Microbes have been engaged in an arms race with one another that goes back for millennia. While we have been able to reveal microbial defense systems like CRISPR, and we have been able to exploit some of them for our own uses in the lab, there are probably still untold numbers of microbial defense systems still waiting to be discovered. Some of them have significance for us in other ways as well; they have helped shape human biology.
While archaeal microbes are similar to bacteria in some ways, these relatively recent additions to the tree of life are unique, and ancient. Some research has indicated that some forms of archaea can be considered the simple ancestors of all complex life.
A new study has shown that Asgard archaea also carry antiviral defense systems that are related to those found in complex life forms. This work, which was reported in Nature Communications, has found that viperin proteins and argonaute proteins, which are found in eukaryotes including humans, have probably evolved from viperin and argonaute proteins proteins that are carried by Asgard archaea. We may get our antiviral capabilities from ancient microbes that lived billions of years ago.
"It adds more support to the fact that the Asgards are our microbial ancestors," said senior study author Brett Baker, an associate professor at the University of Texas at Austin. "It says that not only did eukaryotes get all these rich structural proteins that we've seen before in Asgards, now it's saying that even some of the defense systems in eukaryotes came from Asgards."
Viperin proteins can detect DNA that is not from the host, and which could indicate the presence of a pathogen. Viperins can edit the genome of viruses so they are unable to replicate. Argonaute proteins are also antiviral, but they work by detecting and degrading viral DNA, which also stops the virus from exerting any effect. Argonautes can also work against viruses through a mechanism called RNA silencing, which stops viral proteins from being produced.
This study has also shown that Asgard archaea carry defense systems that were once thought to be exclusive to bacteria.
It seems that even ancient, single-celled organisms had to find ways to stop infectious viruses. They found methods that worked, and those stuck around through evolutionary pressures, and remained as antiviral shields, explained co-corresponding study author Pedro Leão, now an assistant professor at Radboud University. "When bacteria and archaea discovered tools that worked, they were passed down and are still part of our first line of defense."
A protein's structure is vital to its function, and scientists can compare the genomes and proteomes of species to see how DNA and proteins may be the same or different. This study showed that there are many immune proteins in the tree of life that are similar to one another. The viperin protein seems to have maintained a similar structure across many different types of organisms.
"This research highlights the integral role cellular defenses must have played from the beginning of both prokaryotic and eukaryotic life," added study co-author Emily Aguilar-Pine.
"It's undeniable at this point that Asgard archaea contributed a lot to the complexity that we see in eukaryotes today," noted Leão. "So why wouldn't they also be involved in the origin of the immune system? We have strong evidence now that this is true."
Sources: University of Texas at Austin, Nature Communications