All cells require the ability to process spatial information to properly position intracellular molecules. Many protein complexes and DNA molecules are actively positioned either at the cell midpoint or cell poles, but the processes which drive intracellular positioning are still poorly understood. Using computational modeling we propose a bimodal centering/segregation mechanism in bacteria which is driven by the dynamic instability of polymerizing filaments, which grow and shrink with regularity. Modeled cell centering via dynamically unstable filaments is confirmed experimentally via in vivo time-lapse, colocalization measurements of a model system of clustered plasmid-DNA centered by the dynamically unstable actin-like protein filaments Alp7A in Bacillus subtilis. Generalizing to any cylindrical cell, we find strong cell-length dependence in the centering ability of dynamically unstable filaments, culminating in pole positioning when cell length decreases significantly below the theoretically predicted average filament length. Modeling dynamic instability-driven positioning mechanisms from multiple anisotropic in vivo systems demonstrates that dynamically unstable filaments are a general mechanism for both midcell and cell-pole (segregation) positioning, and that desired positioning is preferentially selected in vivo by intrinsic filament polymerization rates and number.
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