Lasso peptides are natural products with a unique three dimensional structure resembling a lariat knot. They are from ribosomal origin and are post-translationally modified by two enzymes (B and C), one of which shares little similarity to enzymes outside of lasso peptide biosynthetic gene clusters and as such is a useful target for genome mining. In this study, we demonstrate a B protein-centric genome mining approach through which we were able to identify 102 putative lasso peptide biosynthetic gene clusters from a total of 87 different proteobacterial strains. Ten of these clusters were cloned into the pET41a expression vector, optimized through incorporation of a ribosomal binding site and heterologously expressed in Escherichia coli BL21(DE3). All 12 predicted lasso peptides (namely burhizin, caulonodin I, caulonodin II, caulonodin III, rhodanodin, rubrivinodin, sphingonodin I, sphingonodin II, syanodin I, sphingopyxin I, sphingopyxin II, and zucinodin) were detected by high-resolution Fourier transform mass spectrometry and their proposed primary structure was confirmed through tandem mass spectrometry. High yields (ranging from 0.4 to 5.2 mg/L) were observable for eight of these compounds, while thermostability assays revealed five new representatives of heat labile lasso peptides.

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 belong to the class of ribosomally synthesized and post-translationally modified peptides. Their common distinguishing feature is an N-terminal macrolactam ring that is threaded by the C-terminal tail. This lasso fold is maintained through steric interactions. The isolation and characterization of xanthomonins I-III, the first lasso peptides featuring macrolactam rings consisting of only seven amino acids, is now presented. The crystal structure of xanthomonin I and the NMR structure of xanthomonin II were also determined. A total of 25 variants of xanthomonin II were generated to probe different aspects of the biosynthesis, stability, and fold maintenance. These mutational studies reveal the limits such a small ring imposes on the threading and show that every plug amino acid larger than serine is able to maintain a heat-stable lasso fold in the xanthomonin II scaffold.

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.

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) constitutes a major worldwide public health threat and economic burden. The pandemic is still ongoing and the SARS-CoV-2 variants are still emerging constantly, resulting in an urgent demand for new drugs to treat this disease. Molnupiravir, a biological prodrug of NHC (β-D-N(4)-hydroxycytidine), is a novel nucleoside analogue with a broad-spectrum antiviral activity against SARS-CoV, SARS-CoV-2, Middle East respiratory syndrome coronavirus (MERS-CoV), influenza virus, respiratory syncytial virus (RSV), bovine viral diarrhea virus (BVDV), hepatitis C virus (HCV) and Ebola virus (EBOV). Molnupiravir showed potent therapeutic and prophylactic activity against multiple coronaviruses including SARS-CoV-2, SARS-CoV, and MERS-CoV in animal models. In clinical trials, molnupiravir showed beneficial effects for mild to moderate COVID-19 patients with a favorable safety profile. The oral bioavailability and potent antiviral activity of molnupiravir highlight its potential utility as a therapeutic candidate against COVID-19. This review presents the research progress of molnupiravir starting with its discovery and synthesis, broad-spectrum antiviral effects, and antiviral mechanism. In addition, the preclinical studies, antiviral resistance, clinical trials, safety, and drug tolerability of molnupiravir are also summarized and discussed, aiming to expand our knowledge on molnupiravir and better deal with the COVID-19 epidemic.

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.

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 sphingopyxin I (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.

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