The weak point of the assembly of the scourges


Electron microscope image of Caulobacter crescentus showing the two different daughter cells during cell division. Only one of the daughter cells hosts a flagellum (top right). Photo credit: UNIGE / VIOLLIER

To build the machinery that bacteria can swim with, over 50 proteins must be put together in a logical and well-defined order to form the flagellum, the cellular equivalent of a boat’s offshore engine. To be functional, the flagellum is put together piece by piece and ends in the helix called the flagellar filament, which is made up of six different subunits called flagellines. Microbiologists from the University of Geneva (UNIGE) have shown that the addition of sugar to flagellines is of fundamental importance for the structure and functionality of the flagellum. This glycosylation is performed by a newly discovered FlmG enzyme, the role of which was previously unknown. Based on this observation, which you can read all about in the diary eLife– Researchers followed up on a second discovery published in Development cell. Among the six flagellines of Caulobacter crescentus, the model bacterium in the two studies, one is the special one that plays a signaling role to trigger the final assembly of the flagellum.

The flagellum is the locomotor engine of bacteria. Thanks to the flagellum, bacteria can swim to food during an infection in Lake Geneva (Léman) or in the host. The flail – which is similar to an offshore engine in its complexity – consists of a basic structure, a rotary engine and a screw propeller. It is synthesized within bacteria in their cytosol. “The 50 proteins have to be made one after the other and put together in the right order,” says Patrick Viollier, a researcher at UNIGE’s Department of Microbiology and Molecular Medicine. At the same time, the flagellum must be incorporated into the bacterial envelope, which contains up to three layers of cells, before it can escape. Although flagella subunits are known, many of the subtleties in the control and targeting mechanisms of flagella assembly are still poorly understood.

Sweet surprise

UNIGE microbiologists examined the bacterium Caulobacter crescentus. “These bacteria are very interesting for the study of flagella because they produce two different daughter cells: one has a flagellum and the other does not. They are ideal for understanding what it takes to build a flagellum, “explains Nicolas Kint, co-author of the study. Another peculiarity is that the flagellar filament of this bacterium consists of six flagellin subunits, which means that it is not the result of polymerization of a single protein, as is the case with many other bacteria. “When we analyzed these six flagellines, we found that they were decorated with sugars, indicating that a glycosylation phase was in progress – an enzymatic reaction involving the addition of proteins To proteins – which had to be assembled. It was a surprising discovery as this reaction is not very common and is not well understood in bacteria, “continues Professor Viollier.

Viollier’s research team was able to show that glycosylation of the six flagellins that make up the filament is essential for the formation and functionality of the flagellum. “To demonstrate this, we first identified the gene that produces the FlmG glycosylation enzyme. If it is absent, bacteria without flagellum will develop. Second, we genetically engineered another type of bacterium, Escherichia coli, to express one of the six flagellines. Glycosylation and sugar producing enzymes from Caulobacter crescentus. All these elements are necessary to obtain a modified flagellin “, adds Nicolas Kint.

A versatile black sheep

“The different elements of the flagellum are produced one after the other: first the molecules of the base, then those of the rotor and finally the helix. The scientific literature shows that this sequential process is important. However, we don’t know how the order of creation of the subunits is controlled. “The researcher and his team focused on synthesizing the six flagellines and discovered a black sheep between them: a subunit that has only 50% sequence homology with the other five. ‘This subunit acts as a checkpoint protein, a repressive molecular cop that inhibits the synthesis of other flagellin proteins,’ says Professor Viollier. It is present before the synthesis of the other five subunits and acts as a negative regulator. As long as it is present in the cytosol, synthesis of the other subunits is prevented. As soon as the elements of the flagellum are joined together with the filament, the spool is exported to the membrane and then removed. Then the synthesis of the last five subunits can begin. “It is a sensor for protein synthesis and at the same time part of the flagella filament: a unique double function”, says the microbiologist happily.

This discovery is critical to understanding bacterial motility and protein assembly. ‘It also provides clues to understanding the synthesis and assembly of tubulin, an essential part of the cytoskeleton,’ concludes Professor Viollier.

The study sheds light on the bacterial arrangement of the helix

More information:
Silvia Ardissone et al., Secretion alleviates translational co-repression through a specialized flagellin paralogue, Development cell (2020). DOI: 10.1016 / j.devcel.2020.10.005

Provided by the University of Geneva

Quote: The weak point of the assembly of scourges (2020, October 27) was consulted on October 27, 2020 from

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