generating a site-specific O-glycosylation system by combining an O-glycosylation tag and a metabolically engineered cell line.

A modular toolbox to generate complex polymeric ubiquitin architectures using orthogonal sortase enzymes

The post-translational modification of proteins with ubiquitin (Ub) and Ub-like modifiers (Ubls) represents one of the most important regulators in eukaryotic biology. Polymeric Ub/Ubl chains of distinct topologies control the activity, stability, interaction and localization of almost all cellular proteins and elicit a variety of biological outputs. Our ability to characterize the roles of distinct Ub/Ubl topologies and to identify enzymes and receptors that create, recognize and remove these modifications is however hampered by the difficulty to prepare them. Here we introduce a modular toolbox (Ubl-tools) that allows the stepwise assembly of Ub/Ubl chains in a flexible and user-defined manner facilitated by orthogonal sortase enzymes. We demonstrate the universality and applicability of Ubl-tools by generating distinctly linked Ub/Ubl hybrid chains, and investigate their role in DNA damage repair. Importantly, Ubl-tools guarantees straightforward access to target proteins, site-specifically modified with distinct homo- and heterotypic (including branched) Ub chains, providing a powerful approach for studying the functional impact of these complex modifications on cellular processes.

Engineering an acetyllysine reader with a photocrosslinking amino acid for interactome profiling

The site-specific installation of light-activable crosslinker unnatural amino acids offers a powerful approach to trap transient protein-protein interactions both in vitro and in vivo. Herein, we engineer a bromodomain to introduce 4-benzoyl-L-phenylalanine (BzF) using amber suppressor mutagenesis without compromising its ability to recognize the acetylated histone proteins. We demonstrate the high crosslinking efficiency of the engineered reader towards the interacting partners and its suitability for profiling the transient bromodomain interactome.

Fast and Selective Reaction of 2-Benzylacrylaldehyde with 1,2-Aminothiol for Stable N-Terminal Cysteine Modification and Peptide Cyclization

N-terminal cysteine (Cys)-specific reactions have been exploited for protein and peptide modifications. However, existing reactions for N-terminal Cys suffer from low reaction rate, unavoidable side reactions, or poor stability for reagents or products. Herein we report a fast, efficient, and selective conjugation between 2-benzylacrylaldehyde (BAA) and 1,2-aminothiol, which involves multistep reactions including aldimine condensation, Michael addition, and reduction of imine by NaBH3CN. This conjugation proceeds with a rate constant of ∼2700 M-1 s-1 under neutral condition at room temperature to produce a pair of seven-membered ring diastereoisomers, which are stable under neutral and acidic conditions. This method enables the selective modifications of the N-terminal Cys residue without interference from the internal Cys and lysine residues, providing a useful alternative to existing approaches for site-specific peptide or protein modifications and synthesis of cyclic peptides.

Precise optical control of gene expression in C. elegans using improved genetic code expansion and Cre recombinase

Synthetic strategies for optically controlling gene expression may enable the precise spatiotemporal control of genes in any combination of cells that cannot be targeted with specific promoters. We develop an improved genetic code expansion system in Caenorhabditis elegans and use it to create a photoactivatable Cre recombinase. We laser-activate Cre in single neurons within a bilaterally symmetric pair to selectively switch on expression of a loxP-controlled optogenetic channel in the targeted neuron. We use the system to dissect, in freely moving animals, the individual contributions of the mechanosensory neurons PLML/PLMR to the C. elegans touch response circuit, revealing distinct and synergistic roles for these neurons. We thus demonstrate how genetic code expansion and optical targeting can be combined to break the symmetry of neuron pairs and dissect behavioural outputs of individual neurons that cannot be genetically targeted.

Animal behaviour and movement emerges from the stimulation of nerve cells that are connected together like a circuit. Researchers use various tools to investigate these neural networks in model organisms such as roundworms, fruit flies and zebrafish. The trick is to activate some nerve cells, but not others, so as to isolate their specific role within the neural circuit.

One way to do this is to switch genes on or off in individual cells as a way to control their neuronal activity. This can be achieved by building a photocaged version of the enzyme Cre recombinase which is designed to target specific genes. The modified Cre recombinase contains an amino acid (the building blocks of proteins) that inactivates the enzyme. When the cell is illuminated with UV light, a part of the amino acid gets removed allowing Cre recombinase to turn on its target gene.

However, cells do not naturally produce these photocaged amino acids. To overcome this, researchers can use a technology called genetic code expansion which provides cells with the tools they need to build proteins containing these synthetic amino acids. Although this technique has been used in live animals, its application has been limited due to the small amount of proteins it produces. Davis et al. therefore set out to improve the efficiency of genetic code expansion so that it can be used to study single nerve cells in freely moving roundworms.

In the new system, named LaserTAC, individual cells are targeted with UV light that ‘uncages’ the Cre recombinase enzyme so it can switch on a gene for a protein that controls neuronal activity. Davis et al. used this approach to stimulate a pair of neurons sensitive to touch to see how this impacted the roundworm’s behaviour. This revealed that individual neurons within this pair contribute to the touch response in different ways. However, input from both neurons is required to produce a robust reaction.

These findings show that the LaserTAC system can be used to manipulate gene activity in single cells, such as neurons, using light. It allows researchers to precisely control in which cells and when a given gene is switched on or off. Also, with the improved efficiency of the genetic code expansion, this technology could be used to modify proteins other than Cre recombinase and be applied to other artificial amino acids that have been developed in recent years.

Identification of permissive amber suppression sites for efficient non-canonical amino acid incorporation in mammalian cells

The genetic code of mammalian cells can be expanded to allow the incorporation of non-canonical amino acids (ncAAs) by suppressing in-frame amber stop codons (UAG) with an orthogonal pyrrolysyl-tRNA synthetase (PylRS)/tRNAPylCUA (PylT) pair. However, the feasibility of this approach is substantially hampered by unpredictable variations in incorporation efficiencies at different stop codon positions within target proteins. Here, we apply a proteomics-based approach to quantify ncAA incorporation rates at hundreds of endogenous amber stop codons in mammalian cells. With these data, we compute iPASS (Identification of Permissive Amber Sites for Suppression; available at www.bultmannlab.eu/tools/iPASS), a linear regression model to predict relative ncAA incorporation efficiencies depending on the surrounding sequence context. To verify iPASS, we develop a dual-fluorescence reporter for high-throughput flow-cytometry analysis that reproducibly yields context-specific ncAA incorporation efficiencies. We show that nucleotides up- and downstream of UAG synergistically influence ncAA incorporation efficiency independent of cell line and ncAA identity. Additionally, we demonstrate iPASS-guided optimization of ncAA incorporation rates by synonymous exchange of codons flanking the amber stop codon. This combination of in silico analysis followed by validation in living mammalian cells substantially simplifies identification as well as adaptation of sites within a target protein to confer high ncAA incorporation rates.

A split protease-E. coli ClpXP system quantifies protein-protein interactions in Escherichia coli cells

Characterizing protein–protein interactions (PPIs) is an effective method to help explore protein function. Here, through integrating a newly identified split human Rhinovirus 3 C (HRV 3 C) protease, super-folder GFP (sfGFP), and ClpXP-SsrA protein degradation machinery, we developed a fluorescence-assisted single-cell methodology (split protease-E. coli ClpXP (SPEC)) to explore protein–protein interactions for both eukaryotic and prokaryotic species in E. coli cells. We firstly identified a highly efficient split HRV 3 C protease with high re-assembly ability and then incorporated it into the SPEC method. The SPEC method could convert the cellular protein-protein interaction to quantitative fluorescence signals through a split HRV 3 C protease-mediated proteolytic reaction with high efficiency and broad temperature adaptability. Using SPEC method, we explored the interactions among effectors of representative type I-E and I-F CRISPR/Cas complexes, which combining with subsequent studies of Cas3 mutations conferred further understanding of the functions and structures of CRISPR/Cas complexes.

Wang et al. developed a fluorescence-assisted single-cell methodology (split protease-E. coli ClpXP (SPEC)) to characterise protein-protein interactions for both eukaryotic and prokaryotic species in E. coli cells. Their method can quantify these interactions with high sensitivity, easy manipulation, and broad temperature adaptability