I lied to you.
In my post AB Toxins - Botox’s Family I claimed that “AB toxins are produced by bacteria”. Saying that completely covers up the crimes of one organism that regularly kills its brothers and sisters in cold blood: The killer yeast!
But how? Why?
Killer yeasts can be found in many yeast species, not just in Saccharomyces cerevisiae, which is the one used to bake. That means a harmless yeast can suddenly start to produce toxins and kill all the other yeast cells nearby - without killing itself!
This part is especially important. You see, toxins are mostly produced to kill organisms that are different (exceptions apply).
The mystery is solved if one has a closer look at the yeasts that produce the toxin: They contain two different viruses.
One virus, the ”Helper virus” (L-A virus) lives in many cells and happily reproduces without bothering the yeast cell it uses as a host. Its genetic code contains everything necessary to build copy after copy of itself. 
The second virus, the ”Killer virus” (M virus) can only persist in a cell if the L-A virus is present too. Why? Because it’s lazy! This virus doesn’t have the right tools to replicate itself. It uses the tools and building blocks provided by the L-A virus.
And this is when the fun part happens.
During the replication of the M virus, a toxin is produced: The A/B toxin of the yeast.
The protein that is built by following the information in the genetic information of the M virus has three parts, α, β and γ. α and β are combined to the actual toxin, γ helps the cell (in combination with α) to not be killed by its own toxin. After all, the virus doesn’t want to kill its host!
And what does the toxin do? That’s actually pretty neat.
If you read the post about AB toxins I linked above, you know that the toxin needs the B part to enter the cell and the A part is toxic. How exactly both those things work varies from case to case.
The killer yeast has something called “HDEL” attached to the B part. HDEL is normally used by the cell to keep proteins where they are supposed to be, humans have something similar called “KDEL”.
It works like this:
The ER (Proteins go in here as a long string of amino acids and come out as a beautifully folded, very complex structure) needs certain proteins to keep functioning as it’s supposed to be. From time to time, these proteins are accidentally shipped away to the Golgi (The beautiful, complex protein structures come in here after being processed by the ER, are packaged and then shipped away to where they are needed in the cell). The ER wants those proteins back, badly. Luckily, the proteins are marked with the mentioned “HDEL” tag which the Golgi recognizes and as a result, ships the proteins back to the ER. Everyone is happy!
Until someone abuses this system, like the toxin of the M virus.
The toxin looks like it’s supposed to be in the ER because of its HDEL tag. As a result, it’s moved there. And then the cell … kills itself.
You read that right: The yeast cell starts a process called apoptosis which is equivalent to suicide.
I’m just glad it doesn’t work on humans.
Yet.
Source:
Lecture “Characteristics of Killer yeasts” by Dr. Frank Breinig
First picture was taken from pixabay.com, sketches are made by me.
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