Flagellar Proteins Control Bacteria Cell Division

Endosymbiosis, the process by which symbiotic organisms live inside each other, has remained a complicated history for scientists to understand. Certain bacteria, like the single-celled flagellate Angomonas deanei, contains a bacterium that was taken up relatively recently within its evolutionary timeframe, making it an excellent candidate for learning how cells gain control over bacteria. Researchers from the Institute of Microbial Cell Biology at Heinrich Heine University Düsseldorf (HHU) examined A. deanei in more detail in their work, recently published in the journal Current Biology.

Cells that take up bacteria from the environment are thought to eventually utilize these species for essential functions, such as metabolic processes. These organelles, whose bacterial predecessors contained their own genetic material, have significantly reduced their genome over time, making them increasingly dependent on their host. This means that functions such as metabolism, protein composition, and reproduction are all now controlled mainly by the host eukaryote. But how and why exactly does this relationship occur?

That’s precisely what the team at HHU wanted to uncover, with work spearheaded by Professor Dr Eva Nowack from the HHU Institute of Microbial Cell Biology. The team wanted to take a closer look at A. deanei, a flagellate that lives inside insect intestines, because these organisms contain one symbiotic bacterium, which was incorporated about 40-120 million years ago. While this may seem like a long stretch, this is relatively recent according to its evolutionary timeframe. Since its integration, this bacterial species supplies the host with vitamins and certain metabolites.

Similarly to other organelles that had evolutionary origins as bacteria, such as mitochondria and chloroplasts, the bacteria’s genome is significantly reduced but not to the same extent as conventional organelles. However, this doesn’t hinder sufficient cell division – when the host cell divides, so does the bacterium.

To learn more about how the host cell controls this endosymbiont, they examined its protein composition in more detail. They discovered that a certain number of proteins from the host cell are transferred into the endosymbiont, three of which form a ring around its division site.

The team was also able to predict the function of two of these proteins by comparing them with previously identified protein sequences. One of them was similar to a protein called “dynamin”, which can polymerize into contractile helical chains. The other protein, dubbed as a “peptidoglycan hydrolase”, helps break down bacterial cell walls.

“Our work shows that a eukaryotic host cell can transfer certain proteins to the endosymbiont at a relatively early stage in the evolution of an endosymbiotic relationship,” explains Professor Nowack. “These proteins enable the cell to gain control over the symbiont.”