Many bacteria have sophisticated molecular injection devices that are used to do some amazing things. For example, a bacterium inoculates certain molecules into a worm larva via such a nanomachine composed of proteins, which triggers the transformation of the larva into an adult worm. Other bacteria use such molecular weapons to kill foreign strains of bacteria or insect larvae, or they defend themselves against scavenger cells.
Researchers in the group of Martin Pilhofer, Professor at the Institute of Molecular Biology and Biophysics at ETH Zurich, who specializes in such molecular injection machines, have just described two novel injection systems in the journal
Nature Microbiology
: one made by cyanobacteria, also known as blue-green algae, and one by the marine bacterium
Algoriphagus machipongonensis
.
The newly discovered so-called contractile injection systems (CISs) work fundamentally differently than previously described devices and have a few unique features. As a result, they also provide information about the evolutionary differences between different injection system classes.
These CISs work like molecular syringes. When the outer sheath module of the nanomachine contracts, an internal, hidden tube filled with proteins is shot out. These proteins are either injected into the environment or directly into a target cell.
A surprising anchoring in the cell
One novel CIS, which the researchers found in cyanobacteria, was not anchored in the cell membrane or loosely floating inside the cell, as expected, but was attached to the so-called thylakoid membrane, where photosynthesis takes place in these bacteria.
"That was the biggest surprise for us," says Gregor Weiss, lead author of the study on the cyanobacterial injection system. Despite this unusual localization, the CIS anchored in the thylakoid membrane – referred to as tCIS – fulfill their purpose. If cyanobacteria are stressed, for example by excessive salt concentrations in the water or ultraviolet light, the outer cell layers detach. This exposes the outward-facing tCIS, ready to fire upon contact with potential target cells.
The molecular spear guns are also unexpectedly common, which according to Weiss indicates an important role in the life cycle of cyanobacteria. He suspects that the tCIS could play a role in the programmed cell death of individual cells in these multicellular cyanobacteria.
