Every cell carries fuel without which it stalls, like an engine with no gasoline. That molecule is NAD+, a carrier of energy and electrons. Bacteria under viral attack have learned to burn this fuel so the virus cannot replicate. It turns out that one of these kill switches, the NIR domain, was handed down to humans as well.
Scientists started with the bacterium Pseudomonas chlororaphis and its protein Avs2. Inside it was a region of unknown function that resembled known NAD+ destroyers in shape. When the bacterium was infected with a phage, this region drained NAD+ stores and accumulated the breakdown product. If a single amino acid in it was broken, the protection vanished completely.
The NIR domain turned up in 8.7% of bacterial genomes and often sits next to other defense systems. It is as if identical fire hydrants stood in different buildings, hidden behind the wall. In animals, NIR is present in 88% of species, and in humans it is built into the TEP1 protein, which is part of vault particles, large complexes with an unclear function.
This means the bacterial weapon has not rusted away over billions of years of evolution. It still works in our cells, but why is not yet clear. Perhaps vault particles use it as a signaling device rather than as a weapon.
A fire hydrant that once put out a viral fire may now simply be opening a door. But if it is still connected to the cell's fuel system, its role could be more important than it seems.
The new defense systems Vulcan and Vesta are named after the Roman gods of fire and the hearth, reflecting their ability to "burn" NAD+ during a viral attack.
The idea of a universal molecular switch that destroys a cell's fuel upon invasion is reminiscent of the self-destruct mechanism in "Star Wars," where a ship shuts down all systems so it won't fall into enemy hands.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.