Ribosomal Synthesis of Backbone-Cyclic Peptides Compatible with In Vitro Display

Engineering Ribosomal Machinery for Noncanonical Amino Acid Incorporation

The introduction of noncanonical amino acids into proteins has enabled researchers to modify fundamental physicochemical and functional properties of proteins. While the alteration of the genetic code, via the introduction of orthogonal aminoacyl-tRNA synthetase:tRNA pairs, has driven many of these efforts, the various components involved in the process of translation are important for the development of new genetic codes. In this review, we will focus on recent advances in engineering ribosomal machinery for noncanonical amino acid incorporation and genetic code modification. The engineering of the ribosome itself will be considered, as well as the many factors that interact closely with the ribosome, including both tRNAs and accessory factors, such as the all-important EF-Tu. Given the success of genome re-engineering efforts, future paths for radical alterations of the genetic code will require more expansive alterations in the translation machinery.

Engineering tRNAs for the Ribosomal Translation of Non-proteinogenic Monomers

Ribosome-dependent protein biosynthesis is an essential cellular process mediated by transfer RNAs (tRNAs). Generally, ribosomally synthesized proteins are limited to the 22 proteinogenic amino acids (pAAs: 20 l-α-amino acids present in the standard genetic code, selenocysteine, and pyrrolysine). However, engineering tRNAs for the ribosomal incorporation of non-proteinogenic monomers (npMs) as building blocks has led to the creation of unique polypeptides with broad applications in cellular biology, material science, spectroscopy, and pharmaceuticals. Ribosomal polymerization of these engineered polypeptides presents a variety of challenges for biochemists, as translation efficiency and fidelity is often insufficient when employing npMs. In this Review, we will focus on the methodologies for engineering tRNAs to overcome these issues and explore recent advances both in vitro and in vivo. These efforts include increasing orthogonality, recruiting essential translation factors, and creation of expanded genetic codes. After our review on the biochemical optimizations of tRNAs, we provide examples of their use in genetic code manipulation, with a focus on the in vitro discovery of bioactive macrocyclic peptides containing npMs. Finally, an analysis of the current state of tRNA engineering is presented, along with existing challenges and future perspectives for the field.

An anticodon-sensing T-boxzyme generates the elongator nonproteinogenic aminoacyl-tRNA in situ of a custom-made translation system for incorporation

In the hypothetical RNA world, ribozymes could have acted as modern aminoacyl-tRNA synthetases (ARSs) to charge tRNAs, thus giving rise to the peptide synthesis along with the evolution of a primitive translation apparatus. We previously reported a T-boxzyme, Tx2.1, which selectively charges initiator tRNA with N-biotinyl-phenylalanine (BioPhe) in situ in a Flexible In-vitro Translation (FIT) system to produce BioPhe-initiating peptides. Here, we performed in vitro selection of elongation-capable T-boxzymes (elT-boxzymes), using para-azido-l-phenylalanine (PheAZ) as an acyl-donor. We implemented a new strategy to enrich elT-boxzyme-tRNA conjugates that self-aminoacylated on the 3'-terminus selectively. One of them, elT32, can charge PheAZ onto tRNA in trans in response to its cognate anticodon. Further evolution of elT32 resulted in elT49, with enhanced aminoacylation activity. We have demonstrated the translation of a PheAZ-containing peptide in an elT-boxzyme-integrated FIT system, revealing that elT-boxzymes are able to generate the PheAZ-tRNA in response to the cognate anticodon in situ of a custom-made translation system. This study, together with Tx2.1, illustrates a scenario where a series of ribozymes could have overseen aminoacylation and co-evolved with a primitive RNA-based translation system.

Cheminformatics-Guided Cell-Free Exploration of Peptide Natural Products

There have been significant advances in the flexibility and power of in vitro cell-free translation systems. The increasing ability to incorporate noncanonical amino acids and complement translation with recombinant enzymes has enabled cell-free production of peptide-based natural products (NPs) and NP-like molecules. We anticipate that many more such compounds and analogs might be accessed in this way. To assess the peptide NP space that is directly accessible to current cell-free technologies, we developed a peptide parsing algorithm that breaks down peptide NPs into building blocks based on ribosomal translation logic. Using the resultant data set, we broadly analyze the biophysical properties of these privileged compounds and perform a retrobiosynthetic analysis to predict which peptide NPs could be directly synthesized in augmented cell-free translation reactions. We then tested these predictions by preparing a library of highly modified peptide NPs. Two macrocyclases, PatG and PCY1, were used to effect the head-to-tail macrocyclization of candidate NPs. This retrobiosynthetic analysis identified a collection of high-priority building blocks that are enriched throughout peptide NPs, yet they had not previously been tested in cell-free translation. To expand the cell-free toolbox into this space, we established, optimized, and characterized the flexizyme-enabled ribosomal incorporation of piperazic acids. Overall, these results demonstrate the feasibility of cell-free translation for peptide NP total synthesis while expanding the limits of the technology. This work provides a novel computational tool for exploration of peptide NP chemical space, that could be expanded in the future to allow design of ribosomal biosynthetic pathways for NPs and NP-like molecules.

Engineered mRNA-ribosome fusions for facile biosynthesis of selenoproteins

Ribosomes are often used in synthetic biology as a tool to produce desired proteins with enhanced properties or entirely new functions. However, repurposing ribosomes for producing designer proteins is challenging due to the limited number of engineering solutions available to alter the natural activity of these enzymes. In this study, we advance ribosome engineering by describing a novel strategy based on functional fusions of ribosomal RNA (rRNA) with messenger RNA (mRNA). Specifically, we create an mRNA-ribosome fusion called RiboU, where the 16S rRNA is covalently attached to selenocysteine insertion sequence (SECIS), a regulatory RNA element found in mRNAs encoding selenoproteins. When SECIS sequences are present in natural mRNAs, they instruct ribosomes to decode UGA codons as selenocysteine (Sec, U) codons instead of interpreting them as stop codons. This enables ribosomes to insert Sec into the growing polypeptide chain at the appropriate site. Our work demonstrates that the SECIS sequence maintains its functionality even when inserted into the ribosome structure. As a result, the engineered ribosomes RiboU interpret UAG codons as Sec codons, allowing easy and site-specific insertion of Sec in a protein of interest with no further modification to the natural machinery of protein synthesis. To validate this approach, we use RiboU ribosomes to produce three functional target selenoproteins in Escherichia coli by site-specifically inserting Sec into the proteins' active sites. Overall, our work demonstrates the feasibility of creating functional mRNA-rRNA fusions as a strategy for ribosome engineering, providing a novel tool for producing Sec-containing proteins in live bacterial cells.

Validation of a New Methodology to Create Oral Drugs beyond the Rule of 5 for Intracellular Tough Targets

Establishing a technological platform for creating clinical compounds inhibiting intracellular protein-protein interactions (PPIs) can open the door to many valuable drugs. Although small molecules and antibodies are mainstream modalities, they are not suitable for a target protein that lacks a deep cavity for a small molecule to bind or a protein found in intracellular space out of an antibody's reach. One possible approach to access these targets is to utilize so-called middle-size cyclic peptides (defined here as those with a molecular weight of 1000-2000 g/mol). In this study, we validated a new methodology to create oral drugs beyond the rule of 5 for intracellular tough targets by elucidating structural features and physicochemical properties for drug-like cyclic peptides and developing library technologies to afford highly N-alkylated cyclic peptide hits. We discovered a KRAS inhibitory clinical compound (LUNA18) as the first example of our platform technology.

Aminobenzoic Acid Derivatives Obstruct Induced Fit in the Catalytic Center of the Ribosome

The Escherichia coli (E. coli) ribosome can incorporate a variety of non-l-α-amino acid monomers into polypeptide chains in vitro but with poor efficiency. Although these monomers span a diverse set of compounds, there exists no high-resolution structural information regarding their positioning within the catalytic center of the ribosome, the peptidyl transferase center (PTC). Thus, details regarding the mechanism of amide bond formation and the structural basis for differences and defects in incorporation efficiency remain unknown. Within a set of three aminobenzoic acid derivatives-3-aminopyridine-4-carboxylic acid (Apy), ortho-aminobenzoic acid (oABZ), and meta-aminobenzoic acid (mABZ)-the ribosome incorporates Apy into polypeptide chains with the highest efficiency, followed by oABZ and then mABZ, a trend that does not track with the nucleophilicity of the reactive amines. Here, we report high-resolution cryo-EM structures of the ribosome with each of these three aminobenzoic acid derivatives charged on tRNA bound in the aminoacyl-tRNA site (A-site). The structures reveal how the aromatic ring of each monomer sterically blocks the positioning of nucleotide U2506, thereby preventing rearrangement of nucleotide U2585 and the resulting induced fit in the PTC required for efficient amide bond formation. They also reveal disruptions to the bound water network that is believed to facilitate formation and breakdown of the tetrahedral intermediate. Together, the cryo-EM structures reported here provide a mechanistic rationale for differences in reactivity of aminobenzoic acid derivatives relative to l-α-amino acids and each other and identify stereochemical constraints on the size and geometry of non-monomers that can be accepted efficiently by wild-type ribosomes.

Atomistic simulations of the Escherichia coli ribosome provide selection criteria for translationally active substrates

As genetic code expansion advances beyond L-α-amino acids to backbone modifications and new polymerization chemistries, delineating what substrates the ribosome can accommodate remains a challenge. The Escherichia coli ribosome tolerates non-L-α-amino acids in vitro, but few structural insights that explain how are available, and the boundary conditions for efficient bond formation are so far unknown. Here we determine a high-resolution cryogenic electron microscopy structure of the E. coli ribosome containing α-amino acid monomers and use metadynamics simulations to define energy surface minima and understand incorporation efficiencies. Reactive monomers across diverse structural classes favour a conformational space where the aminoacyl-tRNA nucleophile is <4 Å from the peptidyl-tRNA carbonyl with a Bürgi-Dunitz angle of 76-115°. Monomers with free energy minima that fall outside this conformational space do not react efficiently. This insight should accelerate the in vivo and in vitro ribosomal synthesis of sequence-defined, non-peptide heterooligomers.

Expanding the substrate scope of pyrrolysyl-transfer RNA synthetase enzymes to include non-α-amino acids in vitro and in vivo

The absence of orthogonal aminoacyl-transfer RNA (tRNA) synthetases that accept non-L-α-amino acids is a primary bottleneck hindering the in vivo translation of sequence-defined hetero-oligomers and biomaterials. Here we report that pyrrolysyl-tRNA synthetase (PylRS) and certain PylRS variants accept α-hydroxy, α-thio and N-formyl-L-α-amino acids, as well as α-carboxy acid monomers that are precursors to polyketide natural products. These monomers are accommodated and accepted by the translation apparatus in vitro; those with reactive nucleophiles are incorporated into proteins in vivo. High-resolution structural analysis of the complex formed between one PylRS enzyme and a m-substituted 2-benzylmalonic acid derivative revealed an active site that discriminates prochiral carboxylates and accommodates the large size and distinct electrostatics of an α-carboxy substituent. This work emphasizes the potential of PylRS-derived enzymes for acylating tRNA with monomers whose α-substituent diverges substantially from the α-amine of proteinogenic amino acids. These enzymes or derivatives thereof could synergize with natural or evolved ribosomes and/or translation factors to generate diverse sequence-defined non-protein heteropolymers.

Using display technologies to identify macrocyclic peptide antibiotics

Antibiotic resistant bacterial infections are now a leading cause of global mortality. While drug resistance continues to spread, the clinical antibiotic pipeline has become bare. This discord has focused attention on developing new strategies for antimicrobial discovery. Natural macrocyclic peptide-based products have provided novel antibiotics and antibiotic scaffolds targeting several essential bacterial cell envelope processes, but discovery of such natural products remains a slow and inefficient process. Synthetic strategies employing peptide display technologies can quickly screen large libraries of macrocyclic sequences for specific target binding and general antibacterial potential providing alternative approaches for new antibiotic discovery. Here we review cell envelope processes that can be targeted with macrocyclic peptide therapeutics, outline important macrocyclic peptide display technologies, and discuss future strategies for both library design and screening.

Cell-free Biosynthesis of Peptidomimetics

A wide variety of peptidomimetics (peptide analogs) possessing innovative biological functions have been brought forth as therapeutic candidates through cell-free protein synthesis (CFPS) systems. A key feature of these peptidomimetic drugs is the use of non-canonical amino acid building blocks with diverse biochemical properties that expand functional diversity. Here, we summarize recent technologies leveraging CFPS platforms to expand the reach of peptidomimetics drugs. We also offer perspectives on engineering the translational machinery that may open new opportunities for expanding genetically encoded chemistry to transform drug discovery practice beyond traditional boundaries.

Programmable Synthesis of Biobased Materials Using Cell-Free Systems

Motivated by the intricate mechanisms underlying biomolecule syntheses in cells that chemistry is currently unable to mimic, researchers have harnessed biological systems for manufacturing novel materials. Cell-free systems (CFSs) utilizing the bioactivity of transcriptional and translational machineries in vitro are excellent tools that allow supplementation of exogenous materials for production of innovative materials beyond the capability of natural biological systems. Herein, recent studies that have advanced the ability to expand the scope of biobased materials using CFS are summarized and approaches enabling the production of high-value materials, prototyping of genetic parts and modules, and biofunctionalization are discussed. By extending the reach of chemical and enzymatic reactions complementary to cellular materials, CFSs provide new opportunities at the interface of materials science and synthetic biology.

Post-translational backbone-acyl shift yields natural product-like peptides bearing hydroxyhydrocarbon units

Hydroxyhydrocarbon (Hhc) moieties in the backbone of peptidic natural products can exert a substantial influence on the bioactivities of the products, making Hhc units an attractive class of building blocks for de novo peptides. However, despite advances in in vitro genetic code reprogramming, the ribosomal incorporation of Hhc units remains challenging. Here we report a method for in vitro ribosomal synthesis of natural-product-like peptides bearing Hhc units. A series of azide/hydroxy acids were designed as chemical precursors of Hhc units and incorporated into the nascent peptide chain by means of genetic code reprogramming. Post-translational reduction of the azide induced an O-to-N acyl shift to rearrange the peptide backbone. This method allows for one-pot ribosomal synthesis of designer macrocycles bearing various β-, γ- and δ-type Hhc units. We also report the synthesis of a statine-containing peptidomimetic inhibitor of β-secretase 1 as a showcase example.

Ribosome-mediated biosynthesis of pyridazinone oligomers in vitro

The ribosome is a macromolecular machine that catalyzes the sequence-defined polymerization of L-α-amino acids into polypeptides. The catalysis of peptide bond formation between amino acid substrates is based on entropy trapping, wherein the adjacency of transfer RNA (tRNA)-coupled acyl bonds in the P-site and the α-amino groups in the A-site aligns the substrates for coupling. The plasticity of this catalytic mechanism has been observed in both remnants of the evolution of the genetic code and modern efforts to reprogram the genetic code (e.g., ribosomal incorporation of non-canonical amino acids, ribosomal ester formation). However, the limits of ribosome-mediated polymerization are underexplored. Here, rather than peptide bonds, we demonstrate ribosome-mediated polymerization of pyridazinone bonds via a cyclocondensation reaction between activated γ-keto and α-hydrazino ester monomers. In addition, we demonstrate the ribosome-catalyzed synthesis of peptide-hybrid oligomers composed of multiple sequence-defined alternating pyridazinone linkages. Our results highlight the plasticity of the ribosome's ancient bond-formation mechanism, expand the range of non-canonical polymeric backbones that can be synthesized by the ribosome, and open the door to new applications in synthetic biology.

Mobile Translation Systems Generate Genomically Engineered Escherichia coli Cells with Improved Growth Phenotypes

Cellular translation is responsible for the synthesis of proteins, a highly diverse class of macromolecules that form the basis of biological function. In Escherichia coli, harnessing and engineering of the biomolecular components of translation, such as ribosomes, transfer RNAs (tRNAs), and aminoacyl-tRNA synthetases, has led to both biotechnology products and an expanded genetic code. However, the engineering potential of molecular translation is hampered by the limited capabilities for rapidly sampling the large genomic space necessary to evolve well-coordinated synthetic translation networks inside cells. To address this limitation, we developed a genome engineering method inspired by the action of mobile genetic elements termed mobilization. Mobilization utilizes the stochastic action of the recombinase flippase (FLP) to generate up to ∼400 million genomic insertions, deletions, or rearrangements at flippase recognition target sites per milliliter of culture per OD in living E. coli cells. As a model, we applied our approach to evolve faster-growing E. coli strains living exclusively off genomically expressed tethered ribosomes. In an iterative "pulse-passaging scheme," we generated genomic libraries of cells via induction of FLP recombinase (pulse) followed by passaging the population without induction of FLP to enrich the resulting population for cells with higher fitness. We observed large structural genomic diversity across these cells, with the fastest growing strains exhibiting a 71% increase in growth rate compared to the ancestral strain. We anticipate that both these strains and the mobilization method will be useful tools for synthetic biology efforts to engineer translation systems.

In Vitro Genetic Code Reprogramming for the Expansion of Usable Noncanonical Amino Acids

Genetic code reprogramming has enabled us to ribosomally incorporate various nonproteinogenic amino acids (npAAs) into peptides in vitro. The repertoire of usable npAAs has been expanded to include not only l-α-amino acids with noncanonical sidechains but also those with noncanonical backbones. Despite successful single incorporation of npAAs, multiple and consecutive incorporations often suffer from low efficiency or are even unsuccessful. To overcome this stumbling block, engineering approaches have been used to modify ribosomes, EF-Tu, and tRNAs. Here, we provide an overview of these in vitro methods that are aimed at optimal expansion of the npAA repertoire and their applications for the development of de novo bioactive peptides containing various npAAs.

Chemical insights into flexizyme-mediated tRNA acylation

A critical step in repurposing the cellular translation machinery for the synthesis of polymeric products is the acylation of transfer RNA (tRNA) with unnatural monomers. Toward this goal, flexizymes, ribozymes capable of aminoacylation, have emerged as a uniquely adept tool for charging tRNA with ever increasingly diverse substrates. In this review, we present a library of monomer substrates that have been tested for tRNA acylation with the flexizyme system. From this mile-high view, we provide insights for understanding the chemical factors that influence flexizyme-mediated tRNA acylation. We conclude that flexizymes are primitive esterification catalysts that display a modest binding affinity to the monomer's aromatic recognition element. Together, these robust, yet flexible, flexizyme systems provide researchers with unprecedented access for preparing unnatural acyl-tRNA and the opportunity to repurpose the translation machinery for the synthesis of novel biologically derived structures beyond native proteins and peptides.

Improving genomically recoded Escherichia coli to produce proteins containing non-canonical amino acids

A genomically recoded Escherichia coli strain that lacks all amber codons and release factor 1 (C321.∆A) enables efficient genetic encoding of chemically diverse non-canonical amino acids (ncAAs) into proteins. While C321.∆A has opened new opportunities in chemical and synthetic biology, this strain has not been optimized for protein production, limiting its utility in widespread industrial and academic applications. To address this limitation, the construction of a series of genomically recoded organisms that are optimized for cellular protein production is described. It is demonstrated that the functional deactivation of nucleases (e.g., rne, endA) and proteases (e.g., lon) increases production of wild-type superfolder green fluorescent protein (sfGFP) and sfGFP containing two ncAAs up to ≈5-fold. Additionally, a genomic IPTG-inducible T7 RNA polymerase (T7RNAP) cassette into these strains is introduced. Using an optimized platform, the ability to introduce two identical N6 -(propargyloxycarbonyl)-L -Lysine residues site specifically into sfGFP with a 17-fold improvement in production relative to the parent strain is demonstrated. The authors envision that their library of organisms will provide the community with multiple options for increased expression of proteins with new and diverse chemistries.

Cyclic peptide drugs approved in the last two decades (2001-2021)

In contrast to the major families of small molecules and antibodies, cyclic peptides, as a family of synthesizable macromolecules, have distinct biochemical and therapeutic properties for pharmaceutical applications. Cyclic peptide-based drugs have increasingly been developed in the past two decades, confirming the common perception that cyclic peptides have high binding affinities and low metabolic toxicity as antibodies, good stability and ease of manufacture as small molecules. Natural peptides were the major source of cyclic peptide drugs in the last century, and cyclic peptides derived from novel screening and cyclization strategies are the new source. In this review, we will discuss and summarize 18 cyclic peptides approved for clinical use in the past two decades to provide a better understanding of cyclic peptide development and to inspire new perspectives. The purpose of the present review is to promote efforts to resolve the challenges in the development of cyclic peptide drugs that are more effective.

Inner residues of macrothiolactone in autoinducer peptides-I/IV circumvents S-to-O acyl transfer to the upstream serine residue

Autoinducing peptides I and IV (AIP-I/IV) are naturally occurring cyclic thiodepsipeptides (CTPs) bearing a Ser–Thr–Cys–Asp/Tyr (STC[D/Y]) tetrapeptide motif, where the Cys thiol (^HSC) in the side-chain is linked to the Met C-terminal carboxylic acid (M_COOH) to form 5-residue macrothiolactones,^−SC(D/Y)FIM_CO−. We have recently reported that CTPs containing SX₁CX₂ motifs spontaneously undergo macrolactonization to yield cyclic depsipeptides (CDPs) by an unprecedented rapid S-to-O acyl transfer to the upstream Ser hydroxyl group. Interestingly, even though the STC[D/Y] motif in AIP-I/IV is a member of the SX₁CX₂ motif family, it maintains the CTP form. This suggests that AIP-I/IV have a structural or chemical motive for avoiding such an S-to-O acyl transfer, thus retaining the CTP form intact. Here we have used genetic code reprogramming to ribosomally synthesize various AIP-I analogs and studied what the determinant is to control the formation of CTP vs. CDP products. The study revealed that a Gly substitution of the inner Asp/Tyr or Met residues in the thiolactone drastically alters the resistance to the promotion of the S-to-O acyl transfer, giving the corresponding CDP product. This suggests that the steric hindrances originating from the α-substituted sidechain in these two amino acids in the AIP-I/IV thiolactone likely play a critical role in controlling the resistance against macrolactone rearrangement to the upstream Ser residue.

In AIP-I/IV, single Gly mutation at the thiolactone induces S-to-O acyl shift to yield a corresponding ring-expanded lactone form.

An Ultrapotent and Selective Cyclic Peptide Inhibitor of Human β-Factor XIIa in a Cyclotide Scaffold

Cyclotides are plant-derived peptides with complex structures shaped by their head-to-tail cyclic backbone and cystine knot core. These structural features underpin the native bioactivities of cyclotides, as well as their beneficial properties as pharmaceutical leads, including high proteolytic stability and cell permeability. However, their inherent structural complexity presents a challenge for cyclotide engineering, particularly for accessing libraries of sufficient chemical diversity to design potent and selective cyclotide variants. Here, we report a strategy using mRNA display enabling us to select potent cyclotide-based FXIIa inhibitors from a library comprising more than 10¹² members based on the cyclotide scaffold of Momordica cochinchinensis trypsin inhibitor-II (MCoTI-II). The most potent and selective inhibitor, cMCoFx1, has a pM inhibitory constant toward FXIIa with greater than three orders of magnitude selectivity over related serine proteases, realizing specific inhibition of the intrinsic coagulation pathway. The cocrystal structure of cMCoFx1 and FXIIa revealed interactions at several positions across the contact interface that conveyed high affinity binding, highlighting that such cyclotides are attractive cystine knot scaffolds for therapeutic development.

In Vitro Selection of Thioether-Closed Macrocyclic Peptide Ligands by Means of the RaPID System

The Random nonstandard Peptides Integrated Discovery (RaPID) system enables efficient screening of macrocyclic peptides with high affinities against target molecules. Random peptide libraries are prepared by in vitro translation using the Flexible In vitro Translation (FIT) system, which allows for incorporation of diverse nonproteinogenic amino acids into peptides by genetic code reprogramming. By introducing an N-chloroacetyl amino acid at the N-terminus and a Cys at the downstream, macrocyclic peptide libraries can be readily generated via posttranslational thioether formation. Here, we describe how to prepare a thioether-closed macrocyclic peptide library, and its application to the RaPID screening.

Directing evolution of novel ligands by mRNA display

mRNA display is a powerful biological display platform for the directed evolution of proteins and peptides. mRNA display libraries covalently link the displayed peptide or protein (phenotype) with the encoding genetic information (genotype) through the biochemical activity of the small molecule puromycin. Selection for peptide/protein function is followed by amplification of the linked genetic material and generation of a library enriched in functional sequences. Iterative selection cycles are then performed until the desired level of function is achieved, at which time the identity of candidate peptides can be obtained by sequencing the genetic material. The purpose of this review is to discuss the development of mRNA display technology since its inception in 1997 and to comprehensively review its use in the selection of novel peptides and proteins. We begin with an overview of the biochemical mechanism of mRNA display and its variants with a particular focus on its advantages and disadvantages relative to other biological display technologies. We then discuss the importance of scaffold choice in mRNA display selections and review the results of selection experiments with biological (e.g., fibronectin) and linear peptide library architectures. We then explore recent progress in the development of "drug-like" peptides by mRNA display through the post-translational covalent macrocyclization and incorporation of non-proteogenic functionalities. We conclude with an examination of enabling technologies that increase the speed of selection experiments, enhance the information obtained in post-selection sequence analysis, and facilitate high-throughput characterization of lead compounds. We hope to provide the reader with a comprehensive view of current state and future trajectory of mRNA display and its broad utility as a peptide and protein design tool.

Incorporation of backbone modifications in mRNA-displayable peptides

Here we comprehensively summarize the most recent efforts in our research team, aiming at installing N-methyl and azole backbones into peptides expressed in translation. The genetic code reprogramming using the Flexible In-vitro Translation system (FIT system) has proven to be the most reliable and versatile approach for ribosomally installing various exotic amino acids. However, it had been yet difficult in translating diverse kinds of multiple and consecutive sequences of N-methyl amino acids (^MeAAs). We have recently reported that a semi-rational fine tuning of ^MeAA-tRNA affinities for EF-Tu by altering tRNA T-stem sequence achieves efficient delivery of ^MeAA-tRNAs to the ribosome. Indeed, this approach has made it possible to express N-methyl-peptides containing multiple ^MeAAs with a remarkably high fidelity. Another interesting backbone modification in peptides is azole moieties often found in natural products, but they are explicitly installed by post-translational modifying enzymes. We have recently devised a method to bypass such enzymatic processes where a bromovinyl group-containing amino acid is incorporated into the peptide by genetic code reprogramming and then chemically converted to an azole group via an intramolecular heterocyclization reaction. These methods will grant more drug-like properties to peptides than ordinary peptides in terms of protease resistance and cell membrane permeability. Particularly when they can be integrated with in vitro mRNA display, such as the RaPID system, the discovery of de novo bioactive peptides can be realized.

Initiating protein synthesis with noncanonical monomers in vitro and in vivo

With few exceptions, ribosomal protein synthesis begins with methionine (or its derivative N-formyl-methionine) across all domains of life. The role of methionine as the initiating amino acid is dictated by the unique structure of its cognate tRNA known as tRNA^fMet. By mis-acylating tRNA^fMet, we and others have shown that protein synthesis can be initiated with a variety of canonical and noncanonical amino acids both in vitro and in vivo. Furthermore, because the α-amine of the initiating amino acid is not required for peptide bond formation, translation can be initiated with a variety of structurally disparate carboxylic acids that bear little resemblance to traditional α-amino acids. Herein, we provide a detailed protocol to initiate in vitro protein synthesis with substituted benzoic acid and 1,3-dicarbonyl compounds. These moieties are introduced at the N-terminus of peptides by mis-acylated tRNA^fMet, prepared by flexizyme-catalyzed tRNA acylation. In addition, we describe a protocol to initiate in vivo protein synthesis with aromatic noncanonical amino acids (ncAAs). This method relies on an engineered chimeric initiator tRNA that is acylated with ncAAs by an orthogonal aminoacyl-tRNA synthetase. Together, these systems are useful platforms for producing N-terminally modified proteins and for engineering the protein synthesis machinery of Escherichia coli to accept additional nonproteinogenic carboxylic acid monomers.

Engineering molecular translation systems

Molecular translation systems provide a genetically encoded framework for protein synthesis, which is essential for all life. Engineering these systems to incorporate non-canonical amino acids (ncAAs) into peptides and proteins has opened many exciting opportunities in chemical and synthetic biology. Here, we review recent advances that are transforming our ability to engineer molecular translation systems. In cell-based systems, new processes to synthesize recoded genomes, tether ribosomal subunits, and engineer orthogonality with high-throughput workflows have emerged. In cell-free systems, adoption of flexizyme technology and cell-free ribosome synthesis and evolution platforms are expanding the limits of chemistry at the ribosome's RNA-based active site. Looking forward, innovations will deepen understanding of molecular translation and provide a path to polymers with previously unimaginable structures and functions.

Approaches for peptide and protein cyclisation

The cyclisation of polypeptides can play a crucial role in exerting biological functions, maintaining stability under harsh conditions and conferring proteolytic resistance, as demonstrated both in nature and in the laboratory. To date, various approaches have been reported for polypeptide cyclisation. These approaches range from the direct linkage of N- and C- termini to the connection of amino acid side chains, which can be applied both in reaction vessels and in living systems. In this review, we categorise the cyclisation approaches into chemical methods (e.g. direct backbone cyclisation, native chemical ligation, aldehyde-based ligations, bioorthogonal reactions, disulphide formation), enzymatic methods (e.g. subtiligase variants, sortases, asparaginyl endopeptidases, transglutaminases, non-ribosomal peptide synthetases) and protein tags (e.g. inteins, engineered protein domains for isopeptide bond formation). The features of each approach and the considerations for selecting an appropriate method of cyclisation are discussed.

One-Pot In Vitro Ribosomal Synthesis of Macrocyclic Depsipeptides

Here, we report a method for the one-pot ribosomal synthesis of macrocyclic depsipeptides. This method is based on a Ser-Pro-Cys-Gly (SPCG) motif discovered by in vitro selection of peptides for the function of self-acylation in the presence of a thioester acyl donor, which forms an O-acyl isopeptide bond via intramolecular S-to-O acyl transfer. Ribosomal synthesis of linear peptides containing the SPCG motif and a backbone "acyl donor" thioester at a downstream position results in spontaneous conversion to the corresponding cyclic depsipeptides (CDPs) in a nearly independent manner of ring size and sequence context. Mutational analysis of the SPCG motif revealed that the P and G residues are dispensable to some extent, but the arrangement of residues in SXCX is crucial for efficient acyl transfer, e.g., CPSG is much less efficient. Finally, one-pot ribosomal synthesis of macrocyclic depsipeptides with various ring sizes and sequences has been demonstrated. This synthetic method can facilitate the ribosomal construction of highly diverse CDP libraries for the discovery of de novo bioactive CDPs.

Discovery of De Novo Macrocyclic Peptides by Messenger RNA Display

Macrocyclic peptides are a promising class of compounds that can often engage challenging therapeutic targets. Display technologies, such as mRNA display, allow for the efficient discovery of macrocyclic peptides. This article reviews the current approaches for generating macrocyclic peptide libraries using mRNA display and highlights some recent examples of ribosomal incorporation of nonproteinogenic amino acids into macrocyclic peptides.

Posttranslational chemical installation of azoles into translated peptides

Azoles are five-membered heterocycles often found in the backbones of peptidic natural products and synthetic peptidomimetics. Here, we report a method of ribosomal synthesis of azole-containing peptides involving specific ribosomal incorporation of a bromovinylglycine derivative into the nascent peptide chain and its chemoselective conversion to a unique azole structure. The chemoselective conversion was achieved by posttranslational dehydrobromination of the bromovinyl group and isomerization in aqueous media under fairly mild conditions. This method enables us to install exotic azole groups, oxazole and thiazole, at designated positions in the peptide chain with both linear and macrocyclic scaffolds and thereby expand the repertoire of building blocks in the mRNA-templated synthesis of designer peptides.

Azoles are five-membered heterocycles found in peptidic natural products and synthetic peptiodomimetics. Here the authors demonstrate a posttranslational chemical modification method for in vitro ribosomal synthesis of peptides with exotic azole groups at specific positions.

Expanding the Chemical Diversity of Genetically Encoded Libraries

The power of ribosomes has increasingly been harnessed for the synthesis and selection of molecular libraries. Technologies, such as phage display, yeast display, and mRNA display, effectively couple genotype to phenotype for the molecular evolution of high affinity epitopes for many therapeutic targets. Genetic code expansion is central to the success of these technologies, allowing researchers to surpass the intrinsic capabilities of the ribosome and access new, genetically encoded materials for these selections. Here, we review techniques for the chemical expansion of genetically encoded libraries, their abilities and limits, and opportunities for further development. Importantly, we also discuss methods and metrics used to assess the efficiency of modification and library diversity with these new techniques.

Cell-Free Approach for Non-canonical Amino Acids Incorporation Into Polypeptides

Synthetic biology holds promise to revolutionize the life sciences and biomedicine via expansion of macromolecular diversity outside the natural chemical space. Use of non-canonical amino acids (ncAAs) via codon reassignment has found diverse applications in protein structure and interaction analysis, introduction of post-translational modifications, production of constrained peptides, antibody-drug conjugates, and novel enzymes. However, simultaneously encoding multiple ncAAs in vivo requires complex engineering and is sometimes restricted by the cell's poor uptake of ncAAs. In contrast the open nature of cell-free protein synthesis systems offers much greater freedom for manipulation and repurposing of the biosynthetic machinery by controlling the level and identity of translational components and reagents, and allows simultaneous incorporation of multiple ncAAs with non-canonical side chains and even backbones (N-methyl, D-, β-amino acids, α-hydroxy acids etc.). This review focuses on the two most used Escherichia coli-based cell-free protein synthesis systems; cell extract- and PURE-based systems. The former is a biological mixture with >500 proteins, while the latter consists of 38 individually purified biomolecules. We delineate compositions of these two systems and discuss their respective advantages and applications. Also, we dissect the translational components required for ncAA incorporation and compile lists of ncAAs that can be incorporated into polypeptides via different acylation approaches. We highlight the recent progress in using unnatural nucleobase pairs to increase the repertoire of orthogonal codons, as well as using tRNA-specific ribozymes for in situ acylation. We summarize advances in engineering of translational machinery such as tRNAs, aminoacyl-tRNA synthetases, elongation factors, and ribosomes to achieve efficient incorporation of structurally challenging ncAAs. We note that, many engineered components of biosynthetic machinery are developed for the use in vivo but are equally applicable to the in vitro systems. These are included in the review to provide a comprehensive overview for ncAA incorporation and offer new insights for the future development in cell-free systems. Finally, we highlight the exciting progress in the genomic engineering, resulting in E. coli strains free of amber and some redundant sense codons. These strains can be used for preparation of cell extracts offering multiple reassignment options.

A Macrocyclic Peptide Library with a Structurally Constrained Cyclopropane-containing Building Block Leads to Thiol-independent Inhibitors of Phosphoglycerate Mutase

Here we report the construction of an mRNA-encoded library of thioether-closed macrocyclic peptides by using an N-chloroacetyl-cyclopropane-containing exotic initiator whose structure is more constrained than the ordinary N-chloroacetyl-α-amino acid initiators. The use of such an initiator has led to a macrocycle library with significantly suppressed population of lariat-shaped species compared with the conventional libraries. We previously used a conventional library and identified a small lariat thioether-macrocycle with a tail peptide with a C-terminal free Cys whose sidechain plays an essential role in potent inhibitory activity against a parasitic model enzyme, phosphoglycerate mutase. On the other hand, the cyclopropane-containing macrocycle library has yielded a larger thioether-macrocycle lacking a free Cys residue, which exhibits potent inhibitory activity to the same enzyme with a different mode of action. This result indicates that such a cyclopropane-containing macrocycle library would allow us to access mechanistically distinct macrocycles.

Methodologies for Backbone Macrocyclic Peptide Synthesis Compatible With Screening Technologies

Backbone macrocyclic structures are often found in diverse bioactive peptides and contribute to greater conformational rigidity, peptidase resistance, and potential membrane permeability compared to their linear counterparts. Therefore, such peptide scaffolds are an attractive platform for drug-discovery endeavors. Recent advances in synthetic methods for backbone macrocyclic peptides have enabled the discovery of novel peptide drug candidates against diverse targets. Here, we overview recent technical advancements in the synthetic methods including 1) enzymatic synthesis, 2) chemical synthesis, 3) split-intein circular ligation of peptides and proteins (SICLOPPS), and 4) in vitro translation system combined with genetic code reprogramming. We also discuss screening methodologies compatible with those synthetic methodologies, such as one-beads one-compound (OBOC) screening compatible with the synthetic method 2, cell-based assay compatible with 3, limiting-dilution PCR and mRNA display compatible with 4.

Hijacking Translation Initiation for Synthetic Biology

Genetic code expansion (GCE) has revolutionized the field of protein chemistry. Over the past several decades more than 150 different noncanonical amino acids (ncAAs) have been co-translationally installed into proteins within various host organisms. The vast majority of these ncAAs have been incorporated between the start and stop codons within an open reading frame. This requires that the ncAA be able to form a peptide bond at the α-amine, limiting the types of molecules that can be genetically encoded. In contrast, the α-amine of the initiating amino acid is not required for peptide bond formation. Therefore, including the initiator position in GCE allows for co-translational insertion of more diverse molecules that are modified, or completely lacking an α-amine. This review explores various methods which have been used to initiate protein synthesis with diverse molecules both in vitro and in vivo.

Initiating ribosomal peptide synthesis with exotic building blocks

Ribosomal peptide synthesis begins almost exclusively with the amino acid methionine, across all domains of life. The ubiquity of methionine initiation raises the question; to what extent could polypeptide synthesis be realized with other amino acids, proteinogenic or otherwise? This highlight describes the breadth of building blocks now known to be accepted by the ribosome initiation machinery, from subtle methionine analogues to large exotic non-proteinogenic structures. We outline the key methodological developments that have enabled these discoveries, including the exploitation of methionyl-tRNA synthetase promiscuity, synthetase and tRNA engineering, and the utilization of artificial tRNA-loading ribozymes, flexizymes. Using these methods, the number and diversity of validated initiation building blocks is rapidly expanding permitting the use of the ribosome to synthesize ever more artificial polymers in search of new functional molecules.

Ribosomal Elongation of Cyclic γ-Amino Acids using a Reprogrammed Genetic Code

Because γ-amino acids generally undergo rapid self-cyclization upon esterification on the carboxyl group, for example, γ-aminoacyl-tRNA, there are no reports of the ribosomal elongation of γ-amino acids to the best of our knowledge. To avoid such self-cyclization, we utilized cyclic γ-amino acids and demonstrated their elongation into a peptide chain. Although the incorporation of the cyclic γ-amino acids is intrinsically slow, we here show that the combination of elongation factor P and engineered tRNAs improves cyclic γ-amino acid incorporation efficiency. Via this method, thioether-macrocyclic peptides containing not only cyclic γ-amino acids but also d-α-, N-methyl-α-, and cyclic β-amino acids were expressed under the reprogrammed genetic code. Ribosomally synthesized macrocyclic peptide libraries containing cyclic γ-amino acids should be applicable to in vitro screening methodologies such as mRNA display for discovering novel peptide drugs.

In Vitro Selection of Peptides and Proteins-Advantages of mRNA Display

mRNA display is a robust in vitro selection technique that allows the selection of peptides and proteins with desired functions from libraries of trillions of variants. mRNA display relies upon a covalent linkage between a protein and its encoding mRNA molecule; the power of the technique stems from the stability of this link, and the large degree of control over experimental conditions afforded to the researcher. This article describes the major advantages that make mRNA display the method of choice among comparable in vivo and in vitro methods, including cell-surface display, phage display, and ribosomal display. We also describe innovative techniques that harness mRNA display for directed evolution, protein engineering, and drug discovery.

Methods for generating and screening libraries of genetically encoded cyclic peptides in drug discovery

Drug discovery has traditionally focused on using libraries of small molecules to identify therapeutic drugs, but new modalities, especially libraries of genetically encoded cyclic peptides, are increasingly used for this purpose. Several technologies now exist for the production of libraries of cyclic peptides, including phage display, mRNA display and split-intein circular ligation of peptides and proteins. These different approaches are each compatible with particular methods of screening libraries, such as functional or affinity-based screening, and screening in vitro or in cells. These techniques allow the rapid preparation of libraries of hundreds of millions of molecules without the need for chemical synthesis, and have therefore lowered the entry barrier to generating and screening for inhibitors of a given target. This ease of use combined with the inherent advantages of the cyclic-peptide scaffold has yielded inhibitors of targets that have proved difficult to drug with small molecules. Multiple reports demonstrate that cyclic peptides act as privileged scaffolds in drug discovery, particularly against 'undruggable' targets such as protein-protein interactions. Although substantial challenges remain in the clinical translation of hits from screens of cyclic-peptide libraries, progress continues to be made in this area, with an increasing number of cyclic peptides entering clinical trials. Here, we detail the various platforms for producing and screening libraries of genetically encoded cyclic peptides and discuss and evaluate the advantages and disadvantages of each approach when deployed for drug discovery.

An overview of lantibiotic biosynthetic machinery promiscuity and its impact on antimicrobial discovery

Introduction: The continued emergence of drug resistant bacteria within the nosocomial and community environment recalcitrant to conventional antimicrobial therapies has enforced the requirement for novel therapeutics. This has led to a renewed interest in peptide antimicrobials, including ribosomally synthesized peptides termed lantibiotics. Lantibiotics represent a novel class of agents that many studies have highlighted as effective against a range of pathogenic bacteria.Areas covered: In this review, the modular nature of lantibiotic synthesis is discussed and how this can be exploited not only to improve known lantibiotics but also for the creation of new to nature lantibiotics exhibiting improved pharmacological properties, antimicrobial activity and ability to bypass bacterial resistance mechanisms.Expert opinion: The use of combinatorial biosynthetic systems to combine different modules or ring structures of known lantibiotics have also been utilized to create new to nature lantibiotics. To fully exploit the available information and its application to lantibiotic engineering, additional structure activity relationship (SAR) analysis is required to fully understand the impact of certain post-translational modifications and the impact they have upon the activity, stability and pharmacological properties.

The Road Ahead for the Development of Macrocyclic Peptide Ligands

Macrocyclic peptides make up an emerging class of candidate therapeutics and chemical probes, with properties that make them potentially applicable to a wide range of targets that are intractable using current pharmacological agents. Additionally, a number of biochemical screening strategies have been developed, particularly over the past decade, that allow for the massively parallel screening of cyclic peptide libraries of up to 1 trillion compounds or more, leading to the isolation of molecules with exceptional target affinity, selectivity, and bioactivity. Clinical development of compounds derived from such screens is already underway, but the nature of these molecules means that such development is likely to follow pathways different from those of traditional small molecule drugs or well-established biologics such as monoclonal antibodies. In addition, recent work has shown that the biochemical techniques used to identify macrocyclic peptides can also be used to rapidly characterize and optimize them. These findings are likely to facilitate the development of these compounds as chemical probes and as therapeutics for areas of unmet medical need.

Feasability of Introducing a Thioether Ring in Vasopressin by nisBTC Co-expression in Lactococcus lactis

Introducing one or more intramolecular thioether bridges in a peptide provides a promising approach to create more stable molecules with improved pharmacodynamic properties and especially to protect peptides against proteolytic degradation. Lanthipeptides are compounds that naturally possess thioether bonds in their structure. The model lanthipeptide, nisin, is produced by Lactococcus lactis as a core peptide fused to a leader peptide. The modification machinery responsible for nisin production, including the Ser/Thr-dehydratase NisB and the cyclase NisC, can be applied for introducing a thioether bridge into peptides fused to the nisin leader peptide, e.g., to replace a disulfide bond. Vasopressin plays a key role in water homeostasis in the human body and helps to constrict blood vessels. There are two cysteine residues in the structure of wild type vasopressin, which form a disulfide bridge in the mature peptide. Here, we show it is possible to direct the biosynthesis of vasopressin variants in such a way that the disulfide bridge is replaced by a thioether bridge using the nisin modification machinery NisBTC, albeit at low efficiency. Vasopressin mutants were fused either to the nisin leader peptide directly (Type A), after the first three rings of nisin (Type B/C), or after full nisin (Type D). The type B strategy was optimal for expression. LC-MS/MS data verified the formation of a thioether bridge, which provides proof of principle for this modification in vasopressin. This is a first step prior to the necessary increase of the production yield and further purification of these peptides to finally test their biological activity in tissue and animal models.