| Abstract | mages of several cytoskeletal protein polymers in E. coli, B. subtilis, C. crescentus and other bacteria show that they form coils or rings on the inside of the cell membrane. Some of these structures extend the whole length of the cell (e.g. the proteins MreB, Mbl, and TubZ), some are localized primarily to a pole (e.g. MinD), and some form rings (e.g. FtsZ). Using stochastic mechanical simulations, I found that these shapes can arise from the combination of inherent protein polymer mechanics and the constraints that the curved cell membrane imposes. Five polymer morphologies are possible on rod-shaped cells: rings, lines, helices, loops, and polar-targeted circles, where the particular one that arises depends on the polymer’s bending stiffnesses. Each morphology has been observed experimentally. This mechanical explanation is sufficient to explain a wide variety of observed structures. It also provides a simple explanation for the dynamics of the Z-ring in a sporulating B. subtilis: the polymer transforms from a ring to a coil, to 2 rings, and finally constricts. In addition, the theory enables cytoskeletal polymer mechanical parameters to be estimated from fluorescence microscope images. |
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