The bacterium Photorhabdus luminescens is a constant companion of some roundworms. These worms assault insect larvae, thereby infecting them with the bacteria; the pathogens then attack the cells of their victims with a deadly cocktail of various toxins. Scientists at the Max Planck Institute of Molecular Physiology in Dortmund working together with colleagues from Freiburg University and Jacobs University Bremen, have discovered that the bacteria use an important toxin complex like a syringe. It makes its way into the host cells via constricted vesicles in the cell membranes, and modifies their structure from within.
Part of the toxin complex then forces its way inside the cell through the vesicle membrane by means of a vuvuzela-like protein channel, and kills the cell. Important toxins of Photorhabdus luminescens are counted among the ABC toxins, which consist of the three protein components TcA, TcB and TcC. The toxin complex first docks at receptor molecules on the membrane of the host cell and is sucked inside the cell in small membrane blisters called vesicles. The TcC components then make their way into the cell fluid and demolish the cell’s protein skeleton. What has remained unclear to date, however, was how the protein managed to get through the vesicle membrane.
Now, for the first time, scientists have been able to decode the structure ofPhotorhabdus luminescens’ ABC toxins using cryoelectron microscopy and single particle analysis. This shows that the bacterium’s TcA protein consists of five subunits that together form the shape of a bell. “Inside the bell, the subunits form a channel that has one wide and one narrow aperture, so that it looks like the notorious vuvuzela horn used by South African football fans”, explains Stefan Raunser of the Max Planck Institute of Molecular Physiology.