Tertiary amide bond formation by an engineered asparaginyl ligase

Transpeptidases are powerful tools for site-specific protein modification, enabling the production of tailored biologics to investigate protein function and aiding the development of next-generation therapeutics and diagnostics. Although protein labelling at the N- or C-terminus is readily accomplished using a range of established transpeptidases, these reactions are generally limited to forming products that are linked by a standard (secondary) amide bond. Here we show that, unlike other widely used transpeptidases, an engineered asparaginyl ligase is able to efficiently synthesise tertiary amide bonds by accepting diverse secondary amine nucleophiles. These reactions proceed efficiently under mild conditions (near-neutral pH) and allow the optimal recognition elements for asparaginyl ligases (P1 Asn and P2'' Leu) to be preserved. Certain products, particularly proline-containing products, were found to be protected from recognition by the enzyme, allowing for straightforward sequential labelling of proteins. Additionally, incorporation of 4-azidoproline enables one-pot dual labelling directly at the ligation junction. These capabilities further expand the chemical diversity of asparaginyl ligase-catalysed reactions and provide an alternative approach for straightforward, successive modification of protein substrates.

Sortase-Catalyzed Protein Domain Inversion

Topological transformations and permutations of proteins have attracted significant interest as strategies to generate new protein functionalities or stability. These efforts have mainly been inspired by naturally occurring post-translational modifications, such as head-to-tail cyclization, circular permutation, or lasso-like entanglement. Such approaches can be realized experimentally via genetic encoding, in the case of circular permutation, or via enzymatic processing, in the case of cyclization. Notably, these previously described strategies leave the polypeptide backbone orientation unaltered. Here we describe an unnatural protein permutation, the protein domain inversion, whereby a C-terminal portion of a protein is enzymatically inverted from the canonical N-to-C to a C-to-C configuration with respect to the N-terminal part of the protein. The closest conceptually analogous biological process is perhaps the inversion of DNA segments as catalyzed by recombinases. We achieve these inversions using an engineered sortase A, a widely used transpeptidase. Our reactions proceed efficiently under mild conditions at 4-25 °C and are compatible with entirely heterologously-produced protein substrates.

Establishing a synthetic orthogonal replication system enables accelerated evolution in E. coli

The evolution of new function in living organisms is slow and fundamentally limited by their critical mutation rate. Here, we established a stable orthogonal replication system in Escherichia coli. The orthogonal replicon can carry diverse cargos of at least 16.5 kilobases and is not copied by host polymerases but is selectively copied by an orthogonal DNA polymerase (O-DNAP), which does not copy the genome. We designed mutant O-DNAPs that selectively increase the mutation rate of the orthogonal replicon by two to four orders of magnitude. We demonstrate the utility of our system for accelerated continuous evolution by evolving a 150-fold increase in resistance to tigecycline in 12 days. And, starting from a GFP variant, we evolved a 1000-fold increase in cellular fluorescence in 5 days.

Continuous synthesis of E. coli genome sections and Mb-scale human DNA assembly

Whole-genome synthesis provides a powerful approach for understanding and expanding organism function1-3. To build large genomes rapidly, scalably and in parallel, we need (1) methods for assembling megabases of DNA from shorter precursors and (2) strategies for rapidly and scalably replacing the genomic DNA of organisms with synthetic DNA. Here we develop bacterial artificial chromosome (BAC) stepwise insertion synthesis (BASIS)-a method for megabase-scale assembly of DNA in Escherichia coli episomes. We used BASIS to assemble 1.1 Mb of human DNA containing numerous exons, introns, repetitive sequences, G-quadruplexes, and long and short interspersed nuclear elements (LINEs and SINEs). BASIS provides a powerful platform for building synthetic genomes for diverse organisms. We also developed continuous genome synthesis (CGS)-a method for continuously replacing sequential 100 kb stretches of the E. coli genome with synthetic DNA; CGS minimizes crossovers1,4 between the synthetic DNA and the genome such that the output for each 100 kb replacement provides, without sequencing, the input for the next 100 kb replacement. Using CGS, we synthesized a 0.5 Mb section of the E. coli genome-a key intermediate in its total synthesis1-from five episomes in 10  days. By parallelizing CGS and combining it with rapid oligonucleotide synthesis and episome assembly5,6, along with rapid methods for compiling a single genome from strains bearing distinct synthetic genome sections1,7,8, we anticipate that it will be possible to synthesize entire E. coli genomes from functional designs in less than 2 months.

Neurotoxic and cytotoxic peptides underlie the painful stings of the tree nettle Urtica ferox

The stinging hairs of plants from the family Urticaceae inject compounds that inflict pain to deter herbivores. The sting of the New Zealand tree nettle (Urtica ferox) is among the most painful of these and can cause systemic symptoms that can even be life-threatening; however, the molecular species effecting this response have not been elucidated. Here we reveal that two classes of peptide toxin are responsible for the symptoms of U. ferox stings: Δ-Uf1a is a cytotoxic thionin that causes pain via disruption of cell membranes, while β/δ-Uf2a defines a new class of neurotoxin that causes pain and systemic symptoms via modulation of voltage-gated sodium (Na_V) channels. We demonstrate using whole-cell patch-clamp electrophysiology experiments that β/δ-Uf2a is a potent modulator of human Na_V1.5 (EC₅₀: 55 nM), Na_V1.6 (EC₅₀: 0.86 nM), and Na_V1.7 (EC₅₀: 208 nM), where it shifts the activation threshold to more negative potentials and slows fast inactivation. We further found that both toxin classes are widespread among members of the Urticeae tribe within Urticaceae, suggesting that they are likely to be pain-causing agents underlying the stings of other Urtica species. Comparative analysis of nettles of Urtica, and the recently described pain-causing peptides from nettles of another genus, Dendrocnide, indicates that members of tribe Urticeae have developed a diverse arsenal of pain-causing peptides.

Cystine Knot Peptides with Tuneable Activity and Mechanism

Knottins are topologically complex peptides that are stabilised by a cystine knot and have exceptionally diverse functions, including protease inhibition. However, approaches for tuning their activity in situ are limited. Here, we demonstrate separate approaches for tuning the activity of knottin protease inhibitors using light or streptavidin. We show that the inhibitory activity and selectivity of an engineered knottin can be controlled with light by activating a second mode of action that switches the inhibitor ON against new targets. Guided by a knottin library screen, we also identify a position in the inhibitor's binding loop that permits insertion of a biotin tag without impairing activity. Using streptavidin, biotinylated knottins with nanomolar affinity can be switched OFF in activity assays, and the anticoagulant activity of a factor XIIa inhibitor can be rapidly switched OFF in human plasma. Our findings expand the scope of engineered knottins for precisely controlling protein function.

Enzymatic C-to-C Protein Ligation

Transpeptidase‐catalyzed protein and peptide modifications have been widely utilized for generating conjugates of interest for biological investigation or therapeutic applications. However, all known transpeptidases are constrained to ligating in the N‐to‐C orientation, limiting the scope of attainable products. Here, we report that an engineered asparaginyl ligase accepts diverse incoming nucleophile substrate mimetics, particularly when a means of selectively quenching the reactivity of byproducts released from the recognition sequence is employed. In addition to directly catalyzing formation of l‐/d‐ or α‐/β‐amino acid junctions, we find C‐terminal Leu‐ethylenediamine (Leu‐Eda) motifs to be bona fide mimetics of native N‐terminal Gly‐Leu sequences. Appending a C‐terminal Leu‐Eda to synthetic peptides or, via an intein‐splicing approach, to recombinant proteins enables direct transpeptidase‐catalyzed C‐to‐C ligations. This work significantly expands the synthetic scope of enzyme‐catalyzed protein transpeptidation reactions.

Enzymatic C-Terminal Protein Engineering with Amines

Chemoenzymatic protein and peptide modification is a powerful means of generating defined, homogeneous conjugates for a range of applications. However, the use of transpeptidases is limited by the need to prepare synthetic peptide conjugates to be ligated, bulky recognition tags remaining in the product, and inefficient substrate turnover. Here, we report a peptide/protein labeling strategy that utilizes a promiscuous, engineered transpeptidase to irreversibly incorporate diverse, commercially available amines at a C-terminal asparagine. To demonstrate the utility of this approach, we prepare a protein-drug conjugate, generate a genetically inaccessible C-to-C protein fusion, and site specifically label both termini of a single protein in sequential steps.

Asparaginyl Ligase-Catalyzed One-Step Cell Surface Modification of Red Blood Cells

Red blood cells (RBCs) can serve as vascular carriers for drugs, proteins, peptides, and nanoparticles. Human RBCs remain in the circulation for ∼120 days, are biocompatible, and are immunologically largely inert. RBCs are cleared by the reticuloendothelial system and can induce immune tolerance to foreign components attached to the RBC surface. RBC conjugates have been pursued in clinical trials to treat cancers and autoimmune diseases and to correct genetic disorders. Still, most methods used to modify RBCs require multiple steps, are resource-intensive and time-consuming, and increase the risk of inflicting damage to the RBCs. Here, we describe direct conjugation of peptides and proteins onto the surface of RBCs in a single step, catalyzed by a highly efficient, recombinant asparaginyl ligase under mild, physiological conditions. In mice, the modified RBCs remain intact in the circulation, display a normal circulatory half-life, and retain their immune tolerance-inducing properties, as shown for protection against an accelerated model for type 1 diabetes. We conjugated different nanobodies to RBCs with retention of their binding properties, and these modified RBCs can target cancer cells in vitro. This approach provides an appealing alternative to current methods of RBC engineering. It provides ready access to more complex RBC constructs and highlights the general utility of asparaginyl ligases for the modification of native cell surfaces.

Asparaginyl Ligases: New Enzymes for the Protein Engineer's Toolbox

Enzyme-catalysed site-specific protein modifications enable the precision manufacture of conjugates for the study of protein function and/or for therapeutic or diagnostic applications. Asparaginyl ligases are a class of highly efficient transpeptidases with the capacity to modify proteins bearing only a tripeptide recognition motif. Herein, we review the types of protein modification that are accessible using these enzymes, including N- and C-terminal protein labelling, head-to-tail cyclisation, and protein-protein conjugation. We describe the progress that has been made to engineer highly efficient ligases as well as efforts to chemically manipulate the enzyme reaction to favour product formation. These enzymes are powerful additions to the protein engineer's toolbox.

Neurotoxic peptides from the venom of the giant Australian stinging tree

Stinging trees from Australasia produce remarkably persistent and painful stings upon contact of their stiff epidermal hairs, called trichomes, with mammalian skin. Dendrocnide-induced acute pain typically lasts for several hours, and intermittent painful flares can persist for days and weeks. Pharmacological activity has been attributed to small-molecule neurotransmitters and inflammatory mediators, but these compounds alone cannot explain the observed sensory effects. We show here that the venoms of Australian Dendrocnide species contain heretofore unknown pain-inducing peptides that potently activate mouse sensory neurons and delay inactivation of voltage-gated sodium channels. These neurotoxins localize specifically to the stinging hairs and are miniproteins of 4 kDa, whose 3D structure is stabilized in an inhibitory cystine knot motif, a characteristic shared with neurotoxins found in spider and cone snail venoms. Our results provide an intriguing example of inter-kingdom convergent evolution of animal and plant venoms with shared modes of delivery, molecular structure, and pharmacology.

A bifunctional asparaginyl endopeptidase efficiently catalyzes both cleavage and cyclization of cyclic trypsin inhibitors

Asparaginyl endopeptidases (AEPs) catalyze the key backbone cyclization step during the biosynthesis of plant-derived cyclic peptides. Here, we report the identification of two AEPs from Momordica cochinchinensis and biochemically characterize MCoAEP2 that catalyzes the maturation of trypsin inhibitor cyclotides. Recombinantly produced MCoAEP2 catalyzes the backbone cyclization of a linear cyclotide precursor (MCoTI-II-NAL) with a k_cat/K_m of 620 mM⁻¹ s⁻¹, making it one of the fastest cyclases reported to date. We show that MCoAEP2 can mediate both the N-terminal excision and C-terminal cyclization of cyclotide precursors in vitro. The rate of cyclization/hydrolysis is primarily influenced by varying pH, which could potentially control the succession of AEP-mediated processing events in vivo. Furthermore, MCoAEP2 efficiently catalyzes the backbone cyclization of an engineered MCoTI-II analog with anti-angiogenic activity. MCoAEP2 provides enhanced synthetic access to structures previously inaccessible by direct chemistry approaches and enables the wider application of trypsin inhibitor cyclotides in biotechnology applications.

Asparaginyl endopeptidases (AEPs) catalyze the cyclization step during the biosynthesis of cyclic peptides in plants. Here, the authors report a recombinantly produced AEP that catalyzes the backbone cyclization of a linear cyclotide precursor and an engineered analog with high efficiency and in a pH-dependent manner.

Site-Specific Sequential Protein Labeling Catalyzed by a Single Recombinant Ligase

Protein ligases of defined substrate specificity are versatile tools for protein engineering. Upon completion of the reaction, the products of currently reported protein ligases contain the amino acid sequence that is recognized by that same ligase, resulting in repeated cycles of ligation and hydrolysis as competing reactions. Thus, previous efforts to sequentially label proteins at distinct positions required ligases of orthogonal specificity. A recombinant Oldenlandia affinis asparaginyl endopeptidase, OaAEP1, is promiscuous for incoming nucleophiles. This promiscuity enabled us to define a nucleophile composed of natural amino acids that is ligated efficiently to the substrate yet yields a product that is poorly recognized by OaAEP1. Proteins modified with an efficient recognition module could be readily modified to yield a defined product bearing a cleavage-resistant motif, whereas proteins containing this inferior recognition motif remained essentially unmodified. We demonstrate the versatility of the N- or C-terminal protein modifications obtainable with this approach and modify the N- and C-termini of a single substrate protein in a sequential, site-specific manner in excellent yield.

Ribosomally-synthesised cyclic peptides from plants as drug leads and pharmaceutical scaffolds

Owing to their exceptional stability and favourable pharmacokinetic properties, plant-derived cyclic peptides have recently attracted significant attention in the field of peptide-based drug design. This article describes the three major classes of ribosomally-synthesised plant peptides - the cyclotides, the PawS-derived peptides and the orbitides - and reviews their applications as leads or scaffolds in drug design. These ribosomally-produced peptides have a range of biological activities, including anti-HIV, cytotoxic and immunomodulatory activity. In addition, recent interest has focused on their use as scaffolds to stabilise bioactive peptide sequences, thereby enhancing their biopharmaceutical properties. There are now more than 30 published papers on such 'grafting' applications, most of which have been reported only in the last few years, and several such studies have reported in vivo activity of orally delivered cyclic peptides. In this article, we describe approaches to the synthesis of cyclic peptides and their pharmaceutically-grafted derivatives as well as outlining their biosynthetic routes. Finally, we describe possible bioproduction routes for pharmaceutically active cyclic peptides, involving plants and plant suspension cultures.

Bioengineering toward direct production of immobilized enzymes: A paradigm shift in biocatalyst design

The need for cost-effectively produced and improved biocatalysts for industrial, pharmaceutical and environmental processes is steadily increasing. While enzyme properties themselves can be improved via protein engineering, immobilization by attachment to carrier materials remains a critical step for stabilization and process implementation. A new emerging immobilization approach, the in situ immobilization, enables simultaneous production of highly active enzymes and carrier materials using bioengineering/synthetic biology of microbial cells. In situ enzyme immobilization holds the promise of cost-effective production of highly functional immobilized biocatalysts for uses such as in bioremediation, drug synthesis, bioenergy and food processing.

Enzyme Engineering for In Situ Immobilization

Enzymes are used as biocatalysts in a vast range of industrial applications. Immobilization of enzymes to solid supports or their self-assembly into insoluble particles enhances their applicability by strongly improving properties such as stability in changing environments, re-usability and applicability in continuous biocatalytic processes. The possibility of co-immobilizing various functionally related enzymes involved in multistep synthesis, conversion or degradation reactions enables the design of multifunctional biocatalyst with enhanced performance compared to their soluble counterparts. This review provides a brief overview of up-to-date in vitro immobilization strategies while focusing on recent advances in enzyme engineering towards in situ self-assembly into insoluble particles. In situ self-assembly approaches include the bioengineering of bacteria to abundantly form enzymatically active inclusion bodies such as enzyme inclusions or enzyme-coated polyhydroxyalkanoate granules. These one-step production strategies for immobilized enzymes avoid prefabrication of the carrier as well as chemical cross-linking or attachment to a support material while the controlled oriented display strongly enhances the fraction of accessible catalytic sites and hence functional enzymes.