Nav: Home

Biophysics: Closing the ring

May 03, 2016

How bacterial cells divide in two is not fully understood. Physicists of Ludwig-Maximilians-Universitaet (LMU) in Munich now show that, at high concentrations, a crucial protein can assemble into ring-shaped filaments that constrict the cell, giving rise to two daughter cells.

In the final step in bacterial cell division, constriction of the so-called Z-ring, an annular structure that forms on the plasma membrane near the midpoint of the cell, gives rise to the two daughter cells: A research team led by Erwin Frey, who holds the Chair of Statistical and Biological Physics at LMU, has now used mathematical modelling to understand the mechanism that drives formation of the Z-ring, and in so doing have uncovered a novel class of pattern-forming mechanism in biological systems. Simulations based on the model show that the major constituent of the Z-ring can self-organize into ring-like structures once its local subunit concentration exceeds a certain threshold value. "From a biological standpoint, this is a very interesting observation, because it sheds new light on the previously mysterious protein dynamics that underlies bacterial cell division," says Frey. The results of the new study appear in Physical Review Letters.

The Z-ring is made up of the protein FtsZ, which polymerizes in the form of filaments that have an intrinsic curvature, as confirmed by experiments carried out with artificial membranes. Moreover, the rings form vortex patterns on the membrane as a result of active subunit exchange: This phenomenon arises because FtsZ polymers are polarized: Subunits can be added to only one end of the filament and are lost from the other. This so-called tread-milling makes it appear as if the filaments are actively crawling along the membrane. "Under certain conditions, the polymers begin to form closed ring-shaped clusters that rotate," says Jonas Denk. "And strikingly, the diameter of these rings is equivalent to that of the average bacterial cell."

That FtsZ polymers self-organize has been known for some time, but Frey's group has also developed a mathematical model which takes the intrinsic curvature of the polymers and their swirling, tread-milling behavior into account. In addition, the model includes the stipulation that filament arcs repel one another, which ensures that filaments do not overlap. Numerical simulations based on this model recapitulate the non-linear dynamics observed in experimental model systems. "What we really wanted to know was the underlying mechanism responsible for the formation of the vortex patterns," says Huber. The simulations demonstrated that the critical factor is particle density - in other words, the total concentration of subunits in the system: In a system in which the particle density is low, opportunities for interaction are few, and individual filaments are widely separated. As the total number of particles is increased, the polymers become increasingly likely to collide with each other. As a consequence of such collisions and the rotary motion of each of the curved filaments, the polymers begin to cluster together, forming densely nested arcs.

According to the Munich researchers, these results imply that formation of the Z-ring is a direct consequence of the dynamics of FtsZ self-organization - and the concentration of FtsZ in the cell is the controlling variable that regulates where and when it forms in the cell. Such an autocatalytic system also provides an entirely novel mechanism for the growth of ring-shaped structures, which differs fundamentally from that used for daughter cell segregation in eukaryotic cell division: In eukaryotes, specific motor proteins which attach to the cell membrane and undergo active contraction are essential for this process, Denk points out. Moreover, quite apart from their biological significance, the new findings are of considerable physical and mathematical interest, Huber adds: The underlying phenomenology of our model differs fundamentally from that conventionally used in modelling the behavior of powered or active particle systems. Its mathematical description turns out to lead to a generalized version of a complex equation that plays a role in the context of phenomena such as bacterial turbulence and pattern formation in more general, non-linear systems.

Ludwig-Maximilians-Universität München

Related Polymers Articles:

Researcher develops method to change fundamental architecture of polymers
A Florida State University research team has developed methods to manipulate polymers in a way that changes their fundamental structure, paving the way for potential applications in cargo delivery and release, recyclable materials, shape-shifting soft robots, antimicrobials and more.
Bottom-up synthesis of crystalline 2D polymers
Scientists at TU Dresden and Ulm University have succeeded in synthesizing sheet-like 2D polymers by a bottom-up process for the first time.
Secret messages hidden in light-sensitive polymers
Scientists from the CNRS and Aix-Marseille Université have recently shown how valuable light-sensitive macromolecules are: when exposed to the right wavelength of light, they can be transformed so as to change, erase or decode the molecular message that they contain.
Successful application of machine learning in the discovery of new polymers
As a powerful example of how artificial intelligence (AI) can accelerate the discovery of new materials, scientists in Japan have designed and verified polymers with high thermal conductivity -- a property that would be the key to heat management, for example, in the fifth-generation (5G) mobile communication technologies.
How to capture waste heat energy with improved polymers
By one official estimate, American manufacturing, transportation, residential and commercial consumers use only about 40 percent of the energy they draw on, wasting 60 percent.
More Polymers News and Polymers Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...