The “Rapunzel” Virus: An Evolutionary Oddity

A recent study in the Journal of Biological Chemistry has solved the mystery behind an evolutionary marvel: a bacteriophage with an extremely long tail. This extraordinary tail is part of a bacteriophage that lives in inhospitable hot springs and preys on some of the world’s toughest bacteria.

Bacteriophages are a group of viruses that infect and multiply in bacteria, and they are the most common and diverse things on earth.

“Bacteriophage, or phage for short, are everywhere there is bacteria, including the dirt and water around you and also in the microbial ecosystem of your own body,” said Emily Agnello, a graduate student at the University of Massachusetts Chan Medical School and the Lead author of the study.

Unlike many viruses that infect humans and animals, which contain only one compartment, phages consist of a tail attached to a spiny, prism-like protein shell that contains its DNA.

Like hairstyles, phage tails vary in length and style; Some are long and bouncy while others are short and stiff. While most phages have short, microscopic tails, the “Rapunzel bacteriophage” P74-26 has a tail 10 times longer than most, measuring nearly 1 micron in length, about the width of spider silk. The nickname “Rapunzel” derives from the fairy tale in which a girl with extremely long hair was locked in a tower by a wicked witch.

Brian Kelch, an associate professor of biochemistry and molecular biotechnology at UMass Chan who oversaw the work, described P74-26 as a “tail monster.”

Phage tails are important for piercing bacteria that are coated in a dense, viscous substance. P74-26’s long tail allows it to penetrate and infect the toughest of bacteria. In addition to having an extremely long tail, P74-26 is the most stable phage, allowing it to exist in and infect bacteria that live in hot springs that can reach over 170°F. Researchers have studied P74-26 to find out why and how it can exist in such extreme environments.

To work with a phage that thrives at such high temperatures, Agnello had to adjust the conditions of her experiments to get the phage tail to assemble in a test tube. Kelch said Agnello created a system for her to induce rapid self-assembly of the tail.

“Each phage tail is made up of many small building blocks that come together to form a long tube. Our research shows that these building blocks can change shape or conformation when they come together,” Agnello said. “This shape changing behavior is important to allow the building blocks to fit together and form the proper structure of the tailpipe.”

Using high-performance imaging techniques as well as computer simulations, the researchers found that the building blocks of the tail lean against each other to stabilize themselves.

“We used a technique called cryo-electron microscopy, which is a giant microscope that allows us to take thousands of pictures and short movies at very high magnification,” Agnello explained. “By taking lots of photos of the phage’s tailpipes and stacking them together, we were able to figure out exactly how the building blocks fit together.”

They found that P74-26 uses a “ball head” mechanism to stabilize itself. Also, the tail consists of vertically stacked rings of molecules forming a hollow channel.

“I like to think of these phage building blocks as a kind of Lego,” said Kelch. “The Lego has knobs on one side and the holes or sockets on the other.”

He added: “Imagine a Lego where the bases start closed. But as you start building with the legos, the sockets will start to open up to allow the studs on other legos to build a larger assembly. This movement is an important way that these phage building blocks self-regulate their assembly.”

Kelch pointed out that compared to most phages, P74-26 uses half the building blocks to form stacking rings that make up the tail.

“We think an ancient virus fused its building blocks into one protein. Imagine two small lego bricks being fused together into one big brick without any seams. That long tail is made of bigger, sturdier building blocks,” Kelch explained. “We think that might stabilize the tail at high temperatures.”

The researchers now plan to genetically manipulate the length of the phage tail and see how this changes its behavior.

Phage colonize almost every corner of the world and are important to a variety of industries such as healthcare, environmental protection, and food safety. In fact, long-tailed phages such as P74-26 have been used in preliminary clinical trials to treat certain bacterial infections.

“Bacteriophage is gaining increasing interest as an alternative to antibiotics to treat bacterial infections,” Agnello said. “By studying phage assembly, we can better understand how these viruses interact with bacteria, which could lead to the development of more effective phage-based therapies. … I believe that studying unique, interesting things can lead to insights and applications that we can.” I can’t even imagine it.”