Biophysics of bacterial cell division

[Translation by Dr. Nachmani Moshe]

In the last stage of bacterial cell division, an annular structure formed on top of the plasma membrane near the midpoint of the cell gives birth to the two daughter cells. Researchers from the Ludwig Maximilian University in Munich used mathematical modeling in order to understand the mechanism underlying the formation of this ring structure (Z-ring) and in the process revealed a new type of mechanism in biological systems. Computer simulations based on the new model showed that the main part of the ring structure can self-assemble into ring-like structures after the concentration of a local subunit exceeds a specified threshold value. "From a biological point of view, this is a particularly interesting observation because it sheds new light on the protein activity that was previously unknown, an activity that is at the basis of the division of a bacterial cell," explains the researcher. The research findings were published in the scientific journal Physical Review Letters.

The Z-ring consists of the protein FtsZ, which polymerizes in the form of filaments with structured windings, as demonstrated by experiments performed with artificial membranes. In addition, the rings form winding patterns on the membrane as a result of exchange in the active subunit. This phenomenon is due to the fact that the FtsZ protein is polar: subunits can be added only at one end and subtracted from the other end only. This property makes the thread appear as if it is crawling along the membrane. "Under certain conditions, the polymers begin to form aggregates in the form of a closed ring that moves in a rotating vortex," explains the lead researcher. "And amazingly, the diameter of these rings is equal to the average diameter of a bacterial cell."

The ability of the FtsZ protein to organize itself has been known for some time, but the researchers were also able to develop a mathematical model that takes into account the tortuosity built into the original protein. In addition, the model also takes into account the hypothesis that the arcs of the polymers repel each other, which ensures that the polymers do not overlap each other. "What we really wanted to know was what is the main mechanism responsible for the tortuous patterns in the polymer," says the lead researcher. The simulations showed that the decisive factor is the density of the particle system - that is, the cumulative concentration of the subunits in the system: when the concentration is low, the opportunities for interactions between them are few, and when the concentration increases, the chance that the different polymers collide with each other increases. As a result of these collisions and the rotational movement of each of the coiled polymers, the polymers begin to aggregate together to obtain more and more compact complexes.

According to the researchers, these findings imply that the creation of the Z ring is born directly from the dynamics of the independent organization of the FtsZ units, and the concentration of these units is the controlling and regulating factor where and when it is created in the cell. Such a self-acceleration system also reveals a completely new mechanism for the creation of ring structures, which are fundamentally different from those common in the separation of daughter cells in the framework of eukaryotic cell division. There, the researcher explains, the essential components are specific motor proteins that bind to the cell membrane and undergo active contraction. In addition to the biological importance of the new findings, they are also of interest in the fields of physics and mathematics - the revealed mechanism is significantly different from the behavior of other known active systems.
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