There’s a lot about our genome, and how it was constructed and pieced together and altered over time that we don’t know about. In recent years, though, it’s become clear that silent stowaways have snuck aboard for the ride.
Once upon a time, these sneaky critters were viruses or virus-like ancestors – but somehow, they’ve managed to incorporate their genetic material into our own and become hidden passengers instead. Just last year, a brand new viral fragment was found in embryos and cancers; now, as reported in two new studies in Cell, a protein that’s vital for memory consolidation behaves an awful lot like a virus too.
They were examining Arc – a peculiar neuron-based protein that we suspect we need to form long-term memories. It also appears to ensure that brains remain “plastic”, which refers to their ability to rearrange themselves to optimize learning and cognitive processing. Without Arc, mice become amnesiacs.
Despite these revelations, much remains enigmatic about Arc, particularly with regards to its origins.
Upon closer inspection, these two research teams suddenly noticed that Arc appeared to be able to assemble sizeable structures. Most notably, it was seen producing a case that looked a lot like the protein shell of a virus, something known as a capsid, which protects the virus' genetic material.
Senior author Jason Shepherd – the assistant professor or neurobiology and anatomy at the University of Utah – told IFLScience that he was “blown away” when he first saw the capsid-like structures.
“I have been working on Arc all my career, since grad school, and this came completely as a surprise.”
Both of these characteristics can be found in modern-day retroviruses, like HIV or the human T-cell leukemia virus. Although they all act slightly differently, all retroviruses carry RNA – a cousin to DNA – and a special enzyme. Using said enzyme, they make a DNA copy of the RNA, and they use this copy to infect their host cells.
There's significance to the resemblence of Arc to retroviruses too, not just any old virus type: they're great at sneaking into animals.
“Many viruses can incorporate their DNA into the DNA of the host cell. However, retroviruses have become particularly good at integrating into germline,” Dr Ben Libberton, a microbiologist at the MAX IV Laboratory in Lund, Sweden, told IFLScience.
The germline, in simple terms, is a lineage of cells that pass DNA from generation to generation in an unbroken line.
Libberton, who wasn’t involved in the study, said that this “means that once they infect one person, their DNA has the potential to spread via sexual reproduction of the host.”