June 27, 2017
In the Terminator franchise, the U.S. creates an artificial intelligence (AI)-based defense system called Skynet to, among other things, react more quickly to threats than any general or politician could. What starts out as an interesting idea almost dooms mankind to extinction once Skynet becomes conscious and decides to eliminate its greatest threat—humans.
Our friend Saccharomyces cerevisiae has its own version of Skynet for when it is “attacked” by too many salt ions. No, the system isn’t conscious and it does not threaten this yeast’s very existence but like Skynet, it is designed to react more quickly than more conventional systems based on gene regulation. It basically buys yeast enough time to allow the cells to more stably adapt to their new high salt environment.
Within 1 -5 minutes of being plunked down into high salt, yeast activates Hog1p, a key MAP kinase. The activated Hog1p heads into the nucleus and within 30-60 minutes, it tweaks the expression of a bunch of genes so the yeast can now better deal with its new environment.
This is a lot of time to be languishing in high salt. Luckily, yeast’s “whole hog” approach to high salt is not limited to just Hog1p. According to a recent study by Jin and coworkers in the Journal of Cell Biology, there is another, faster reaction to the high salt. And at least in these experiments, it is critical for yeast’s survival when it is assaulted by too much salt.
This rapid response involves a signaling lipid found in the vacuole called phosphatidylinositol 3,5-bisphosphate, or PI3,5P2. The amount of this lipid goes way up within just five minutes of the high salt shock.
PI3,5P2 is synthesized in yeast by a single enzyme, Fab1p. It stands to reason that if PI3,5P2 is critical to yeast survival in high salt, then deleting FAB1 should affect yeast’s ability to deal with all of those extra ions in its environment. This is just what Jin and coworkers found.
They compared the viability of wild type, hog1Δ, and fab1Δ strains under normal conditions and after a four hour exposure to 0.9M NaCl (high salt). Under low salt conditions, the fab1Δ strain was less viable than the other two. Around 30% of the fab1Δ yeast were dead.
At high salt, less than 10% of the wild type yeast and around 30% of the hog1Δ yeast were dead after four hours. This compares to the greater than 80% dead fab1Δ yeast.
The next steps in the study were to identify how the high salt increases the amount PI3,5P2. They reasoned that they needed something fast and that kinases just might fit the bill, so they started looking for strains that dealt poorly with high salt in the “…knockout haploid yeast mutant collection of 103 nonessential protein kinases.” They found a likely candidate in Pho85p and further work showed that its partner cyclin Pho80p was also involved.
Both the pho85Δ and pho80Δ strains had enlarged vacuoles (a common phenotype in yeast that cannot make PI3,5P2). More importantly, both strains could not make PI3,5P2 either under normal or high salt conditions and were also less viable than wild type under high salt conditions.
Additional experiments provided strong evidence that Pho85p phosphorylated Fab1p and that this phosphorylated Fab1p was important for synthesizing PI3,5P2 under high salt conditions. The final experiments confirmed that something similar happens in mammalian cells.
Jin and coworkers showed that the Pho80p-Pho85p equivalent in mammalian cells, CDK5-p35, phosphorylates the Fab1p equivalent, PIKfyve, in vitro. They also showed that CDK5-p35 is important for mouse fibroblasts to make more PI3,5P2 when exposed to high salt.
These studies suggest that yeast and probably mammals have at least two systems for dealing with high salt. The first is a rapid increase in PI3,5P2 that protects the cells from the environmental insult which gives the cells time to set up the second system—a longer term, more stable adaptation.
If only the folks in the Terminator world were as smart as yeast and had made Skynet a transient system set up to protect the U.S. while humans had time to respond in a more stable way. Think how much easier Sarah Connor’s life would have been!
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight