Modern eukaryotic cells contain numerous so-called organelles, which used to be separate bacteria. In order to understand how these bacteria were integrated into cells over the course of evolution and how they are controlled, a research team from the Institute for Microbial Cell Biology at Heinrich Heine University Düsseldorf (HHU) examined the unicellular flagellate Angomonas deanei, which contains a bacterium that was ingested relatively recently. In the diary Current Biologythe biologists now describe, among other things, how certain proteins in the flagellate control the cell division process of the bacterium.
In the course of evolution, modern eukaryotic cells (cells with a cell nucleus) have absorbed bacteria from their environment. The cells have brought the stored bacteria under their control and are now using them for important functions such as metabolic processes. The bacteria have thus become cell organelles. Organelles that arose from such “endosymbionts” (symbionts within the cell) are mitochondria, the “powerhouses of the cell”, and chloroplasts, in which photosynthesis takes place in plants. This happened about 1.5 to 2 billion years ago.
Organelles whose bacterial precursors had their own genome have greatly reduced their genome, making them increasingly dependent on the host cell over time. Their metabolism, their protein composition and their reproduction are now largely controlled by the host organism.
But how did this adaptation process come about in evolutionary terms? The working group led by Professor Dr. Eva Nowack from the Institute for Microbial Cell Biology at HHU Angomonas deanei, a flagellate that lives in the intestines of insects. This model organism is particularly well suited to answering this question, since each of these protozoa contains only one symbiotic bacterium that was introduced relatively recently (40 to 120 million years ago). This bacterium supplies the host with vitamins and certain metabolites.
Similar to mitochondria and chloroplasts, the bacterium’s genome is already reduced compared to its free-living relatives, but not yet to the extent of conventional organelles. However, integration has progressed so far that cell division occurs synchronously: when the host organism divides, the bacterium also divides – only once, with a portion going into each new flagellate cell.
The Düsseldorf research team wanted to find out how the host cell controls the endosymbiont. They studied its protein composition and discovered that a certain number of proteins are transferred from the host cell to the endosymbiont. Three of these proteins form a ring around its site of division.
The researchers were able to predict the function of two of these proteins by comparing them to known protein sequences. One of them resembles the protein “dynamin”, which can polymerize into contractile helical chains. Another protein known as peptidoglycan hydrolase can break down bacterial cell walls.
In mitochondria and chloroplasts, dynamin-like proteins also form a ring around the site of organelle division, and contraction of this ring aids in organelle cleavage. In addition, the division of some chloroplasts requires a peptidoglycan hydrolase to degrade the remnants of the bacterial cell wall at the site of division of these organelles.
Professor Nowack: “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. These proteins allow the cell to take control of the symbiote.”
Relation: Morales J, Ehrlich G, Poschmann G, et al. Host-symbiont interactions in Angomonas deanei involve the development of a host-derived dynamin ring around the endosymbiont’s site of division. Curr Biol. 2022:S0960982222017766. doi: 10.1016/j.cub.2022.11.020
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