Nonribosomal peptides are a structurally diverse and bioactive class of natural products constructed by multidomain enzymatic assembly lines known as nonribosomal peptide synthetases (NRPSs). While the core catalytic domains and even entire protein subunits of NRPSs have been structurally elucidated, little biophysical work has been reported on the docking domains that promote interactions—and thus transfer of biosynthetic intermediates—between subunits. In the present study, we closely examine the COM domains that mediate COMmunication between donor epimerization (E) and acceptor condensation (C) domains found at the termini of NRPS subunits. Through a combination of X-ray crystallography, circular dichroism spectroscopy, solution- and solid-state NMR spectroscopy, and molecular dynamics (MD) simulations, we provide direct evidence for an intrinsically disordered donor COM region that folds into a dynamic helical motif upon binding to a suitable acceptor. Furthermore, our NMR titration and carbene footprinting experiments illuminate the residues involved at the COM interaction interface, and our MD simulations demonstrate folding consistent with experimental data. Although our results lend credence to the previously proposed helix-hand mode of interaction, they also underscore the importance of viewing COM interfaces as dynamic ensembles rather than single rigid structures and suggest that engineering experiments should account for the interactions which transiently guide folding in addition to those which stabilize the final complex. Through activity assays and affinity measurements, we further substantiate the role of the donor COM region in binding the acceptor C domain and implicate this short motif as readily transposable for noncognate domain crosstalk. Finally, our bioinformatics analyses show that COM domains are widespread in natural product pathways and function at interfaces beyond the canonical type described above, setting a high priority for thorough characterization of these docking domains. Our findings lay the groundwork for future attempts to rationally engineer NRPS domain–domain interactions with the ultimate goal of generating bioactive molecules.

Genome mining identified the fungal-bacterial endosymbiosis Rhizopus microsporus-Mycetohabitans (previously Burkholderia) rhizoxinica as a rich source of novel natural products. However, most of the predicted compounds have remained cryptic. In this study, we employed heterologous expression to isolate and characterize three ribosomally synthesized and post-translationally modified peptides with lariat topology (lasso peptides) from the endosymbiont M. rhizoxinica: burhizin-23, mycetohabin-16, and mycetohabin-15. Through coexpression experiments, it was shown that an orphan gene product results in mature mycetohabin-15, albeit encoded remotely from the core biosynthetic gene cluster. Comparative genomics revealed that mycetohabins are highly conserved among M. rhizoxinica and related endosymbiotic bacteria. Gene knockout and reinfection experiments indicated that the lasso peptides are not crucial for establishing symbiosis; instead, the peptides are exported into the environment during endosymbiosis. This is the first report on lasso peptides from endosymbiotic bacteria.

Lasso peptides constitute a structurally unique class of 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. Tandem mass spectrometry using collision induced dissociation (CID) and electron capture dissociation (ECD) have shown previously different fragmentation patterns for capistruin, microcin J25 and their corresponding branched-cyclic forms in which the C-terminal tail is unthreaded. In order to develop general rules that unambiguously discriminate the lasso and branched-cyclic topologies, this report presents experimental evidence for a set of twenty-one lasso peptides analyzed by CID and electron transfer dissociation (ETD). CID experiments on lasso peptides specifically yielded mechanically interlocked species with associated bi and yj fragments. For class II lasso peptides, these lasso-specific fragments were observed only for peptides in which the loop, located above the macrolactam ring, was strictly longer than four amino acid residues. For class I and III lasso peptides, part of the C-terminal tail remains covalently linked to the macrolactam ring by disulfide bonds; associated bi and yj fragments therefore do not clearly constitute a signature of the lasso topology. ETD experiments of lasso peptides showed a significant increase of hydrogen migration events in the loop region when compared to their branched-cyclic topoisomers, leading to the formation of specific ci˙/z'j fragments for all lasso peptides, regardless of their class and loop size. Our experiments enabled us to establish general rules for obtaining structural details from CID and ETD fragmentation patterns, obviating the need for structure determination by NMR or X-ray crystallography.

Nosocomial pathogens can cause life-threatening infections in neonates and immunocompromised patients. E. bugandensis (EB-247) is a recently described species of Enterobacter, associated with neonatal sepsis. Here we demonstrate that the extended spectrum ß-lactam (ESBL) producing isolate EB-247 is highly virulent in both Galleria mellonella and mouse models of infection. Infection studies in a streptomycin-treated mouse model showed that EB-247 is as efficient as Salmonella Typhimurium in inducing systemic infection and release of proinflammatory cytokines. Sequencing and analysis of the complete genome and plasmid revealed that virulence properties are associated with the chromosome, while antibiotic-resistance genes are exclusively present on a 299 kb IncHI plasmid. EB-247 grew in high concentrations of human serum indicating septicemic potential. Using whole genome-based transcriptome analysis we found 7% of the genome was mobilized for growth in serum. Upregulated genes include those involved in the iron uptake and storage as well as metabolism. The lasso peptide microcin J25 (MccJ25), an inhibitor of iron-uptake and RNA polymerase activity, inhibited EB-247 growth. Our studies indicate that Enterobacter bugandensis is a highly pathogenic species of the genus Enterobacter. Further studies on the colonization and virulence potential of E. bugandensis and its association with septicemic infection is now warranted.

Bacteria under stress produce ribosomally synthesized and post-translationally modified peptides (RiPPs) to target closely related species, such as the lasso peptide microcin J25 (MccJ25). These peptides are also toxic to the producing organisms that utilize dedicated ABC transporters to achieve self-immunity. MccJ25 is exported by the Escherichia coli ABC transporter McjD through a complex mechanism of recognition that has remained elusive. Here, we used biomolecular NMR to study this interaction and identified a region of the toxic peptide that is crucial to its recognition by the ABC transporter. Our study provides evidence that McjD is highly specific to MccJ25 and not to other RiPPs or antibiotics, unlike multidrug ABC transporters. Additionally, we show that MccJ25 is not exported by another natural product ABC transporter. Therefore, we propose that specific interactions between natural product ABC transporters and their substrate provides them with their high degree of specificity. Taken together, these findings suggest that ABC transporters might have acquired structural elements in their binding cavity to recognize and allow promiscuous export of a larger variety of compounds.

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.

Novel tailor-made artificial non-ribosomal peptide syn thetases (NRPSs) for non-ribosomal synthesis and/or modi fication of peptides of a predetermined length and composi tion and/or for modification of individual amino acids are described. The fusion of building units of said peptide syn thetases in particular linker regions makes it possible to spe cifically prepare by means of "modular molecule construc tion kits' NRPSs which are capable of synthesizing peptides of a desired structure. US 2010/0297751A1 Nov. 25, 2010 Patent Application Publication

Nonribosomal peptide synthetases (NRPSs) are a family of multidomain, multimodule enzymes that synthesize structurally and functionally diverse peptides, many of which are of great therapeutic or commercial value. The central chemical step of peptide synthesis is amide bond formation, which is typically catalyzed by the condensation (C) domain. In many NRPS modules, the C domain is replaced by the heterocyclization (Cy) domain, a homologous domain that performs two consecutive reactions by using hitherto unknown catalytic mechanisms. It first catalyzes amide bond formation, and then the intramolecular cyclodehydration between a Cys, Ser, or Thr side chain and the backbone carbonyl carbon to form a thiazoline, oxazoline, or methyloxazoline ring. The rings are important for the form and function of the peptide product. We present the crystal structure of an NRPS Cy domain, Cy2 of bacillamide synthetase, at a resolution of 2.3 Å. Despite sharing the same fold, the active sites of C and Cy domains have important differences. The structure allowed us to probe the roles of active-site residues by using mutational analyses in a peptide synthesis assay with intact bacillamide synthetase. The drastically different effects of these mutants, interpreted by using our structural and bioinformatic results, provide insight into the catalytic mechanisms of the Cy domain and implicate a previously unexamined Asp-Thr dyad in catalysis of the cyclodehydration reaction.

The biosynthesis of iron sulfur (Fe-S) clusters in Bacillus subtilis is mediated by a SUF-type gene cluster, consisting of the cysteine desulfurase SufS, the scaffold protein SufU, and the putative chaperone complex SufB/SufC/SufD. Here, we present the high-resolution crystal structure of the SufS homodimer in its product-bound state (i.e., in complex with pyrodoxal-5'-phosphate, alanine, Cys361-persulfide). By performing hydrogen/deuterium exchange (H/DX) experiments, we characterized the interaction of SufS with SufU and demonstrate that SufU induces an opening of the active site pocket of SufS. Recent data indicate that frataxin could be involved in Fe-S cluster biosynthesis by facilitating iron incorporation. H/DX experiments show that frataxin indeed interacts with the SufS/SufU complex at the active site. Our findings deepen the current understanding of Fe-S cluster biosynthesis, a complex yet essential process, in the model organism B. subtilis.

Lasso peptides are characterized by their peculiar lariat knot‐like structure. Except for maturation of this fold, post‐translational modifications of lasso peptides are rare. However, we recently delineated the biosynthetic pathway of a post‐translationally phosphorylated lasso peptide, paeninodin. In this study, further investigation of two kinases revealed their ability to transfer multiple phosphate groups onto precursor peptide substrates, ultimately leading to polyphosphorylated lasso peptides. We found that this polyphosphorylating activity depended on the identity of the phosphate donor and the sequence of the precursor peptide. Our investigations provide new insight into the remarkable strategies for chemical diversification employed by the lasso peptide biosynthetic machinery.

Lasso peptides are a class of ribosomally synthesized and post-translationally modified peptides (RiPPs) with a unique lariat knot-like fold that endows them with extraordinary stability and biologically relevant activity. However, the biosynthetic mechanism of these fascinating molecules remains largely speculative. Generally, two enzymes (B for processing and C for cyclization) are required to assemble the unusual knot-like structure. Several subsets of lasso peptide gene clusters feature a "split" B protein on separate open reading frames (B1 and B2), suggesting distinct functions for the B protein in lasso peptide biosynthesis. Herein, we provide new insights into the role of the RiPP recognition element (RRE) PadeB1, characterizing its capacity to bind the paeninodin leader peptide and deliver its peptide substrate to PadeB2 for processing.

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.

Lasso peptides are natural products that assume a unique lariat knot topology. Lasso peptide isopeptidases (IsoPs) eliminate this topology through isopeptide bond cleavage. To probe how these enzymes distinguish between substrates and hydrolyze only isopeptide bonds, we examined the structure and mechanism of a previously uncharacterized IsoP from the proteobacterium Sphingopyxis alaskensis RB2256 (SpI-IsoP). We demonstrate that SpI-IsoP efficiently and specifically linearizes the lasso peptide sphingopyxinI (SpI) and variants thereof. We also present crystal structures of SpI and SpI-IsoP, revealing a threaded topology for the former and a prolyl oligopeptidase (POP)-like fold for the latter. Subsequent structure-guided mutational analysis allowed us to propose roles for active-site residues. Our study sheds light on lasso peptide catabolism and expands the engineering potential of these fascinating molecules.

This viewpoint article focuses on the structures of the dissected catalytic domains of non-ribosomal peptide synthetases (NRPSs) associated with substrate selection and activation (A domain), substrate shuttling among the active sites (PCP domain), peptide bond formation (C domain) and product release (TE domain). Structural details of these essential components of the NRPS machinery, integrated in a didomain (PCP-C) and an elongation module (C-A-PCP), were used to generate a model for a multimodular NRPS assembly line.

Lasso peptides are fascinating natural products with a unique structural fold that can exhibit tremendous thermal stability. Here, we investigate factors responsible for the thermal stability of caulosegnin II. By employing X-ray crystallography, mutational analysis and molecular dynamics simulations, the ring residue proline 8 was proven to be crucial for thermal stability.