Structural homology reveals cerato-platanins as conserved antimicrobials repeatedly co-opted for fungal host colonization

The Invisible War and the Shape of Secrets

Imagine a world where plants are like bustling cities, and the air around them is filled with microscopic travelers. Some of these travelers are helpful neighbors, while others are invaders trying to break in. To survive, plants have built immune systems, but the invaders have their own secret weapons called "effectors." Think of effectors as special keys or spy gadgets that fungi use to sneak past the plant's security, trick the plant into opening the gates, or even shut down the plant's alarm system.

For a long time, scientists thought these fungal keys were unique to each type of fungus, designed specifically to hack a particular plant. But recently, a new idea has been growing: maybe these "keys" were originally just weapons used to fight other microbes in the soil, and fungi only later learned to use them against plants. It's like finding out that a master lock-pick was originally just a tool for opening jars. The big question is: can we find a common blueprint that links these different tools together? This paper dives into that mystery by looking at the 3D shapes of these proteins, rather than just their chemical recipes, to see if they share a hidden family history.

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The Shape-Shifting Family of Fungal Spies

In the microscopic world of fungi, there is a family of tiny, secreted proteins called cerato-platanins (or CPs for short). Imagine them as the Swiss Army knives of the fungal world. They are small, about 12 kilodaltons in weight (which is like a very light feather in the world of molecules), and they show up everywhere fungi go. Sometimes they help fungi stick to surfaces, sometimes they help them grow, and sometimes they act as "spies" that mess with a plant's immune system.

For years, scientists were puzzled. These CPs do so many different things that it was hard to figure out what their main job was. Was it glue? Was it a shield? Was it a weapon? The paper suggests that the answer might be much more ancient and violent than anyone thought: they are primarily antimicrobial weapons.

The Detective Work: Looking at Shapes, Not Just Letters

To solve this mystery, the researchers looked at a specific fungus called Verticillium dahliae, a notorious plant pathogen. They used a super-smart computer program (AlphaFold2) to predict the 3D shape of every single secreted protein this fungus makes. It's like taking a blueprint of a house and trying to guess what the furniture looks like just by looking at the floor plan.

They found that most of the proteins grouped together in obvious families, like a bunch of scissors or a bunch of hammers. But then, they found a weird group. This group contained a protein called Ave1, which was already known to be a weapon that kills bacteria, and CP1, a cerato-platanin that was thought to be a "spy" for plants.

Here is the twist: if you look at the chemical letters (the sequence) of Ave1 and CP1, they look nothing alike. They are like two people who speak different languages. But when you look at their 3D shapes, they are practically twins! They both have a specific structure called a "double-ψ β-barrel" (imagine a sturdy, double-layered barrel made of folded sheets) surrounded by some helical springs. This discovery was a huge clue. It suggested that even though they do different jobs now, they share a common ancestor and a common structural design.

The "Killer" Test: Do They Actually Fight?

If these proteins share a shape, do they share a function? The team tested this by making pure versions of CP1, CP2, and CP3 in the lab and dropping them onto a petri dish filled with different types of bacteria and fungi.

The results were striking. CP1 turned out to be a selective killer.

• It stopped the growth of some bacteria (like Sphingomonas and Bacillus) but ignored others.
• It also attacked fungi, specifically killing a yeast called Cyberlindnera jadinii very effectively.
• When they compared CP1 to its structural twin, Ave1, they found they had overlapping "kill lists," but they weren't identical. It's like two different security guards at a club; they both stop trouble, but they might stop different people.

The team also tested CP2 and CP3. These cousins were also killers, but they were a bit more picky. They didn't kill as many types of microbes as CP1 did. The researchers noticed that CP2 and CP3 have an "extra tail" at the end of their structure that CP1 doesn't have. They chopped off this tail to see if it was the secret weapon, but it turned out the tail wasn't needed for the killing power. The main body of the protein was doing the heavy lifting.

The Big Picture: Ancient Weapons Repurposed

The researchers didn't stop at just one fungus. They looked at a massive database of 150 different fungi, ranging from plant pathogens to harmless soil dwellers. They found that this "double-ψ β-barrel" shape is everywhere.

• They tested CPs from a fungus that lives inside plants (Colletotrichum tofieldiae), one that rots plants (Fusarium redolens), and one that eats dead stuff (Aspergillus campestris).
• The result: Almost all of them showed some ability to kill microbes.

This suggests a powerful story. Long ago, before fungi were even thinking about attacking plants, they used these proteins to fight bacteria and other fungi in the soil. It was a tool for survival in a crowded, competitive world. Over millions of years, as some fungi decided to become plant pathogens, they kept these weapons but "repurposed" them. They tweaked them to also mess with plant cells or help the fungus stick to leaves.

The paper argues that the "antimicrobial" ability is the core, ancient function of this protein family. The ability to manipulate plants is a newer, secondary skill that evolved on top of this ancient weapon.

How Do They Kill?

So, how does this barrel-shaped protein kill a bacterium? The researchers found that these proteins have "sticky" patches that are positively charged (like a magnet). Bacteria and fungi have cell walls that are negatively charged.

• The Mechanism: The protein likely sticks to the cell wall first (like a magnet to a fridge) and then uses its positive charge to poke holes in the cell membrane, causing the microbe to burst.
• The Difference: While a related protein called Ave1 sticks to a specific part of the bacterial wall called lipoteichoic acid, CP1 seems to stick to other things like chitin (a common fungal building block) or peptidoglycan (a bacterial building block). It's a slightly different approach to the same goal: getting close enough to pop the cell.

What This Means

This paper doesn't just tell us about one fungus; it changes how we view the evolution of disease. It suggests that many of the "spies" fungi use to attack plants today were originally just "soldiers" fighting in the microbial mud. The shape of the protein is the key that links these different roles together.

The authors are careful to say this is a strong suggestion based on structural and functional evidence, not a final, unchangeable law. But the evidence is compelling: the "double-ψ β-barrel" is an ancient, robust design that nature has kept and tweaked over and over again. It's a reminder that in the microscopic world, the tools for fighting neighbors often become the tools for conquering kingdoms.