Israeli scientists find forgotten immune strategy to fight viruses - study

Different species have a forgotten innate protein that has the ability to fight off viruses, according to a study by the Weizmann Institute of Science.

 An illustrative image of a virus. (photo credit: INGIMAGE)
An illustrative image of a virus.
(photo credit: INGIMAGE)

Everything needs energy. Every time we use our smartphones, send a text message, or watch a video, some of the energy stored in the battery is depleted. Living cells also store energy in “currencies” they can cash in when needed to fuel life processes. 

The main energy “currency” of all living things on Earth – the lithium battery that powers the living world, as it were – is a molecule known as adenosine triphosphate (ATP). However, even though charging cells with ATP molecules is of vital importance, it has now been discovered that discharging them could be no less crucial and even lifesaving. 

In a study just published in the journal Cell under the title “A conserved family of immune effectors cleaves cellular ATP upon viral infection,” researchers from the Weizmann Institute of Science in Rehovot have described a new family of proteins that deplete cells of their energy, thereby protecting the cells from invaders. 

The body's forgotten innate army to fend off viruses

The scientists showed that this previously unknown immune mechanism doesn’t exist only in single-celled organisms; it has been preserved for more than a billion years of evolution and is used by many living creatures, from corals to bees.

The newly revealed strategy is the latest of more than 100 recently discovered sophisticated mechanisms used by bacteria in their heroic battle against bacteriophages – the viruses that attack bacteria. Many of these mechanisms were discovered and described in the lab of Prof. Rotem Sorek in Weizmann’s molecular genetics department. 

A DNA double helix is seen in an undated artist's illustration released by the National Human Genome Research Institute to Reuters on May 15, 2012. (credit: REUTERS/NATIONAL HUMAN GENOME RESEARCH INSTITUTE/HANDOUT)
A DNA double helix is seen in an undated artist's illustration released by the National Human Genome Research Institute to Reuters on May 15, 2012. (credit: REUTERS/NATIONAL HUMAN GENOME RESEARCH INSTITUTE/HANDOUT)

Just like the viruses that attack us, phages are also made up of a small quantity of proteins and a lot of genetic material – DNA or RNA – that they inject into the bacteria they are taking over. Having accomplished this task, these viruses use the bacteria’s cellular machinery to replicate themselves over and over. When they have depleted a single bacterium’s resources, they tear down its membrane, break out, and spread throughout the entire bacterial colony.

In their study, the researchers focused on a gene that had caught their attention – one that had mysteriously canceled out a phage’s ability to replicate itself and infect the rest of the colony. The scientists revealed that the mystery gene encodes a protein that cuts up and permanently destroys ATP molecules, thereby denying the invasive phage the energy it needs to reproduce itself. The result is an effective immune strategy. The researchers reached the conclusion that the gene plays a key role in bacterial immune systems: In its absence, phages that infected the bacteria replicated 100 times faster.

“Lowering the levels of ATP in the cell is a strategy that’s brilliant in its simplicity,” said Dr. François Rousset, who led the research team in Sorek’s lab. “The phage cannot replicate without energy, and it is better for the bacterium that in any case is infected and about to die to deplete its own battery, preventing the phage from replicating and spreading to the rest of the colony.” 

The researchers also discovered that in some cases, depleting ATP affects the phage’s control systems, causing it to breach the bacterial cell’s membrane too soon – before it has had a chance to replicate. This prevents much more widespread damage to the colony.

Rather surprisingly, this strategy is much more common than one would imagine. The scientists scanned genome databases of tens of thousands of bacteria and discovered more than 1,000 immune genes that work in a similar way. They were surprised to find ATP-depleting capabilities in one family of proteins that, until now, was not even known to belong to the immune system. This suggests that the researchers have discovered a new immune strategy that exists in hundreds of different bacteria and grants them effective protection against viruses.

The scientists did not, however, stop at bacteria; they carried out a comprehensive analysis showing that far-more-advanced organisms – fungi, insects like bees, corals, sponges and many other organisms – also produce immune ATP-cutting proteins. Although this type of immune protein does not exist in humans, the researchers believe it is the ancient predecessor of the proteins that make up our innate immune systems.

“Many studies in the past few years have used the knowledge of advanced organisms’ immune systems to reveal immune strategies used by bacteria,” concluded Sorek. “Our new study shows that the extensive knowledge accumulated about bacterial immune systems enables us to follow the opposite logic – we can learn about the immune systems of advanced organisms by studying those of bacteria. ATP molecules are among the most widely occurring in nature, so clarifying their role in immunity can greatly contribute to understanding the defense strategies that countless organisms use when attacked by viruses.”