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How multicellular cyanobacteria transport molecules

Also known as blue-green algae, cyanobacteria are a special class of bacteria that can carry out photosynthesis. In evolutionary terms, they are ancient. Their predecessors – who first appeared on earth about 2.5 billion years ago – paved the way for higher forms of life thanks to their ability to produce oxygen through photosynthesis.

Some cyanobacterial species are filaments, multicellular organisms that have developed different cell functions. Some cells carry out photosynthesis, while others absorb atmospheric nitrogen. Cyanobacteria obtain energy in the form of glucose through photosynthesis; they use nitrogen to produce amino acids, protein building materials.

For cyanobacteria, this raises the problem of how individual cells can communicate and exchange substances. Photosynthetic cells must keep your nitrogen-fixing cells supplied with glucose; in the same way, amino acids need to be transported in the opposite direction. For this purpose, cyanobacteria have developed special cell junctions that allow the exchange of nutrients and messengers across cell boundaries, without the cells joining together.

Explain structure in a cellular context

Until now, very little was known about the detailed structure and functioning of cell connections in multicellular and multicellular cyanobacteria. In the latest issue of the scientific journal Cell, a group of researchers from ETH Zurich and the University of Tübingen present an unprecedented level of detail on the structure and function of cell-to-cell connections, referred to as septal junctions, in the genus Anabaena.

The researchers revealed that the connecting channel consists of a protein tube that is closed with a plug at both ends. In addition, this tube is covered with a five-armed protein element, which is arranged like a camera opening.

These channels connect the cytoplasm of two neighboring cells by passing through different membranes and cell walls. The cells are separated by an ultra-thin slit, only a few nanometers wide.

"Researchers have so far failed to clarify these details with conventional electron microscopes. By expanding cryo-electron microscopes, we can obtain a level of accuracy that has never been achieved before," said Professor Martin Pilhofer of the Institute of Molecular Biology and Biophysics at ETH Zurich.

Gregor Weiss, Pilhofer doctoral student, developed the process of preparing cyanobacteria such that the channels can be visualized through cryo-electron microscopy. Using frozen cyanobacteria, Weiss "grinds" the junction between two cells, layer by layer, until the sample is thin enough. Without this pre-processing, round cells will be too thick for cryo-electron microscopy.

Mechanism to prevent leaking

"Because of the complex structure of the connecting channel, we suspect there is a mechanism for opening and closing it," said Karl Forchhammer, Professor of Microbiology at the University of Tubingen. He and his team were actually able to show how complex cells communicate with each other under different stress conditions. They stain the cyanobacteria chain with fluorescent dyes and then whiten individual cells with a laser. The researchers then measured the entry of dyes from neighboring cells.

Using this method, they can show that the channel is completely closed when treated with chemicals or in the dark. The filigree lid structure of the channel closes like an iris and interferes with the exchange of substances between cells; the researchers recognized this phenomenon through the various levels of fluorescence they observed.

"This closure mechanism protects all multicellular organisms," Forchhammer said. For example, it can prevent cells from transmitting harmful substances to neighboring cells, which can destroy entire organisms. Cyanobacteria can also use channels to prevent cell contents from all tissues leaking if each cell is damaged mechanically.

Preserved structure

With their research, researchers can show that in the course of evolution, multicellular organisms from different lineages repeatedly and independently "discovered" cell junctions. "This emphasizes how important it is for multicellular organisms to be able to monitor the transport of substances between their individual cells," Pilhofer said. By explaining the channel structure and its function in cyanobacteria, ETH researchers added another piece to the puzzle. "As far as we know, this is fundamental biological research, without focusing on potential applications. This new data gives us a greater understanding of the evolution of complex life forms," ​​the ETH professor explained.



Weiss GL, Kieninger A-K, Maldener I, Forchhammer K, Pilhofer M. Structure and function of bacterial slit analogues. Cell, 2019, July 11. DOI 10.1016 / j.cell.2019.05.055

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