Structure, biosynthesis, and bioactivity of nostolysamides

This study elucidates the structure, biosynthesis, and bioactivity of nostolysamides, a novel class of acylated lanthipeptides from *Nostoc punctiforme*, revealing their unique ring topology, the non-essential role of acylation for their membrane-disrupting antibacterial and antifungal activities, and the broad substrate specificity of the acyltransferase NpuN.

The Gist

The Big Picture: Finding a New "Fungus-Fighter" in the Microscopic World

Imagine the human body as a bustling city. Sometimes, unwanted guests like Candida (a type of yeast that causes infections) move in and cause trouble. We need new weapons to fight them, especially because old weapons (antifungal drugs) are becoming less effective.

Scientists went on a digital treasure hunt through the genomes of bacteria (specifically a blue-green algae called Nostoc) to find new natural weapons. They found a set of instructions (a gene cluster) that looked promising but had never been fully understood. They named the resulting weapon Nostolysamide.

The Story of the "Lanthipeptide" Factory

Think of the bacteria as a tiny factory. Inside this factory, there is a specific assembly line designed to build a special kind of peptide (a small protein chain).

1. The Blueprint (NpuA): The factory starts with a raw blueprint called a precursor peptide. It's like a long, floppy piece of clay with instructions written all over it.
2. The Sculptor (NpuM): A special machine called an enzyme (NpuM) comes along. Its job is to take that floppy clay and twist it into a rigid, complex shape. It does this by:
   • Drying it out: Removing water molecules to create double bonds (like drying clay to make it hard).
   • Stitching it: Sewing the ends of the clay together to form loops or rings.
   • The Result: The final product is a "lanthipeptide"—a peptide with a unique, knotted structure made of sulfur bridges. Think of it like a molecular pretzel that is very hard to break apart.

The Mystery of the Rings

The scientists had a puzzle: How exactly are these rings tied?

• They knew there were four rings in the final knot.
• To figure out the pattern, they played a game of "what if." They used genetic engineering to remove specific "stitching points" (cysteine amino acids) in the blueprint.
• The Analogy: Imagine trying to figure out how a complex knot is tied by cutting one string at a time. When they cut the string at the very end (Cys25), the knot fell apart in a specific way that revealed the pattern of the other three rings.
• The Discovery: They found the knot has one small, non-overlapping ring at the start, and three overlapping rings at the end, creating a tight, complex cage.

The "Lipid" Question: Does the Tail Matter?

The original gene instructions also included a second machine (NpuN), which is like a painter. Its job is to attach a long, greasy "tail" (a fatty acid) to the front of the peptide.

• The Expectation: Usually, adding a greasy tail helps a drug stick to cell membranes better, making it more powerful. The scientists expected this tail to be essential for the weapon to work.
• The Surprise: When they tested the peptide with the tail and without the tail, they found no difference in how well it killed the fungus.
• The Takeaway: The "greasy tail" isn't needed for the weapon to work in the lab. It might be there for other reasons in nature (like helping the bacteria survive in its own environment), but for killing the fungus, the knotted shape is what matters.

How It Kills the Fungus

So, how does this molecular pretzel actually kill the Candida fungus?

• The Mechanism: It doesn't attack the fungus's internal machinery. Instead, it acts like a molecular crowbar.
• The Action: It pokes holes in the fungus's outer skin (cell membrane).
• The Result: The fungus leaks its insides out and dies. This is a very effective way to kill cells because it's hard for them to develop resistance to having their walls smashed in.

Why This Matters

1. New Weapon: This is the first time a "Class II" lanthipeptide has been found to kill fungi. Most of these molecules are known for killing bacteria, not fungi.
2. Structure Confirmed: They finally figured out the exact 3D shape of the molecule, which is crucial for designing better drugs in the future.
3. Future Potential: Even though the "greasy tail" didn't help in the lab, understanding how this molecule works gives scientists a new blueprint. They can now try to build synthetic versions of this "molecular crowbar" to treat stubborn fungal infections in humans.

In a nutshell: Scientists found a new, naturally occurring "molecular pretzel" made by a cyanobacterium. They figured out its exact shape, discovered it kills fungi by punching holes in their cell walls, and learned that a fatty tail attached to it isn't necessary for the killing power. It's a promising new lead in the fight against drug-resistant fungal infections.

Technical Summary

1. Problem Statement

• Global Health Context: Invasive fungal infections, particularly those caused by Candida species, pose a significant global health threat with high mortality rates and rising antifungal resistance. There is an urgent need for novel antifungal agents.
• Scientific Gap: While ribosomally synthesized and post-translationally modified peptides (RiPPs), specifically lanthipeptides, are a rich source of bioactive compounds, most known antifungal lanthipeptides are rare.
• Specific Challenge: A previous genome mining study identified a biosynthetic gene cluster (BGC) in the cyanobacterium Nostoc punctiforme PCC 73102 (the npu cluster) encoding a class II lanthipeptide. This cluster includes a precursor peptide (NpuA), a synthetase (NpuM), and a GNAT acetyltransferase (NpuN) predicted to lipidate the peptide. However, the structure, ring pattern, and bioactivity of the resulting peptide (termed "nostolysamides") remained undetermined due to extremely low production levels in heterologous expression systems.

2. Methodology

The authors employed a combination of synthetic biology, structural biology, and biochemical assays to characterize the nostolysamides:

• Heterologous Expression & Production Optimization:
  • Co-expressed the precursor peptide NpuA with the class II lanthipeptide synthetase NpuM in Escherichia coli.
  • To overcome low yields and solubility issues, an N-terminal SUMO tag was fused to NpuA (His6-SUMO-NpuA).
  • The leader peptide was removed in vitro using the protease LahT150 (a homolog of the native PCAT protease NpuT).
• Structural Elucidation:
  • Mass Spectrometry: Used MALDI-TOF and LC-MS/MS (Collision-Activated Dissociation) with the Interactive Peptide Spectral Annotator (IPSA) to determine dehydration and cyclization patterns.
  • Site-Directed Mutagenesis: Created variants (e.g., C25A, C14A, C20A, S4A) to disrupt specific rings and deduce the connectivity of overlapping rings via fragmentation analysis.
  • Stereochemistry: Employed Marfey's analysis (using L-FDLA derivatization) to determine the stereochemical configuration (D/L) of the lanthionine (Lan) and methyllanthionine (MeLan) residues.
• Acylation Studies:
  • Co-expressed NpuN (a GNAT acetyltransferase) with NpuA/NpuM to study lipidation.
  • Performed in vitro acylation assays using His-SUMO-NpuN and various acyl-CoA substrates to determine substrate specificity and regioselectivity (specifically targeting Lys1).
• Bioactivity Assays:
  • Antimicrobial/Antifungal Screening: Agar diffusion and broth dilution assays against Gram-positive/negative bacteria and Candida species.
  • Mode of Action:
    • LiaRS Assay: To test for lipid II binding (cell wall targeting).
    • Time-Kill Assays: To determine fungistatic vs. fungicidal activity.
    • Membrane Depolarization: Used the lipophilic cationic dye DiSC3(5) to assess membrane integrity disruption.

3. Key Contributions & Results

A. Structural Characterization of Nostolysamide C

• Production: The SUMO-fusion strategy successfully improved production yields, enabling structural analysis.
• Ring Pattern: The peptide contains four rings formed from four Cysteine residues and three Serine/Threonine residues.
  • Ring A: Non-overlapping N-terminal ring formed between Cys5 and Thr9 (MeLan).
  • Rings B, C, D: Three overlapping rings at the C-terminus. Mutagenesis and MS data propose the pattern: Cys14–Ser18, Cys20–Ser23, and Cys25–Thr12 (MeLan).
  • Stereochemistry: All four (methyl)lanthionine residues possess the DL configuration.
• NpuM Specificity: NpuM belongs to the ProcM-clade of class II LanM enzymes. Unlike typical LanMs that cyclize C-to-N, NpuM demonstrates bidirectional cyclization (both C-to-N and N-to-C), dictated by the substrate sequence rather than the enzyme alone.

B. Biosynthesis of Lipidated Variants (Nostolysamides A & B)

• Regioselectivity: The GNAT enzyme NpuN specifically acylates the side chain of Lys1 in the core peptide. Mutations of Lys1 to Ala abolished acylation, while Lys3 mutations did not.
• Substrate Scope: In vitro, NpuN accepts various acyl-CoAs (acetyl, lauroyl, myristoyl, oleoyl, palmitoyl).
• Native Products: Co-expression in E. coli yielded peptides with hexadecenoyl and hydroxyhexadecenoyl groups (C16), suggesting these are the physiological products.

C. Bioactivity and Mode of Action

• Antifungal Activity: The non-acylated product (Nostolysamide C) exhibits potent antifungal activity against Candida albicans, C. glabrata, and C. tropicalis (MIC ~6.25 µM). It is fungicidal, reducing colony-forming units over time.
• Antibacterial Activity: Shows activity against Bacillus subtilis but weak activity against Gram-negatives.
• Mechanism:
  • Not Lipid II: The LiaRS assay confirmed Nostolysamide C does not bind lipid II, distinguishing it from many antibacterial lantibiotics like nisin.
  • Membrane Disruption: The compound causes rapid membrane depolarization in Candida tropicalis, indicating a membrane-targeting mechanism similar to the known antifungal lanthipeptide Pinensins.
• Role of Acylation: Surprisingly, acylation is not required for antifungal or antibacterial activity. Acylated variants showed similar potency to the non-acylated form, suggesting the lipid tail does not enhance bioactivity in the tested conditions.
• Structure-Activity Relationship: The C25A mutant (lacking the largest macrocycle) retained activity but with a >10-fold increase in MIC, indicating the tetracyclic structure is crucial for full potency.

4. Significance

• First Class II Antifungal Lanthipeptide: This study identifies the first class II lanthipeptide with significant antifungal activity against Candida species, expanding the known functional diversity of this class.
• Novel Ring Topology: The discovery of bidirectional cyclization by NpuM (ProcM-clade) and the specific overlapping ring pattern adds to the understanding of lanthipeptide structural diversity.
• Lipidation Insights: The work challenges the general assumption that lipidation is essential for the bioactivity of RiPPs. While NpuN is a functional GNAT, its acylation of Lys1 appears dispensable for the observed antimicrobial effects, suggesting potential roles in self-resistance, stability in the native environment, or interactions with specific co-habitants of Nostoc.
• Therapeutic Potential: Given the rise of antifungal resistance, nostolysamides represent a promising new scaffold for developing membrane-targeting antifungal drugs.

Conclusion

The authors successfully characterized the structure and biosynthesis of nostolysamides, a novel class of lipidated class II lanthipeptides. They elucidated a unique bidirectional ring formation mechanism, determined the stereochemistry, and demonstrated potent fungicidal activity via membrane disruption. Crucially, they showed that while the biosynthetic machinery includes a lipidating enzyme, the lipid modification is not strictly required for the observed bioactivity, opening new avenues for the engineering and optimization of lanthipeptide therapeutics.