The pearl jubilee of microcin J25: thirty years of research on an exceptional lasso peptide

Covering: 1992 up to 2023Since their discovery, lasso peptides went from peculiarities to be recognized as a major family of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products that were shown to be spread throughout the bacterial kingdom. Microcin J25 was first described in 1992, making it one of the earliest known lasso peptides. No other lasso peptide has since then been studied to such an extent as microcin J25, yet, previous review articles merely skimmed over all the research done on this exceptional lasso peptide. Therefore, to commemorate the 30th anniversary of its first report, we give a comprehensive overview of all literature related to microcin J25. This review article spans the early work towards the discovery of microcin J25, its biosynthetic gene cluster, and the elucidation of its three-dimensional, threaded lasso structure. Furthermore, the current knowledge about the biosynthesis of microcin J25 and lasso peptides in general is summarized and a detailed overview is given on the biological activities associated with microcin J25, including means of self-immunity, uptake into target bacteria, inhibition of the Gram-negative RNA polymerase, and the effects of microcin J25 on mitochondria. The in vitro and in vivo models used to study the potential utility of microcin J25 in a (veterinary) medicine context are discussed and the efforts that went into employing the microcin J25 scaffold in bioengineering contexts are summed up.

Mutexa: A Computational Ecosystem for Intelligent Protein Engineering

Protein engineering holds immense promise in shaping the future of biomedicine and biotechnology. This Review focuses on our ongoing development of Mutexa, a computational ecosystem designed to enable "intelligent protein engineering". In this vision, researchers will seamlessly acquire sequences of protein variants with desired functions as biocatalysts, therapeutic peptides, and diagnostic proteins through a finely-tuned computational machine, akin to Amazon Alexa's role as a versatile virtual assistant. The technical foundation of Mutexa has been established through the development of a database that combines and relates enzyme structures and their respective functions (e.g., IntEnzyDB), workflow software packages that enable high-throughput protein modeling (e.g., EnzyHTP and LassoHTP), and scoring functions that map the sequence-structure-function relationship of proteins (e.g., EnzyKR and DeepLasso). We will showcase the applications of these tools in benchmarking the convergence conditions of enzyme functional descriptors across mutants, investigating protein electrostatics and cavity distributions in SAM-dependent methyltransferases, and understanding the role of nonelectrostatic dynamic effects in enzyme catalysis. Finally, we will conclude by addressing the future steps and fundamental challenges in our endeavor to develop new Mutexa applications that assist the identification of beneficial mutants in protein engineering.

LassoHTP: A High-Throughput Computational Tool for Lasso Peptide Structure Construction and Modeling

Lasso peptides are a subclass of ribosomally synthesized and post-translationally modified peptides with a slipknot conformation. With superior thermal stability, protease resistance, and antimicrobial activity, lasso peptides are promising candidates for bioengineering and pharmaceutical applications. To enable high-throughput computational prediction and design of lasso peptides, we developed a software, LassoHTP, for automatic lasso peptide structure construction and modeling. LassoHTP consists of three modules, including the scaffold constructor, mutant generator, and molecular dynamics (MD) simulator. With a user-provided sequence and conformational annotation, LassoHTP can either generate the structure and conformational ensemble as is or conduct random mutagenesis. We used LassoHTP to construct eight known lasso peptide structures de novo and to simulate their conformational ensembles for 100 ns MD simulations. For benchmarking, we calculated the root mean square deviation (RMSD) of these ensembles with reference to their experimental crystal or NMR PDB structures; we also compared these RMSD values against those of the MD ensembles that are initiated from the PDB structures. Dihedral principal component analysis was also conducted. The results show that the LassoHTP-initiated ensembles are similar to those of the PDB-initiated ensembles. LassoHTP offers a computational platform to develop strategies for lasso peptide prediction and design.

Cloacaenodin, an Antimicrobial Lasso Peptide with Activity against Enterobacter

Using genome mining and heterologous expression, we report the discovery and production of a new antimicrobial lasso peptide from species related to the Enterobacter cloacae complex. Using NMR and mass spectrometric analysis, we show that this lasso peptide, named cloacaenodin, employs a threaded lasso fold which imparts proteolytic resistance that its unthreaded counterpart lacks. Cloacaenodin has selective, low micromolar, antimicrobial activity against species related to the E. cloacae complex, including species implicated in nosocomial infections and against clinical isolates of carbapenem-resistant Enterobacterales. We further used site-directed mutagenesis to probe the importance of specific residues to the peptide's biosynthesis, stability, and bioactivity.

Discovery and Characterization of Rubrinodin Provide Clues into the Evolution of Lasso Peptides

Lasso peptides are unique natural products that comprise a class of ribosomally synthesized and post-translationally modified peptides. Their defining three-dimensional structure is a lariat knot, in which the C-terminal tail is threaded through a macrolactam ring formed between the N-terminal amino group and an Asp or Glu side chain (i.e., an isopeptide bond). Recent genome mining strategies have revealed various types of lasso peptide biosynthetic gene clusters and have thus redefined the known chemical space of lasso peptides. To date, over 20 different types of these gene clusters have been discovered, including several different clades from Proteobacteria. Despite the diverse architectures of these gene clusters, which may or may not encode various tailoring enzymes, most currently known lasso peptides are synthesized by two discrete clades defined by the presence of an ATP-binding cassette transporter or its absence and (sometimes) concurrent appearance of an isopeptidase, raising questions about their evolutionary history. Herein, we discovered and characterized the lasso peptide rubrinodin, which is assembled by a gene cluster encoding both an ATP-binding cassette transporter and an isopeptidase. Our bioinformatics analyses of this and other representative cluster types provided new clues into the evolutionary history of lasso peptides. Furthermore, our structural and biochemical investigations of rubrinodin permitted the conversion of this thermolabile lasso peptide into a more thermostable scaffold.

Combined thermal and carboxypeptidase Y stability assays for probing the threaded fold of lasso peptides

Lasso peptides are natural products belonging to the superfamily of ribosomally synthesized and post-translationally modified peptides (RiPPs). The defining characteristic of lasso peptides is their threaded structure, which is reminiscent of a lariat knot. When working with lasso peptides, it is therefore of major importance to understand and evidence their threaded folds. While the full elucidation of their three-dimensional structures via NMR spectroscopy or crystallization remains the gold standard, these methods are time-consuming, require large quantities of highly pure lasso peptides, and therefore might not always be applicable. Instead, the unique properties of lasso peptides in context of their behavior at elevated temperatures and toward carboxypeptidase Y treatment can be leveraged as a tool to investigate and evidence the threaded lasso fold using only minute amounts of compound that does not need to be purified first. This chapter will provide insights into the thermal stability properties of lasso peptides and their behavior when treated with carboxypeptidase Y in comparison to a branched-cyclic peptide with the same amino acid sequence. Furthermore, it will be described in detail how to set up a combined thermal and carboxypeptidase Y stability assay and how to analyze its outcomes.

Rational generation of lasso peptides based on biosynthetic gene mutations and site-selective chemical modifications

Lasso peptides are a unique family of natural products whose structures feature a specific threaded fold, which confers these peptides the resistance to thermal and proteolytic degradation. This stability gives lasso peptides excellent pharmacokinetic properties, which together with their diverse reported bioactivities have garnered extensive attention because of their drug development potential. Notably, the threaded fold has proven quite inaccessible by chemical synthesis, which has hindered efficient generation of structurally diverse lasso peptides. We herein report the discovery of a new lasso peptide stlassin (1) by gene activation based on a Streptomyces heterologous expression system. Site-directed mutagenesis on the precursor peptide-encoding gene is carried out systematically, generating 17 stlassin derivatives (2–17 and 21) with residue-replacements at specific positions of 1. The solution NMR structures of 1, 3, 4, 14 and 16 are determined, supporting structural comparisons that ultimately enabled the rational production of disulfide bond-containing derivatives 18 and 19, whose structures do not belong to any of the four classes currently used to classify lasso peptides. Several site-selective chemical modifications are first applied on 16 and 21, efficiently generating new derivatives (20, 22–27) whose structures bear various decorations beyond the peptidyl monotonicity. The high production yields of these stlassin derivatives facilitate biological assays, which show that 1, 4, 16, 20, 21 and 24 possess antagonistic activities against the binding of lipopolysaccharides to toll-like receptor 4 (TLR4). These results demonstrate proof-of-concept for the combined mutational/chemical generation of lasso peptide libraries to support drug lead development.

A new class II lasso peptide stlassin (1) was discovered and stlassin derivatives (2–27) were rationally generated by biosynthetic gene mutations and site-selective chemical modifications, expanding the structural diversity of lasso peptides.

Streptomyces griseocarneus R132 expresses antimicrobial genes and produces metabolites that modulate Galleria mellonella immune system

Actinobacteria is a phylum composed of aerobic, Gram-positive, and filamentous bacteria with a broad spectrum of biological activity, including antioxidant, antitumor, and antibiotic. The crude extract of Streptomyces griseocarneus R132 was fractionated on a C18 silica column and the isolated compound was identified by ¹H and ¹³C nuclear magnetic resonance as 3-(phenylprop-2-enoic acid), also known as trans-cinnamic acid. Antimicrobial activity against human pathogens was assayed in vitro (disk-diffusion qualitative test) and in vivo using Galleria mellonella larvae (RT-qPCR). The methanol fractions 132-F30%, 132-F50%, 132-F70%, and 132-F100% inhibited the Escherichia coli (ATCC 25922) and Staphylococcus aureus (MRSA) growth in vitro the most effectively. Compared with the untreated control (60-80% of larvae death), the fractions and isolated trans-cinnamic acid increased the survival rate and modulated the immune system of G. mellonella larvae infected with pathogenic microorganisms. The anti-infection effect of the S. griseocarneus R132 fermentation product led us to sequence its genome, which was assembled and annotated using the Rast and antiSMASH platforms. The assembled genome consisted of 227 scaffolds represented on a linear chromosome of 8.85 Mb and 71.3% of GC. We detected conserved domains typical of enzymes that produce molecules with biological activity, such as polyketides and non-ribosomal and ribosomal peptides, indicating a great potential for obtaining new antibiotics and molecules with biotechnological application.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02942-1.

Structural Insights from Tandem Mass Spectrometry, Ion Mobility-Mass Spectrometry, and Infrared/Ultraviolet Spectroscopy on Sphingonodin I: Lasso vs Branched-Cyclic Topoisomers

Lasso peptides form a class of ribosomally synthesized and post-translationally modified peptides (RiPPs) characterized by a mechanically interlocked topology, where the C-terminal tail of the peptide is threaded and trapped within an N-terminal macrolactam ring. Sphingonodin I is a lasso peptide that has not yet been structurally characterized using the traditional structural biology tools (e.g., NMR and X-ray crystallography), and its biological function has not yet been elucidated. In the present work, we describe structural signatures characteristic of the class II lasso peptide sphingonodin I and its branched-cyclic analogue using a combination of gas-phase ion tools (e.g., tandem mass spectrometry, MS/MS, trapped ion mobility spectrometry, TIMS, and infrared, IR, and ultraviolet, UV, spectroscopies). Tandem MS/MS CID experiments on sphingonodin I yielded mechanically interlocked species with associated b_i and y_j fragments demonstrating the presence of a lasso topology, while tandem MS/MS ECD experiments on sphingonodin I showed a significant increase in hydrogen migration in the loop region when compared to the branched-cyclic analogue. The high-mobility resolving power of TIMS permitted the separation of both topoisomers, where sphingonodin I adopted a more compact structure than its branched-cyclic analogue. Cryogenic and room-temperature IR spectroscopy experiments evidenced a different hydrogen bond network between the two topologies, while cryogenic UV spectroscopy experiments clearly demonstrated a distinct phenylalanine environment for the lasso peptide.

New developments in RiPP discovery, enzymology and engineering

Covering: up to June 2020Ribosomally-synthesized and post-translationally modified peptides (RiPPs) are a large group of natural products. A community-driven review in 2013 described the emerging commonalities in the biosynthesis of RiPPs and the opportunities they offered for bioengineering and genome mining. Since then, the field has seen tremendous advances in understanding of the mechanisms by which nature assembles these compounds, in engineering their biosynthetic machinery for a wide range of applications, and in the discovery of entirely new RiPP families using bioinformatic tools developed specifically for this compound class. The First International Conference on RiPPs was held in 2019, and the meeting participants assembled the current review describing new developments since 2013. The review discusses the new classes of RiPPs that have been discovered, the advances in our understanding of the installation of both primary and secondary post-translational modifications, and the mechanisms by which the enzymes recognize the leader peptides in their substrates. In addition, genome mining tools used for RiPP discovery are discussed as well as various strategies for RiPP engineering. An outlook section presents directions for future research.

Class IV Lasso Peptides Synergistically Induce Proliferation of Cancer Cells and Sensitize Them to Doxorubicin

Heterologous expression of a biosynthesis gene cluster from Amycolatopsis sp. resulted in the discovery of two unique class IV lasso peptides, felipeptins A1 and A2. A mixture of felipeptins stimulated proliferation of cancer cells, while having no such effect on the normal cells. Detailed investigation revealed, that pre-treatment of cancer cells with a mixture of felipeptins resulted in downregulation of the tumor suppressor Rb, making the cancer cells to proliferate faster. Pre-treatment with felipeptins made cancer cells considerably more sensitive to the anticancer agent doxorubicin and re-sensitized doxorubicin-resistant cells to this drug. Structural characterization and binding experiments showed an interaction between felipeptins resulting in complex formation, which explains their synergistic effect. This discovery may open an alternative avenue in cancer treatment, helping to eliminate quiescent cells that often lead to cancer relapse.

How to harness biosynthetic gene clusters of lasso peptides

Lasso peptides produced by bacteria have a very unique cyclic structure (“lasso” structure) and are resistant to protease. To date, a number of lasso peptides have been isolated from proteobacteria and actinobacteria. Many lasso peptides exhibit various biological activities, such as antibacterial activity, and are expected to have various applications. Based on study of genome mining, large numbers of biosynthetic gene cluster of lasso peptides are revealed to distribute over genomes of proteobacteria and actinobacteria. However, the biosynthetic gene clusters are cryptic in most cases. Therefore, the combination of genome mining and heterologous production is efficient method for the production of lasso peptides. To utilize lasso peptide as fine chemical, there have been several attempts to add new function to lasso peptide by genetic engineering. Currently, a more efficient lasso peptide production system is being developed to harness cryptic biosynthetic gene clusters of lasso peptide. In this review, the overview of lasso peptide study is discussed.

Lasso Peptides: Heterologous Production and Potential Medical Application

Lasso peptides are natural products found in bacteria. They belong to a specific family of ribosomally-synthesized and posttranslationally-modified peptides with an unusual lasso structure. Lasso peptides possess remarkable thermal and proteolytic stability and various biological activities, such as antimicrobial activity, enzyme inhibition, receptor blocking, anticancer properties and HIV antagonism. They have promising potential therapeutic effects on gastrointestinal diseases, tuberculosis, Alzheimer’s disease, cardiovascular disease, fungal infections and cancer. Lasso peptides with high stability have been shown to be good carriers for other bioactive peptides. These make them attractive candidates for pharmaceutical research. This review aimed to describe the strategies used for the heterologous production of lasso peptides. Also, it indicated their therapeutical potential and their capacity to use as an efficient scaffold for epitope grafting.

Evidence of Cis/Trans-Isomerization at Pro7/Pro16 in the Lasso Peptide Microcin J25

Microcin J25 is a ribosomal synthesized and post-translationally modified peptide (RiPP) characterized by a mechanically interlocked topology called the lasso fold. This structure provides microcin J25 a potent antimicrobial activity resulting from internalization via the siderophore receptor FhuA and further inhibition of the RNA polymerase. In the present work, nuclear magnetic resonance (NMR) and trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) were used to investigate the lasso structure of microcin J25. NMR experiments showed that the lasso peptide microcin J25 can adopt conformational states where Pro16 can be found in the cis- and trans-orientations. The high-resolution mobility analysis, aided by site-directed mutagenesis ([P7A], [P16A], and [P7A/P16A] variants), demonstrated that microcin J25 can adopt cis/cis-, cis/trans-, trans/cis-, and trans/trans-conformations at the Pro7 and Pro16 peptide bonds. It was also shown that interconversion between the conformers can occur as a function of the starting solvent conditions and ion heating (collision-induced activation, CIA) despite the lasso topology. Complementary to NMR findings, the cis-conformations at Pro7 were assigned using TIMS-MS. This study highlights the analytical power of TIMS-MS and site-directed mutagenesis for the study of biological systems with large micro-heterogeneity as a way to further increase our understanding of the receptor-binding dynamics and biological activity.

Assessing the Flexibility of the Prochlorosin 2.8 Scaffold for Bioengineering Applications

Cyclization is a common strategy to confer proteolytic resistance to peptide scaffolds. Thus, cyclic peptides have been the focus of extensive bioengineering efforts. Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a superfamily of peptidic natural products that often contain macrocycles. In the RiPP family of lanthipeptides, macrocyclization is accomplished through formation of thioether cross-links between cysteines and dehydrated serines/threonines. The recent production of lanthipeptide libraries and development of methods to display lanthipeptides on yeast or phage highlights their potential for bioengineering and synthetic biology. In this regard, the prochlorosins are especially promising as the corresponding class II lanthipeptide synthetase ProcM matures numerous precursor peptides with diverse core peptide sequences. To facilitate future bioengineering projects, one of its native substrates, ProcA2.8, was subjected in this study to in-depth mutational analysis to test the limitations of ProcM-mediated cyclization. Alanine scan mutagenesis was performed on all residues within the two rings, and multiple prolines were introduced at various positions. Moreover, mutation, deletion, and insertion of residues in the region linking the two lanthionine rings was tested. Additional residues were also introduced or deleted from either ring, and inversion of ring forming residues was attempted to generate diastereomers. The findings were used for epitope grafting of the RGD integrin binding epitope within prochlorosin 2.8, resulting in a low nanomolar affinity binder of the αvβ3 integrin that was more stable toward proteolysis and displayed higher affinity than the linear counterpart.

Heterologous and in Vitro Reconstitution of Fuscanodin, a Lasso Peptide from Thermobifida fusca

Lasso peptides are a class of ribosomally derived natural products typified by their threaded rotaxane structure. The conversion of a linear precursor peptide into a lasso peptide structure requires two enzymatic activities: cleavage of the precursor via a cysteine protease and cyclization via isopeptide bond formation. In vitro studies of lasso peptide enzymology have been hampered by difficulties in obtaining pure, soluble enzymes. We reasoned that thermophilic bacteria would be a good source for well-behaved lasso peptide biosynthetic enzymes. The genome of the thermophilic actinobacterium Thermobifida fusca encodes for a lasso peptide with an unprecedented Trp residue at its N-terminus, a peptide we have named fuscanodin. Here we reconstitute fuscanodin biosynthesis in vitro with purified components, establishing a minimal fuscanodin synthetase. These experiments have allowed us to probe the kinetics of lasso peptide biosynthesis for the first time, and we report initial rates of fuscanodin biosynthesis. The fuscanodin biosynthetic enzymes are insensitive to substrate concentration and operate in a near single-turnover regime in vitro. While lasso peptides are often touted for their stability to both chaotropic and thermal challenges, fuscanodin is found to undergo a conformational change consistent with lasso peptide unthreading in organic solvents at room temperature.

Enzymatic Reconstitution and Biosynthetic Investigation of the Lasso Peptide Fusilassin

Lasso peptides are a class of ribosomally synthesized and post-translationally modified natural product which possess a unique lariat knot conformation. The low entropy "threaded" conformation endows lasso peptides with considerable resistance to heat and proteolytic degradation, which are attractive properties for the development of peptide-based therapeutics. Despite their discovery nearly 30 years ago, the molecular mechanism underlying lasso peptide biosynthesis remains poorly characterized due to the low stability of the purified biosynthetic enzymes. Here, we report the biosynthetic reconstitution of a lasso peptide derived from Thermobifida fusca, termed fusilassin. Beyond robust catalytic activity, the fusilassin enzymes demonstrate extraordinary substrate tolerance during heterologous expression in E. coli and upon purification in cell-free biosynthetic reconstitution reactions. We provide evidence that the fusilassin biosynthetic enzymes are not capable of forming branched-cyclic products but can produce entirely unrelated lasso peptides. Finally, we leveraged our bioinformatic survey of all lasso peptides identified in GenBank to perform coevolutionary analysis of two requisite biosynthetic proteins. This effort correctly identified residues governing an important protein-protein interaction, illustrating how genomic insight can accelerate the characterization of natural product biosynthetic pathways. The fusilassin enzymes described within represent a model system for both designing future lasso peptides of biomedical importance and also for elucidating the molecular mechanisms that govern lasso peptide biosynthesis.

Isolation and structure determination of a new lasso peptide specialicin based on genome mining

Based on genome mining, a new lasso peptide specialicin was isolated from the extract of Streptomyces specialis. The structure of specialicin was established by ESI-MS and NMR analyses to be a lasso peptide with the length of 21 amino acids, containing an isopeptide bond and two disulfide bonds in the molecule. The stereochemistries of the constituent amino acids except for Trp were determined to be L and the stereochemistry of Trp at C-terminus was determined to be D. Three dimensional structure of specialicin was determined based on NOE experimental data, which indicated that specialicin possessed the similar conformational structure with siamycin I. Specialicin showed the antibacterial activity against Micrococcus luteus and the moderate anti-HIV activity against HIV-1 NL4-3. The biosynthetic gene cluster of specialicin was proposed from the genome sequence data of S. specialis.

Heterologous production of a new lasso peptide brevunsin in Sphingomonas subterranea

A shuttle vector pHSG396Sp was constructed to perform gene expression using Sphingomonas subterranea as a host. A new lasso peptide biosynthetic gene cluster, derived from Brevundimonas diminuta, was amplified by PCR and integrated to afford a expression vector pHSG396Sp-12697L. The new lasso peptide brevunsin was successfully produced by S. subterranea, harboring the expression vector, with a high production yield (10.2 mg from 1 L culture). The chemical structure of brevunsin was established by NMR and MS/MS experiments. Based on the information obtained from the NOE experiment, the three-dimensional structure of brevunsin was determined, which indicated that brevunsin possessed a typical lasso structure. This expression vector system provides a new heterologous production method for unexplored lasso peptides that are encoded by bacterial genomes.

Investigation of the Biosynthesis of the Lasso Peptide Chaxapeptin Using an E. coli-Based Production System

Lasso peptides are natural products belonging to the family of ribosomally synthesized and posttranslationally modified peptides (RiPPs) and are defined by their unique topology. Even though lasso peptide biosynthetic gene clusters are found in many different kinds of bacteria, most of the hitherto studied lasso peptides were of proteobacterial or actinobacterial origin. Despite this, no E. coli-based production system has been reported for actinobacterial lasso peptides, while there are numerous examples of this for proteobacterial lasso peptides. Here, a heterologous production system of the lasso peptide chaxapeptin was established in E. coli. Chaxapeptin, originally isolated from Streptomyces leeuwenhoekii strain C58, is closely related to the lasso peptide sungsanpin (produced by a marine Streptomyces sp.) and shares its inhibitory activity against cell invasion by the human lung cancer cell line A549. Our production system not only allowed isolation of the mature lasso peptide outside of the native producer with a yield of 0.1 mg/L (compared to 0.7 mg/L from S. leeuwenhoekii) but also was used for a mutational study to identify residues in the precursor peptide that are important for biosynthesis. In addition to these experiments, the stability of chaxapeptin against thermal denaturation and proteases was assessed.

Metal ions induced secondary structure rearrangements: mechanically interlocked lassovs.unthreaded branched-cyclic topoisomers

Metal ions can play a significant role in a variety of important functions in protein systems including cofactor for catalysis, protein folding, assembly, structural stability and conformational change.

Lasso peptides: chemical approaches and structural elucidation

The diverse functionality and the extraordinary stability of lasso peptides make these molecules attractive scaffolds for drug discovery. The ability to generate lasso peptides chemically remains a challenging endeavor.

General rules of fragmentation evidencing lasso structures in CID and ETD

Lasso peptides are ribosomally synthesized and post-translationally modified peptides (RiPPs) characterized by a mechanically interlocked structure in which the C-terminal tail of the peptide is threaded and trapped within an N-terminal macrolactam ring.

A new genome-mining tool redefines the lasso peptide biosynthetic landscape

Ribosomally synthesized and post-translationally modified peptide (RiPP) natural products are attractive for genome-driven discovery and re-engineering, but limitations in bioinformatic methods and exponentially increasing genomic data make large-scale mining of RiPP data difficult. We report RODEO (Rapid ORF Description and Evaluation Online), which combines hidden-Markov-model-based analysis, heuristic scoring, and machine learning to identify biosynthetic gene clusters and predict RiPP precursor peptides. We initially focused on lasso peptides, which display intriguing physicochemical properties and bioactivities, but their hypervariability renders them challenging prospects for automated mining. Our approach yielded the most comprehensive mapping to date of lasso peptide space, revealing >1,300 compounds. We characterized the structures and bioactivities of six lasso peptides, prioritized based on predicted structural novelty, including one with an unprecedented handcuff-like topology and another with a citrulline modification exceptionally rare among bacteria. These combined insights significantly expand the knowledge of lasso peptides and, more broadly, provide a framework for future genome-mining efforts.

Signatures of Mechanically Interlocked Topology of Lasso Peptides by Ion Mobility-Mass Spectrometry: Lessons from a Collection of Representatives

Lasso peptides are characterized by a mechanically interlocked structure, where the C-terminal tail of the peptide is threaded and trapped within an N-terminal macrolactam ring. Their compact and stable structures have a significant impact on their biological and physical properties and make them highly interesting for drug development. Ion mobility - mass spectrometry (IM-MS) has shown to be effective to discriminate the lasso topology from their corresponding branched-cyclic topoisomers in which the C-terminal tail is unthreaded. In fact, previous comparison of the IM-MS data of the two topologies has yielded three trends that allow differentiation of the lasso fold from the branched-cyclic structure: (1) the low abundance of highly charged ions, (2) the low change in collision cross sections (CCS) with increasing charge state and (3) a narrow ion mobility peak width. In this study, a three-dimensional plot was generated using three indicators based on these three trends: (1) mean charge divided by mass (ζ), (2) relative range of CCS covered by all protonated molecules (ΔΩ/Ω) and (3) mean ion mobility peak width (δΩ). The data were first collected on a set of twenty one lasso peptides and eight branched-cyclic peptides. The indicators were obtained also for eight variants of the well-known lasso peptide MccJ25 obtained by site-directed mutagenesis and further extended to five linear peptides, two macrocyclic peptides and one disulfide constrained peptide. In all cases, a clear clustering was observed between constrained and unconstrained structures, thus providing a new strategy to discriminate mechanically interlocked topologies. Graphical Abstract ᅟ.

Thermal Unthreading of the Lasso Peptides Astexin-2 and Astexin-3

Lasso peptides are a class of knot-like polypeptides in which the C-terminal tail of the peptide threads through a ring formed by an isopeptide bond between the N-terminal amine group and a side chain carboxylic acid. The small size (∼20 amino acids) and simple topology of lasso peptides make them a good model system for studying the unthreading of entangled polypeptides, both with experiments and atomistic simulation. Here, we present an in-depth study of the thermal unthreading behavior of two lasso peptides astexin-2 and astexin-3. Quantitative kinetics and energetics of the unthreading process were determined for variants of these peptides using a series of chromatography and mass spectrometry experiments and biased molecular dynamics (MD) simulations. In addition, we show that the Tyr15Phe variant of astexin-3 unthreads via an unprecedented "tail pulling" mechanism. MD simulations on a model ring-thread system coupled with machine learning approaches also led to the discovery of physicochemical descriptors most important for peptide unthreading.

Insights into the Unique Phosphorylation of the Lasso Peptide Paeninodin

Lasso peptides are a new class of ribosomally synthesized and post-translationally modified peptides and thus far are only isolated from proteo- and actinobacterial sources. Typically, lasso peptide biosynthetic gene clusters encode enzymes for biosynthesis and export but not for tailoring. Here, we describe the isolation of the novel lasso peptide paeninodin from the firmicute Paenibacillus dendritiformis C454 and reveal within its biosynthetic cluster a gene encoding a kinase, which we have characterized as a member of a new class of lasso peptide-tailoring kinases. By employing a wide variety of peptide substrates, it was shown that this novel type of kinase specifically phosphorylates the C-terminal serine residue while ignoring those located elsewhere. These experiments also reveal that no other recognition motif is needed for efficient enzymatic phosphorylation of the C-terminal serine. Furthermore, through comparison with homologous HPr kinases and subsequent mutational analysis, we confirmed the essential catalytic residues. Our study reveals how lasso peptides are chemically diversified and sets the foundation for rational engineering of these intriguing natural products.