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Meeting in the bacterial middle

Cycling proteins determine cell division

Munich, 06/08/2012

When a cell divides, it first doubles in size and then constricts in the middle. A new model explains how self-organizing proteins allow bacterial cells to “sense” their shape and enable them to confine the site of constriction to midcell.

In bacteria, so-called Min proteins act to prevent constriction of the cell wall.They oscillate back and forth in the growing cell, and distribute themselves in such a way that the cell can only divide in the middle.LMU physicist Professor Erwin Frey and his PhD student Jacob Halatek have now developed a model which identifies the mechanisms that determine the distribution ofMin proteins. The model throws new light on the mechanisms that mediate the process of locating midcell.

Cycles of binding to and dissociation from the membrane beneath the cell wall mediate the oscillation of the proteins. Bound MinD captures more MinD and MinE, until the concentration of MinE reaches a threshold. The complex then dissociates, and MinD diffuses to an area where the concentration of MinE is lower. “The geometry of the cell determines the pattern of sites of Min accumulation,” says Frey. “In rod-shaped cells, stripes are formed, and in spherical cells rotating waves.”

The patterning role of geometry
The new model is the first that reproduces the patterns observed experimentally when diverse parameters, such as temperature, are varied. It demonstrates that cell geometry alone, not differences in protein concentration or affinity for the membrane, determines the pattern that emerges.The researchers themselves refer to the mechanism they have defined for the process of finding midcell as “canalized particle transport”.

For the first time, the new model makes it possible to study systematically the effects of varying diffusion rates and membrane affinities. It also allows to predict the spatio-temporal protein patterns for different cell geometries, and has a great potential for applications in artificial membrane systems and synthetic cell modules. The work was performed in the context of the Nanosystems Initiative Munich (NIM), a Cluster of Excellence. (Cell Reports, 7. Juni 2012cr/suwe

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