A moonlighting quorum sensing molecule

Bacteria talk to each other using quorum sensing molecules. Although the first one was found 50 years ago, the field really opened up with the work of Bonnie Bassler at Princeton in the 90s. These are small molecules which bacteria secrete, so that when there are a lot of bacteria around, the concentration of quorum sensors rises, allowing them to get into bacteria (by the law of mass action) changing gene expression for a variety of things, particularly virulence and biofilm formation. They have also been used by bacteria to compete with those of a different species.  There was a lot of hope, that we could control some nasty bugs (such as Pseudomonas) by messing about with their quorum sensors, but it hasn’t panned out. 

The real surprise came in a paper [ Proc. Natl. Acad. Sci. vol. 118 e2012529118  ’21 ]showing that Pseudomonas uses one of its quorum sensing molecules (C12 < N-3-oxo-dodecanoyl) homoserine lactone > ) inside the eukaryotic cells it attacks. 

What it does once inside, is to attack a cellular organelle I’ve (and probably you) never heard of called vaults.  They’s been known since ’86, and given their size (12.9 megaDaltons) I’m surprised I’d never heard of them.  The likely reason is that no one knows what their function is. 

It is made of just 3 proteins

l. MVP — Major Vault Protein, mass 100 kiloDaltons 96 copies/vault

2. VPARP — Vault poly ADP ribose polymerase 190 kiloDaltons

3. Telomerase associated protein 290 kiloDaltons.

Human vaults contain 4 different RNAs (called, naturally enough vault RNAs < vtRNAs >).  They are 88 – 100 nucleotides long.  

Vaults look like a hand grenades and are 670 Angstroms long and 400 Angstroms in maximum diameter. 

[ Cell vol. 176 pp. 1054 – 1067 ’19 ] says that there can be 10,000 to 100,000 vaults/cell.  So why haven’t I seen them?

One of the vtRNAs binds to a protein involved in autophagy inhibiting it. This is an example of an RNA binding to a protein altering its function, something unusual until you think of the ribosome or the spliceosome. Starvation decreases the number of vaults inducing autophagy.

Once pseudomonas C12 gets into a cell it binds to the Major Vault Protein, causing its translocation into lipid rafts, the net effect being attenuation of the p38 protein kinase pathway to attenuate programmed cell death (apoptosis).  

So C12 keeps the cell alive when normally it would die.  A lot of recent work has shown that bacteria infiltrate cancers.  Do they do something similar to cancer cells to keep them alive. 

It really makes you humble (or should) to realize how many separate parts of cellular and molecular biology you must understand to even hope to understand how cells (and bacteria) go about their business. 

Think of how many terms were introduced to understand what the humble quorum sensor C12 is up to. 

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