Sometimes nature is just a bit weird. Today at the SGM Autumn conference, Dr Stefan Raunser, a structural biologist from the Max Planck Institute of Molecular Physiology, told us all about a remarkable new class of bacterial toxin that changes shape to inject itself into insect cells.
Before we get to the toxin, let’s set the scene. I love a three-way bacterial-nematode-insect parasitic relationship as much as the next person, and this one is a beauty.
It all revolves around the bioluminescent bacterial species Photorhabdus luminescens, which lives in the gut of some roundworms. These worms have a very particular life cycle and can only reproduce within insect larvae (making them ‘entomopathogenic’).
The cycle begins with a juvenile worm living in the soil, whose only role in life is to seek out an insect host. Once a suitable target is located, the worm forces its way inside, whereupon contact with haemolymph – insects’ circulatory fluid – makes it throw-up. Far from being a measure of the worm’s disgust of its new environment, the vomiting releases the P. luminescens bacteria into the insect’s innards, which release toxins that rapidly kill the insect.
The remaining insect cadaver is used as shelter and food for the dividing bacteria, who are in turn eaten by the reproducing worms (although some are saved inside the worms for later infection). When the food runs out, new juvenile worms burst out of the corpse and go off to infect other insects.
Dr Raunser’s research focuses on the delivery mechanism of one of the toxins produced by P. luminescens that is used to kill the insects. Like the life cycle it is associated with, it involves several stages and form changes. Until now, nobody knew how it worked.
The mechanism, a construct of three proteins, TcA, TcB and TcC, is a self injecting syringe that acts from within the host’s cell. When first produced, the syringe (and its associated toxin) is surrounded by a water soluble protein shell, which docks with receptors on the host cell surface. The solubility of the shell allows the toxin complex to be absorbed into the cell via a small membrane vesicle.
Once inside the cell, a pH change triggers the syringe to be released from its shell and inserts itself into the membrane of the vesicle. Dr Raunser describes the syringe as, “looking like the notorious vuvuzela horns used by South African football fans.”
Now primed and in position, the final act of the mechanism is to inject its associated toxin out of the vesicle, directly into the heart of the insect cell. This process can be seen in the diagram below. Strikingly, the toxin (coloured orange) is initially folded; through methods currently unknown, it is unfolded and threaded through the vuvuzela, after which it refolds and becomes active. The toxin works by affecting parts of the cytoskeleton scaffold within the cell, causing it to polymerise uncontrollably, rupturing the cell. It is the emptied contents of the cell that the bacteria then eat.
Research has shown that Yersinia pestis and Yersinia pseudotuberculosis – the bacterial causes of plague and Far East scarlet-like fever – both contain similar systems.
Dr Raunser hopes that these findings will give us a greater understanding of the Yersinia armoury and may have other uses too, “Moths are becoming resistant to the toxin in Bt corn; industry is looking for new proteins that affect insects and could be used in future crop varieties. Also, this work gives us a new way to study the transport of proteins across cell membranes – plus it will help us learn more about the complicated lifestyle of these worms and bacteria.”