Site-Specific Protein Labeling and Generation of Defined Ubiquitin-Protein Conjugates Using an Asparaginyl Endopeptidase

Expanding horizons: genetic code expansion technology in the study of PTM functions

Recent advancements in Genetic Code Expansion (GCE) have significantly enhanced our understanding of post-translational modifications (PTMs), which are critical for protein regulation. GCE facilitates the precise incorporation of unnatural amino acids (UAAs) at specific sites within proteins of interest (POIs), making it a powerful tool for modulating PTMs in vivo. This review summarizes the various UAAs utilized to directly incorporate PTMs into proteins through GCE, with a focus on their applications in both histone and non-histone PTMs research. We also discuss the challenges associated with incorporating certain PTMs into target proteins via GCE and provide an overview of the latest strategies developed to overcome these hurdles.

Highly Efficient Transpeptidase-Catalyzed Isopeptide Ligation

Transpeptidases are specialized enzymes that have evolved for site-selective modification of peptides and proteins at their backbone termini. Approaches for adapting transpeptidases to catalyze side chain modifications are substantially more restricted, and typically rely on large recognition tags or require specific reaction conditions that are not easily compatible with broader applications. Here we show that the engineered asparaginyl ligase OaAEP1 catalyzes direct isopeptide ligation by accepting an internal 2,3-diaminopropionic acid (Dap) residue adjacent to Leu, a motif that mimics the canonical N-terminal Gly-Leu substrate. These reactions proceed efficiently at near-neutral pH without any required additives, enabling straightforward formation of diverse isopeptide-linked products under simple reaction conditions. We demonstrate that OaAEP1-catalyzed isopeptide ligation can be utilized for site-selective side chain labeling at an introduced Dap residue with minimal off-target labeling of Lys residues. Additionally, we generate engineered peptide topologies via intramolecular side chain-to-tail cross-links and produce direct protein-cyclic peptide fusions via efficient intermolecular ligation. We also show that OaAEP1-catalyzed isopeptide ligation extends to d-peptide acceptors containing a retro-inverso d-Leu-d-Dap motif. This capability further expands the range and complexity of isopeptide-linked products that can be accessed with OaAEP1, which we exemplify by forming a hybrid d-/l- bicyclic peptide topology where both termini are linked to internal side chains.

Generation of antibody-drug conjugates by proximity-driven acyl transfer and sortase-mediated ligation

We report a sortase-based site-specific antibody-drug conjugation strategy, which involves an affinity peptide-directed acyl transfer reaction and sortase-mediated peptide ligation. Through the affinity peptide-mediated acyl transfer reaction, an LPXTG-containing peptide is conjugated to a specific Lys side chain of an antibody. Under the assistance of sortase, a protein drug bearing a GG motif reacts specifically with the LPXTG moiety to produce an antibody-drug conjugate. Our strategy for antibody conjugation can be applied not only to chemically synthesized drugs, but also to biologically expressed proteins, and will provide a new sortase-based strategy for the preparation of antibody-drug conjugates.

Site-specific incorporation of 19F-nulcei at protein C-terminus to probe allosteric conformational transitions of metalloproteins

Allosteric conformational change is an important paradigm in the regulation of protein function, which is typically triggered by the binding of small cofactors, metal ions or protein partners. Here, we found those conformational transitions can be effectively monitored by 19F NMR, facilitated by a site-specific 19F incorporation strategy at the protein C-terminus using asparaginyl endopeptidase (AEP). Three case studies show that C-terminal 19F-nuclei can reveal protein dynamics not only adjacent but also distal to C-terminus, including those occurring in a hemoprotein neuroglobin (Ngb), calmodulin (CaM), and a cobalt metalloregulator (CoaR) responding to both cobalt and tetrapyrrole. In Ngb, the heme orientation disorder is affected by missense mutations that perturb backbone rigidity or surface charges close to the heme axial ligands. In CaM, the C-terminal 19F-nuclei is an ideal probe for detecting the binding states of Ca2+, peptides and inhibitors. Furthermore, multiple 19F-moieties were incorporated into the two domains of CoaR, revealing the intrinsically disordered C-terminal metal binding tail might be an allosteric conformational switch to maintain cobalt homeostasis and balance corrinoid biosynthesis. This study demonstrates that the AEP-based 19F-modification strategy can be applied to various targets to study allosteric regulation, especially for those biological processes modulated by the protein C-terminus.

Secretase promotes AD progression: simultaneously cleave Notch and APP

Alzheimer's disease (AD) involves complex pathological mechanisms. Secretases include membrane protein extracellular structural domain proteases and intramembrane proteases that cleave the topology to type I or type II. Secretases can effectively regulate the activation of Notch and amyloid precursor protein (APP), key factors in the progression of AD and cancer. This article systematically summarizes the intracellular localization, cleavage sites and products, and biological functions of six subtypes of secretases (α-secretase, β-secretase, γ-secretase, δ-secretase, ε-secretase, and η-secretase), and for the first time, elucidates the commonalities and differences between these subtypes of secretases. We found that each subtype of secretase primarily cleaves APP and Notch as substrates, regulating Aβ levels through APP cleavage to impact the progression of AD, while also cleaving Notch receptors to affect cancer progression. Finally, we review the chemical structures, indications, and research stages of various secretase inhibitors, emphasizing the promising development of secretase inhibitors in the fields of cancer and AD.

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.

Recent Toolboxes for Chemoselective Dual Modifications of Proteins

Site-selective chemical modifications of proteins have emerged as a potent technology in chemical biology, materials science, and medicine, facilitating precise manipulation of proteins with tailored functionalities for basic biology research and developing innovative therapeutics. Compared to traditional recombinant expression methods, one of the prominent advantages of chemical protein modification lies in its capacity to decorate proteins with a wide range of functional moieties, including non-genetically encoded ones, enabling the generation of novel protein conjugates with enhanced or previously unexplored properties. Among these, approaches for dual or multiple modifications of proteins are increasingly garnering attention, as it has been found that single modification of proteins is inadequate to meet current demands. Therefore, in light of the rapid developments in this field, this review provides a timely and comprehensive overview of the latest advancements in chemical and biological approaches for dual functionalization of proteins. It further discusses their advantages, limitations, and potential future directions in this relatively nascent area.

Repurposing a plant peptide cyclase for targeted lysine acylation

Transpeptidases are powerful tools for protein engineering but are largely restricted to acting at protein backbone termini. Alternative enzymatic approaches for internal protein labelling require bulky recognition motifs or non-proteinogenic reaction partners, potentially restricting which proteins can be modified or the types of modification that can be installed. Here we report a strategy for labelling lysine side chain ε-amines by repurposing an engineered asparaginyl ligase, which naturally catalyses peptide head-to-tail cyclization, for versatile isopeptide ligations that are compatible with peptidic substrates. We find that internal lysines with an adjacent leucine residue mimic the conventional N-terminal glycine-leucine substrate. This dipeptide motif enables efficient intra- or intermolecular ligation through internal lysine side chains, minimally leaving an asparagine C-terminally linked to the lysine side chain via an isopeptide bond. The versatility of this approach is demonstrated by the chemoenzymatic synthesis of peptides with non-native C terminus-to-side chain topology and the conjugation of chemically modified peptides to recombinant proteins.

An efficient peptide ligase engineered from a bamboo asparaginyl endopeptidase

In recent years, a few asparaginyl endopeptidases (AEPs) from certain higher plants have been identified as efficient peptide ligases with wide applications in protein labeling and cyclic peptide synthesis. Recently, we developed a NanoLuc Binary Technology (NanoBiT)-based peptide ligase activity assay to identify more AEP-type peptide ligases. Herein, we screened 61 bamboo species from 16 genera using this assay and detected AEP-type peptide ligase activity in the crude extract of all tested bamboo leaves. From a popular bamboo species, Bambusa multiplex, we identified a full-length AEP-type peptide ligase candidate (BmAEP1) via transcriptomic sequencing. After its zymogen was overexpressed in Escherichia coli and self-activated in vitro, BmAEP1 displayed high peptide ligase activity, but with considerable hydrolytic activity. After site-directed mutagenesis of its ligase activity determinants, the mutant zymogen of [G238V]BmAEP1 was normally overexpressed in E. coli, but failed to activate itself. To resolve this problem, we developed a novel protease-assisted activation approach in which trypsin was used to cleave the mutant zymogen and was then conveniently removed via ion-exchange chromatography. After the noncovalently bound cap domain was dissociated from the catalytic core domain under acidic conditions, the recombinant [G238V]BmAEP1 displayed high peptide ligase activity with much lower hydrolytic activity and could efficiently catalyze inter-molecular protein ligation and intramolecular peptide cyclization. Thus, the engineered bamboo-derived peptide ligase represents a novel tool for protein labeling and cyclic peptide synthesis.

Cellular and Biophysical Applications of Genetic Code Expansion

Despite their diverse functions, proteins are inherently constructed from a limited set of building blocks. These compositional constraints pose significant challenges to protein research and its practical applications. Strategically manipulating the cellular protein synthesis system to incorporate novel building blocks has emerged as a critical approach for overcoming these constraints in protein research and application. In the past two decades, the field of genetic code expansion (GCE) has achieved significant advancements, enabling the integration of numerous novel functionalities into proteins across a variety of organisms. This technological evolution has paved the way for the extensive application of genetic code expansion across multiple domains, including protein imaging, the introduction of probes for protein research, analysis of protein-protein interactions, spatiotemporal control of protein function, exploration of proteome changes induced by external stimuli, and the synthesis of proteins endowed with novel functions. In this comprehensive Review, we aim to provide an overview of cellular and biophysical applications that have employed GCE technology over the past two decades.

Reading and Writing the Ubiquitin Code Using Genetic Code Expansion

Deciphering ubiquitin proteoform signaling and its role in disease has been a long-standing challenge in the field. The effects of ubiquitin modifications, its relation to ubiquitin-related machineries, and its signaling output has been particularly limited by its reconstitution and means of characterization. Advances in genetic code expansion have contributed towards addressing these challenges by precision incorporation of unnatural amino acids through site selective codon suppression. This review discusses recent advances in studying the 'writers', 'readers', and 'erasers' of the ubiquitin code using genetic code expansion. Highlighting strategies towards genetically encoded protein ubiquitination, ubiquitin phosphorylation, acylation, and finally surveying ubiquitin interactions, we strive to bring attention to this unique approach towards addressing a widespread proteoform problem.

A Method for Rigorously Selective Capture and Simultaneous Fluorescent Labeling of N-Terminal Glycine Peptides

Although chemical methods for the selective derivatization of amino acid (AA) side chains in peptides and proteins are available, selective N-terminal labeling is challenging, especially for glycine, which has no side chain at the α-carbon position. We report here a double activation at glycine's α-methylene group that allows this AA to be differentiated from the other 19 AAs. A condensation reaction of dibenzoylmethane with glycine results in the formation of an imine, and subsequent tautomerization is followed by intramolecular cyclization, leading to the formation of a fluorescent pyrrole ring. Additionally, the approach exhibits compatibility with AAs possessing reactive side chains. Further, the method allows for selective pull-down assays of N-terminal glycine peptides from mixtures without prior knowledge of the N-terminal peptide distribution.

Engineered reactivity of a bacterial E1-like enzyme enables ATP-driven modification of protein C termini

In biological systems, ATP provides an energetic driving force for peptide bond formation, but protein chemists lack tools that emulate this strategy. Inspired by the eukaryotic ubiquitination cascade, we developed an ATP-driven platform for C-terminal activation and peptide ligation based on E. coli MccB, a bacterial ancestor of ubiquitin-activating (E1) enzymes that natively catalyzes C-terminal phosphoramidate bond formation. We show that MccB can act on non-native substrates to generate an O-AMPylated electrophile that can react with exogenous nucleophiles to form diverse C-terminal functional groups including thioesters, a versatile class of biological intermediates that have been exploited for protein semisynthesis. To direct this activity towards specific proteins of interest, we developed the Thioesterification C-terminal Handle (TeCH)-tag, a sequence that enables high-yield, ATP-driven protein bioconjugation via a thioester intermediate. By mining the natural diversity of the MccB family, we developed two additional MccB/TeCH-tag pairs that are mutually orthogonal to each other and to the E. coli system, facilitating the synthesis of more complex bioconjugates. Our method mimics the chemical logic of peptide bond synthesis that is widespread in biology for high-yield in vitro manipulation of protein structure with molecular precision.

Asparaginyl Endopeptidase-Mediated Peptide Cyclization for Phage Display

We report here an enzymatic strategy for asparaginyl endopeptidase-mediated peptide cyclization. Incorporation of chloroacetyl groups into the recognition sequence of OaAEP1 enabled intramolecular cyclization with Cys residues. Combining this strategy and phage display, we identified nanomolar macrocyclic peptide ligands targeting TEAD4. One of the bicyclic peptides binds to TEAD4 with a KD value of 139 nM, 16 times lower than its linear analogue, demonstrating the utility of this platform in discovering high-affinity macrocyclic peptide ligands.

Sortase mediated protein ubiquitination with defined chain length and topology

Ubiquitination is a key post-translational modification on protein lysine sidechains known to impact protein stability, signal transduction cascades, protein-protein interactions, and beyond. Great strides have been made towards developing new methods to generate discrete chains of polyubiquitin and conjugate them onto proteins site-specifically, with methods ranging from chemical synthetic approaches, to enzymatic approaches and many in between. Previous work has demonstrated the utility of engineered variants of the bacterial transpeptidase enzyme sortase (SrtA) for conjugation of ubiquitin site-specifically onto target proteins. In this manuscript, we've combined the classical E1/E2-mediated polyubiquitin chain extension approach with sortase-mediated ligation and click chemistry to enable the generation of mono, di, and triubiquitinated proteins sfGFP and PCNA. We demonstrate the utility of this strategy to generate both K48-linked and K63-linked polyubiquitins and attach them both N-terminally and site-specifically to the proteins of interest. Further, we highlight differential activity between two commonly employed sortase variants, SrtA 5M and 7M, and demonstrate that while SrtA 7M can be used to conjugate these ubiquitins to substrates, SrtA 5M can be employed to release the ubiquitin from the substrates as well as to cleave C-terminal tags from the ubiquitin variants used. Overall, we envision that this approach is broadly applicable to readily generate discrete polyubiquitin chains of any linkage type that is accessible via E1/E2 systems and conjugate site-specifically onto proteins of interest, thus granting access to bespoke ubiquitinated proteins that are not currently possible.

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.

N-Terminal-Specific Dual Modification of Peptides through Copper-Catalyzed [3+2] Cycloaddition

Site-specific introduction of multiple components into peptides is greatly needed for the preparation of densely functionalized and structurally uniform peptides. In this regard, N-terminal-specific peptide modification is attractive, but it can be difficult due to the presence of highly nucleophilic lysine ϵ-amine. In this work, we developed a method for the N-terminal-specific dual modification of peptides through a three-component [3+2] cycloaddition with aldehydes and maleimides under mild copper catalysis. This approach enables exclusive functionalization at the glycine N-terminus of iminopeptides, regardless of the presence of lysine ϵ-amine, thus affording the cycloadducts in excellent yields. Tolerating a broad range of functional groups and molecules, the present method provides the opportunity to rapidly construct doubly functionalized peptides using readily accessible aldehyde and maleimide modules.

Orthogonal Translation for Site-Specific Installation of Post-translational Modifications

Post-translational modifications (PTMs) endow proteins with new properties to respond to environmental changes or growth needs. With the development of advanced proteomics techniques, hundreds of distinct types of PTMs have been observed in a wide range of proteins from bacteria, archaea, and eukarya. To identify the roles of these PTMs, scientists have applied various approaches. However, high dynamics, low stoichiometry, and crosstalk between PTMs make it almost impossible to obtain homogeneously modified proteins for characterization of the site-specific effect of individual PTM on target proteins. To solve this problem, the genetic code expansion (GCE) strategy has been introduced into the field of PTM studies. Instead of modifying proteins after translation, GCE incorporates modified amino acids into proteins during translation, thus generating site-specifically modified proteins at target positions. In this review, we summarize the development of GCE systems for orthogonal translation for site-specific installation of PTMs.

Structure-guided engineering enables E3 ligase-free and versatile protein ubiquitination via UBE2E1

Ubiquitination, catalyzed usually by a three-enzyme cascade (E1, E2, E3), regulates various eukaryotic cellular processes. E3 ligases are the most critical components of this catalytic cascade, determining both substrate specificity and polyubiquitination linkage specificity. Here, we reveal the mechanism of a naturally occurring E3-independent ubiquitination reaction of a unique human E2 enzyme UBE2E1 by solving the structure of UBE2E1 in complex with substrate SETDB1-derived peptide. Guided by this peptide sequence-dependent ubiquitination mechanism, we developed an E3-free enzymatic strategy SUE1 (sequence-dependent ubiquitination using UBE2E1) to efficiently generate ubiquitinated proteins with customized ubiquitinated sites, ubiquitin chain linkages and lengths. Notably, this strategy can also be used to generate site-specific branched ubiquitin chains or even NEDD8-modified proteins. Our work not only deepens the understanding of how an E3-free substrate ubiquitination reaction occurs in human cells, but also provides a practical approach for obtaining ubiquitinated proteins to dissect the biochemical functions of ubiquitination.

Enzymatic Protein Immobilization for Nanobody Array

Antibody arrays play a pivotal role in the detection and quantification of biomolecules, with their effectiveness largely dependent on efficient protein immobilization. Traditional methods often use heterobifunctional cross-linking reagents for attaching functional residues in proteins to corresponding chemical groups on the substrate surface. However, this method does not control the antibody's anchoring point and orientation, potentially leading to reduced binding efficiency and overall performance. Another method using anti-antibodies as intermediate molecules to control the orientation can be used but it demonstrates lower efficiency. Here, we demonstrate a site-specific protein immobilization strategy utilizing OaAEP1 (asparaginyl endopeptidase) for building a nanobody array. Moreover, we used a nanobody-targeting enhanced green fluorescent protein (eGFP) as the model system to validate the protein immobilization method for building a nanobody array. Finally, by rapidly enriching eGFP, this method further highlights its potential for rapid diagnostic applications. This approach, characterized by its simplicity, high efficiency, and specificity, offers an advancement in the development of surface-modified protein arrays. It promises to enhance the sensitivity and accuracy of biomolecule detection, paving the way for broader applications in various research and diagnostic fields.

Native Semisynthesis of Isopeptide-Linked Substrates for Specificity Analysis of Deubiquitinases and Ubl Proteases

Post-translational modifications with ubiquitin (Ub) and ubiquitin-like proteins (Ubls) are regulated by isopeptidases termed deubiquitinases (DUBs) and Ubl proteases. Here, we describe a mild chemical method for the preparation of fluorescence polarization substrates for these enzymes that is based on the activation of C-terminal Ub/Ubl hydrazides to acyl azides and their subsequent functionalization to isopeptides. The procedure is complemented by native purification routes and thus circumvents the previous need for desulfurization and refolding. Its broad applicability was demonstrated by the generation of fully cleavable substrates for Ub, SUMO1, SUMO2, NEDD8, ISG15, and Fubi. We employed these reagents for the investigation of substrate specificities of human UCHL3, USPL1, USP2, USP7, USP16, USP18, and USP36. Pronounced selectivity of USPL1 for SUMO2/3 over SUMO1 was observed, which we rationalize with crystal structures and biochemical assays, revealing a SUMO paralogue specificity mechanism distinct from SENP family deSUMOylases. Moreover, we investigated the recently identified Fubi proteases USP16 and USP36 and found both to act as bona fide deFubiylases, harboring catalytic activity against isopeptide-linked Fubi. Surprisingly, we also noticed the activity of both enzymes toward ISG15, previously not identified in chemoproteomics, which makes USP16 and USP36 the first human DUBs with specific isopeptidase activity toward three distinct modifiers. The methods described here for the preparation of isopeptide-linked, fully folded substrates will aid in the characterization of further DUBs/Ubl proteases. More broadly, our findings highlight possible limitations associated with fluorogenic substrates and Ubl activity-based probes and stress the importance of isopeptide-containing reagents for validating isopeptidase activities and quantifying substrate specificities.

Synthesis of ubiquitinated proteins for biochemical and functional analysis

Ubiquitination plays a crucial role in controlling various biological processes such as translation, DNA repair and immune response. Protein degradation for example, is one of the main processes which is controlled by the ubiquitin system and has significant implications on human health. In order to investigate these processes and the roles played by different ubiquitination patterns on biological systems, homogeneously ubiquitinated proteins are needed. Notably, these conjugates that are made enzymatically in cells cannot be easily obtained in large amounts and high homogeneity by employing such strategies. Therefore, chemical and semisynthetic approaches have emerged to prepare different ubiquitinated proteins. In this review, we will present the key synthetic strategies and their applications for the preparation of various ubiquitinated proteins. Furthermore, the use of these precious conjugates in different biochemical and functional studies will be highlighted.

Introducing Azomethine Imines to Chemical Biology: Bioorthogonal Reaction with Isonitriles

Azomethine imines are valuable substrates for chemical synthesis in organic solvents that often require anhydrous conditions. Here, we introduce C,N-cyclic-N'-acyl azomethine imines (AMIs) to bioorthogonal reactions in an aqueous environment. These AMIs are stable under physiological conditions and react rapidly (k2 = 0.1-250 M-1 s-1, depending on pH) and chemoselectively with isonitriles in the presence of biological nucleophiles, including thiols. Live-cell imaging of cell-surface-bound isonitriles underlines the biocompatibility of the AMI-isonitrile ligation, and simultaneous one-pot triple-protein labeling demonstrates its orthogonality to commonly used bioorthogonal reactions, such as the SPAAC and iEDDA ligations.

Functional analysis of protein post-translational modifications using genetic codon expansion

Post-translational modifications (PTMs) of proteins not only exponentially increase the diversity of proteoforms, but also contribute to dynamically modulating the localization, stability, activity, and interaction of proteins. Understanding the biological consequences and functions of specific PTMs has been challenging for many reasons, including the dynamic nature of many PTMs and the technical limitations to access homogenously modified proteins. The genetic code expansion technology has emerged to provide unique approaches for studying PTMs. Through site-specific incorporation of unnatural amino acids (UAAs) bearing PTMs or their mimics into proteins, genetic code expansion allows the generation of homogenous proteins with site-specific modifications and atomic resolution both in vitro and in vivo. With this technology, various PTMs and mimics have been precisely introduced into proteins. In this review, we summarize the UAAs and approaches that have been recently developed to site-specifically install PTMs and their mimics into proteins for functional studies of PTMs.

Nature-inspired protein ligation and its applications

The ability to manipulate the chemical composition of proteins and peptides has been central to the development of improved polypeptide-based therapeutics and has enabled researchers to address fundamental biological questions that would otherwise be out of reach. Protein ligation, in which two or more polypeptides are covalently linked, is a powerful strategy for generating semisynthetic products and for controlling polypeptide topology. However, specialized tools are required to efficiently forge a peptide bond in a chemoselective manner with fast kinetics and high yield. Fortunately, nature has addressed this challenge by evolving enzymatic mechanisms that can join polypeptides using a diverse set of chemical reactions. Here, we summarize how such nature-inspired protein ligation strategies have been repurposed as chemical biology tools that afford enhanced control over polypeptide composition.

PSL Chemical Biology Symposia Third Edition: A Branch of Science in its Explosive Phase

This symposium is the third PSL (Paris Sciences & Lettres) Chemical Biology meeting (2016, 2019, 2023) held at Institut Curie. This initiative originally started at Institut de Chimie des Substances Naturelles (ICSN) in Gif-sur-Yvette (2013, 2014), under the directorship of Professor Max Malacria, with a strong focus on chemistry. It was then continued at the Institut Curie (2015) covering a larger scope, before becoming the official PSL Chemical Biology meeting. This latest edition was postponed twice for the reasons that we know. This has given us the opportunity to invite additional speakers of great standing. This year, Institut Curie hosted around 300 participants, including 220 on site and over 80 online. The pandemic has had, at least, the virtue of promoting online meetings, which we came to realize is not perfect but has its own merits. In particular, it enables those with restricted time and resources to take part in events and meetings, which can now accommodate unlimited participants. We apologize to all those who could not attend in person this time due to space limitation at Institut Curie.

OaAEP1 Ligase-Assisted Chemoenzymatic Synthesis of Full Cysteine-Rich Metal-Binding Cyanobacterial Metallothionein SmtA

Among all approaches used for the semisynthesis of natural or chemically modified products, enzyme-assisted ligation is among the most promising and dynamically developing approaches. Applying an efficient C247A mutant of Oldenlandia affinis plant ligase OaAEP1 and solid-phase peptide synthesis chemistry, we present the chemoenzymatic synthesis of a complete sequence of the cysteine-rich and metal-binding cyanobacterial metallothionein Synechococcus metallothionein A (SmtA). Zn(II) and Cd(II) binding to the newly synthesized SmtA showed identical properties to the protein expressed in Escherichia coli. The presented approach is the first example of the use of OaAEP1 mutant for total protein synthesis of metallothionein, which occurs in mild conditions preventing cysteine thiol oxidation. The recognition motif of the applied enzyme could naturally occur in the protein structure or be synthetically or genetically incorporated in some loops or secondary structure elements. Therefore, we envision that this strategy can be used for efficiently obtaining SmtA and for a wide range of proteins and their derivatives.

Expanding Glycomic Investigations through Thiol-Derivatized Glycans

N-(2-thioethyl)-2-aminobenzamide (TEAB), a novel glycan auxiliary, was synthesized and its utility was evaluated. The auxiliary was conjugated to glycans by reductive amination with the water-stable reagent 2-picoline borane complex. Glycan products, which ranged from 1 to 7 linked hexoses, were all isolated in yields ranging from 60% to 90% after purification by reverse-phase chromatography. The novel conjugate introduces a convenient, shelf-stable thiol directly onto the desired free glycans with purification advantages and direct modification with efficient reactions through alkenes, halides, epoxides, disulfides, and carboxylates in yields of 49% to 93%. Subsequently, a thiol-selective modification of the BSA protein was used to generate a neoglycoprotein with a bifunctional PEG-maleimide linker. To further illustrate the utility of a thiol motif, 2-thiopyridine activation of a thiol-containing support facilitated the covalent chromatographic purification of labeled glycans in yields up to 63%. Finally, initial proof of concept of implementation in a light printed microarray was explored and validated through FITC-labeled concanavalin A binding. In conclusion, the thiol-functionalized glycans produced greatly expand the diversity of bioconjugation tools that can be developed with glycans and enable a variety of biological investigations.

One-Step Sortase-Mediated Chemoenzymatic Semisynthesis of Deubiquitinase-Resistant Ub-Peptide Conjugates

Post-translational modifications (PTMs) of proteins increase the functional diversity of the proteome and play crucial regulatory roles in cellular processes. Ubiquitination is a highly regulated and reversible PTM accomplished by a complex multistep process with the sequential action of several specific ubiquitinating (E1-E3) and deubiquitinating enzymes. The different types of ubiquitination (mono-, poly-mono-, and poly-) and the presence of several target sites in a single substrate add to its complexity, which makes the in vitro reconstitution of this ubiquitin (Ub) machinery a quite cumbersome process. Defects in components of the ubiquitination process also contribute to disease pathogenesis, especially cancer and neurodegeneration. This makes them of interest as potential therapeutic targets. Therefore, the development of efficient and reliable methods that will generate a highly homogeneous ubiquitinated peptide and protein conjugate is a topical subject area of research. In this report, we describe the development of a simple and efficient in vitro sortase-mediated chemoenzymatic strategy for semisynthesis of defined and homogeneous ubiquitin conjugates with more than 90% yield. This was achieved by engineering a sortase recognition motif in the dynamic C-terminus of ubiquitin and its conjugation to an isopeptide-linked di-Gly appended peptide LMFK(ε-GG)TEG corresponding to the ubiquitination site residues ₃₈₃LMFKTEG₃₈₉ of p53. The defined and homogeneous ubiquitin conjugates were also weighed for their recognition propensity by deubiquitinating enzymes. This facile semisynthesis of ubiquitin conjugates establishes a simple one-step sortase-mediated chemoenzymatic route for the synthesis of homogeneous and defined isopeptide-linked polypeptides and will help in understanding the complexity of the ubiquitination machinery as well as designing isopeptide drugs and therapeutics.