Peptidase Activation by a Leader Peptide-Bound RiPP Recognition Element

Substrate interactions guide cyclase engineering and lasso peptide diversification

Lasso peptides are a diverse class of naturally occurring, highly stable molecules kinetically trapped in a distinctive [1]rotaxane conformation. How the ATP-dependent lasso cyclase constrains a relatively unstructured substrate peptide into a low entropy product has remained a mystery owing to poor enzyme stability and activity in vitro. In this study, we combined substrate tolerance data with structural predictions, bioinformatic analysis, molecular dynamics simulations and mutational scanning to construct a model for the three-dimensional orientation of the substrate peptide in the lasso cyclase active site. Predicted peptide cyclase molecular contacts were validated by rationally engineering multiple, phylogenetically diverse lasso cyclases to accept substrates rejected by the wild-type enzymes. Finally, we demonstrate the utility of lasso cyclase engineering by robustly producing previously inaccessible variants that tightly bind to integrin αvβ8, which is a primary activator of transforming growth factor β and, thus, an important anti-cancer target.

Advances in lasso peptide discovery, biosynthesis, and function

Lasso peptides are a large and sequence-diverse class of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products characterized by their slip knot-like shape. These unique, highly stable peptides are produced by bacteria for various purposes. Their stability and sequence diversity make them a potentially useful scaffold for biomedically relevant folded peptides. However, many questions remain about lasso peptide biosynthesis, ecological function, and diversification potential for biomedical and agricultural applications. This review discusses new insights and open questions about lasso peptide biosynthesis and biological function. The role that genome mining has played in the development of new methodologies for discovering and diversifying lasso peptides is also discussed.

Micrococcin cysteine-to-thiazole conversion through transient interactions between the scaffolding protein TclI and the modification enzymes TclJ and TclN

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a broad group of compounds mediating microbial competition in nature. Azole/azoline heterocycle formation in the peptide backbone is a key step in the biosynthesis of many RiPPs. Heterocycle formation in RiPP precursors is often carried out by a scaffold protein, an ATP-dependent cyclodehydratase, and an FMN-dependent dehydrogenase. It has generally been assumed that the orchestration of these modifications is carried out by a stable complex including the scaffold, cyclodehydratase, and dehydrogenase. The antimicrobial RiPP micrococcin begins as a precursor peptide (TclE) with a 35-amino acid N-terminal leader and a 14-amino acid C-terminal core containing six Cys residues that are converted to thiazoles. The putative scaffold protein (TclI) presumably presents the TclE substrate to a cyclodehydratase (TclJ) and a dehydrogenase (TclN) to accomplish the two-step installation of the six thiazoles. In this study, we identify a minimal TclE leader region required for thiazole formation, demonstrate complex formation between TclI, TclJ, and TclN, and further define regions of these proteins required for complex formation. Our results point to a mechanism of thiazole installation in which TclI associates with the two enzymes in a mutually exclusive fashion, such that each enzyme competes for access to the peptide substrate in a dynamic equilibrium, thus ensuring complete modification of each Cys residue in the TclE core.

IMPORTANCE

Thiopeptides are a family of antimicrobial peptides characterized for having sulfur-containing heterocycles and for being highly post-translationally modified. Numerous thiopeptides have been identified; almost all of which inhibit protein synthesis in gram-positive bacteria. These intrinsic antimicrobial properties make thiopeptides promising candidates for the development of new antibiotics. The thiopeptide micrococcin is synthesized by the ribosome and undergoes several post-translational modifications to acquire its bioactivity. In this study, we identify key interactions within the enzymatic complex that carries out cysteine to thiazole conversion in the biosynthesis of micrococcin.

Structural features and substrate engagement in peptide-modifying radical SAM enzymes

In recent years, the biological significance of ribosomally synthesized, post-translationally modified peptides (RiPPs) and the intriguing chemistry catalyzed by their tailoring enzymes has garnered significant attention. A subgroup of bacterial radical S-adenosylmethionine (rSAM) enzymes can activate C-H bonds in peptides, which leads to the production of a diverse range of RiPPs. The remarkable ability of these enzymes to facilitate various chemical processes, to generate and harbor high-energy radical species, and to accommodate large substrates with a high degree of flexibility is truly intriguing. The wide substrate scope and diversity of the chemistry performed by rSAM enzymes raise one question: how does the protein environment facilitate these distinct chemical conversions while sharing a similar structural fold? In this review, we discuss recent advances in the field of RiPP-rSAM enzymes, with a particular emphasis on domain architectures and substrate engagements identified by biophysical and structural characterizations. We provide readers with a comparative analysis of six examples of RiPP-rSAM enzymes with experimentally characterized structures. Linking the structural elements and the nature of rSAM-catalyzed RiPP production will provide insight into the functional engineering of enzyme activity to harness their catalytic power in broader applications.

Novel types of RiPP-modifying enzymes

Novel discoveries in natural product biosynthesis reveal hidden bioactive compounds and expand our knowledge in enzymology. Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a rapidly growing class of natural products featuring diverse non-canonical amino acids introduced by maturation enzymes as a class-defining characteristic. Underexplored RiPP sources, such as the human microbiome, the oceans, uncultured microorganisms, and plants are rich hunting grounds for novel enzymology. Unusual α- and β-amino acids, peptide cleavages, lipidations, diverse macrocyclizations, and other features expand the range of chemical groups that are installed in RiPPs by often promiscuous enzymes. This review highlights the search for novelty in RiPP enzymology in the past two years, with respect to the discovery of new biochemical modifications but also towards novel applications.

Cell-free biosynthesis and engineering of ribosomally synthesized lanthipeptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a major class of natural products with diverse chemical structures and potent biological activities. A vast majority of RiPP gene clusters remain unexplored in microbial genomes, which is partially due to the lack of rapid and efficient heterologous expression systems for RiPP characterization and biosynthesis. Here, we report a unified biocatalysis (UniBioCat) system based on cell-free gene expression for rapid biosynthesis and engineering of RiPPs. We demonstrate UniBioCat by reconstituting a full biosynthetic pathway for de novo biosynthesis of salivaricin B, a lanthipeptide RiPP. Next, we delete several protease/peptidase genes from the source strain to enhance the performance of UniBioCat, which then can synthesize and screen salivaricin B variants with enhanced antimicrobial activity. Finally, we show that UniBioCat is generalizable by synthesizing and evaluating the bioactivity of ten uncharacterized lanthipeptides. We expect UniBioCat to accelerate the discovery, characterization, and synthesis of RiPPs.

One Descriptor to Fold Them All: Harnessing Intuition and Machine Learning to Identify Transferable Lasso Peptide Reaction Coordinates

Identifying optimal reaction coordinates for complex conformational changes and protein folding remains an outstanding challenge. This study combines collective variable (CV) discovery based on chemical intuition and machine learning with enhanced sampling to converge the folding free energy landscape of lasso peptides, a unique class of natural products with knot-like tertiary structures. This knotted scaffold imparts remarkable stability, making lasso peptides resistant to proteolytic degradation, thermal denaturation, and extreme pH conditions. Although their direct synthesis would enable therapeutic design, it has not yet been possible due to the improbable occurrence of spontaneous lasso folding. Thus, simulations characterizing the folding propensity are needed to identify strategies for increasing access to the lasso architecture by stabilizing the pre-lasso ensemble before isopeptide bond formation. Herein, harmonic linear discriminant analysis (HLDA) is combined with metadynamics-enhanced sampling to discover CVs capable of distinguishing the pre-lasso fold and converging the folding propensity. Intuitive CVs are compared to iterative rounds of HLDA to identify CVs that not only accomplish these goals for one lasso peptide but also seem to be transferable to others, establishing a protocol for the identification of folding reaction coordinates for lasso peptides.

Tryptophan-Centric Bioinformatics Identifies New Lasso Peptide Modifications

Lasso peptides are a class of ribosomally synthesized and post-translationally modified peptides (RiPPs) defined by a macrolactam linkage between the N-terminus and the side chain of an internal aspartic acid or glutamic acid residue. Instead of adopting a branched-cyclic conformation, lasso peptides are "threaded", with the C-terminal tail passing through the macrocycle to present a kinetically trapped rotaxane conformation. The availability of enhanced bioinformatics methods has led to a significant increase in the number of secondary modifications found on lasso peptides. To uncover new ancillary modifications in a targeted manner, a bioinformatic strategy was developed to discover lasso peptides with modifications to tryptophan. This effort identified numerous putative lasso peptide biosynthetic gene clusters with core regions of the precursor peptides enriched in tryptophan. Parsing of these tryptophan (Trp)-rich biosynthetic gene clusters uncovered several putative ancillary modifying enzymes, including halogenases and dimethylallyltransferases expected to act upon Trp. Characterization of two gene products yielded a lasso peptide with two 5-Cl-Trp modifications (chlorolassin) and another bearing 5-dimethylallyl-Trp and 2,3-didehydro-Tyr modifications (wygwalassin). Bioinformatic analysis of the requisite halogenase and dimethylallyltransferase revealed numerous other putative Trp-modified lasso peptides that remain uncharacterized. We anticipate that the Trp-centric strategy reported herein may be useful in discovering ancillary modifications for other RiPP classes and, more generally, guide the functional prediction of enzymes that act on specific amino acids.

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.

Lasso Peptides: Exploring the Folding Landscape of Nature's Smallest Interlocked Motifs

Lasso peptides make up a class of natural products characterized by a threaded structure. Given their small size and stability, chemical synthesis would offer tremendous potential for the development of novel therapeutics. However, the accessibility of the pre-folded lasso architecture has limited this advance. To better understand the folding process de novo, simulations are used herein to characterize the folding propensity of microcin J25 (MccJ25), a lasso peptide known for its antimicrobial properties. New algorithms are developed to unambiguously distinguish threaded from nonthreaded precursors and determine handedness, a key feature in natural lasso peptides. We find that MccJ25 indeed forms right-handed pre-lassos, in contrast to past predictions but consistent with all natural lasso peptides. Additionally, the native pre-lasso structure is shown to be metastable prior to ring formation but to readily transition to entropically favored unfolded and nonthreaded structures, suggesting that de novo lasso folding is rare. However, by altering the ring forming residues and appending thiol and thioester functionalities, we are able to increase the stability of pre-lasso conformations. Furthermore, conditions leading to protonation of a histidine imidazole side chain further stabilize the modified pre-lasso ensemble. This work highlights the use of computational methods to characterize lasso folding and demonstrates that de novo access to lasso structures can be facilitated by optimizing sequence, unnatural modifications, and reaction conditions like pH.

Proteases Involved in Leader Peptide Removal during RiPP Biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) have received much attention in recent years because of their promising bioactivities and the portability of their biosynthetic pathways. Heterologous expression studies of RiPP biosynthetic enzymes identified by genome mining often leave a leader peptide on the final product to prevent toxicity to the host and to allow the attachment of a genetically encoded affinity purification tag. Removal of the leader peptide to produce the mature natural product is then carried out in vitro with either a commercial protease or a protease that fulfills this task in the producing organism. This review covers the advances in characterizing these latter cognate proteases from bacterial RiPPs and their utility as sequence-dependent proteases. The strategies employed for leader peptide removal have been shown to be remarkably diverse. They include one-step removal by a single protease, two-step removal by two dedicated proteases, and endoproteinase activity followed by aminopeptidase activity by the same protease. Similarly, the localization of the proteolytic step varies from cytoplasmic cleavage to leader peptide removal during secretion to extracellular leader peptide removal. Finally, substrate recognition ranges from highly sequence specific with respect to the leader and/or modified core peptide to nonsequence specific mechanisms.

Bioinformatics-guided discovery of biaryl-linked lasso peptides

Lasso peptides are a class of ribosomally synthesized and post-translationally modified peptides (RiPPs) that feature an isopeptide bond and a distinct lariat fold. A growing number of secondary modifications have been described that further decorate lasso peptide scaffolds. Using genome mining, we have discovered a pair of lasso peptide biosynthetic gene clusters (BGCs) that include cytochrome P450 genes. Using mass spectrometry, stable isotope incorporation, and extensive 2D-NMR spectrometry, we report the structural characterization of two unique examples of (C-N) biaryl-linked lasso peptides. Nocapeptin A, from Nocardia terpenica, is tailored with a Trp-Tyr crosslink, while longipepetin A, from Longimycelium tulufanense, features a Trp-Trp linkage. Besides the unusual bicyclic frame, a Met of longipepetin A undergoes S-methylation to yield a trivalent sulfonium, a heretofore unprecedented RiPP modification. A bioinformatic survey revealed additional lasso peptide BGCs containing P450 enzymes which await future characterization. Lastly, nocapeptin A bioactivity was assessed against a panel of human and bacterial cell lines with modest growth-suppression activity detected towards Micrococcus luteus.

Micrococcin cysteine-to-thiazole conversion through transient interactions between a scaffolding protein and two modification enzymes

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a broad group of compounds mediating microbial competition in nature. Azole/azoline heterocycle formation in the peptide backbone is a key step in the biosynthesis of many RiPPs. Heterocycle formation in RiPP precursors is often carried out by a scaffold protein, an ATP-dependent cyclodehydratase, and an FMN-dependent dehydrogenase. It has generally been assumed that the orchestration of these modifications is carried out by a stable complex including the scaffold, cyclodehydratase and dehydrogenase. The antimicrobial RiPP micrococcin begins as a precursor peptide (TclE) with a 35-amino acid N-terminal leader and a 14-amino acid C-terminal core containing six Cys residues that are converted to thiazoles. The putative scaffold protein (TclI) presumably presents the TclE substrate to a cyclodehydratase (TclJ) and a dehydrogenase (TclN) to accomplish the two-step installation of the six thiazoles. In this study, we identify a minimal TclE leader region required for thiazole formation, we demonstrate complex formation between TclI, TclJ and TclN, and further define regions of these proteins required for complex formation. Our results point to a mechanism of thiazole installation in which TclI associates with the two enzymes in a mutually exclusive fashion, such that each enzyme competes for access to the peptide substrate in a dynamic equilibrium, thus ensuring complete modification of each Cys residue in the TclE core.

Leader peptide removal in lasso peptide biosynthesis based on penultimate isoleucine residue

Lasso peptides are ribosomally synthesized peptides that undergo post-translational modifications including leader peptide removal by B (or the segregated B1 and B2) proteins and core peptide macrolactamization by C proteins to form a unique lariat topology. A conserved threonine residue at the penultimate position of leader peptide is hitherto found in lasso peptide precursors and shown to be a critical recognition element for effective enzymatic processing. We identified a lasso peptide biosynthetic gene cluster (bsf) from Bradymonas sediminis FA350, a Gram-negative and facultatively prey-dependent bacterium that belongs to a novel bacterial order Bradymonadales in the class Deltaproteobacteria. The kinase BsfK specifically catalyzes the phosphorylation of the precursor peptide BsfA on the Ser3 residue. BsfB1 performs dual functions to accelerate the post-translational phosphorylation and assist BsfB2 in leader peptide removal. Most importantly, the penultimate residue of leader peptide is an isoleucine rather than the conserved threonine and this isoleucine has a marked impact on the phosphorylation of Ser3 as well as leader peptide removal, implying that BsfB1 and BsfB2 exhibit a new substrate selectivity for leader peptide binding and excision. This is the first experimentally validated penultimate isoleucine residue in a lasso peptide precursor to our knowledge. In silico analysis reveals that the leader peptide Ile/Val(-2) residue is rare but not uncommon in phosphorylated lasso peptides, as this residue is also discovered in Acidobacteriaceae and Sphingomonadales in addition to Bradymonadales.