ABSTRACT
Disassembly only from the Tip .......................................................................... 80 4.3.3 Comparisons with Experiments on Par-Mediated Chromosome Pulling ...80 4.3.4 Implications for Other Phenomena .......................................................... 81 4.4 Results ............................................................................................................... 83 4.4.1 Simulating ParB Translocation ................................................................ 83 4.4.2 Translocation is most Robust when ParB Binds Side-on to ParA ........... 83 4.4.3 Tip-Only Binding ..................................................................................... 84 4.4.4 Side-Binding with Filament Severing ..................................................... 84 4.4.5 Side-Binding with Tip-Only Disassembly ............................................... 84 4.4.6 Weak Binding .......................................................................................... 84 4.4.7 Fast Hydrolysis and Depolymerization ................................................... 84 4.4.8 The Rate of Disassembly Controls the ParB Translocation Velocity ...... 84 4.4.9 Three Regimes of Translocation Velocity................................................ 88 4.4.10 Dependence of the Translocation Velocity on Binding Energy, Binding
Sites, Applied Load, and Other Physical Variables ........................................... 89 4.4.11 Detachment Force for the Parb Polymer ............................................... 89
4.4.12 The Parb Polymer Translocates even When the Para Bundle is not Anchored ........................................................................................................... 91
Keywords ................................................................................................................... 91 Acknowledgments ...................................................................................................... 91 References .................................................................................................................. 91
4.1 INTRODUCTION
Chromosome segregation is fundamental to all cells, but the force-generating mechanisms underlying chromosome translocation in bacteria remain mysterious. Caulobacter crescentus utilizes a depolymerization-driven process in which a ParA protein structure elongates from the new cell pole, binds to a ParB-decorated chromosome, and then retracts via disassembly, pulling the chromosome across the cell. This poses the question of how a depolymerizing structure can robustly pull the chromosome that disassembles it. Brownian dynamics simulations is performed with a simple, physically consistent model of the ParABS system. The simulations suggest that the mechanism of translocation is “self-diffusiophoretic”: by disassembling ParA, ParB generates a ParA concentration gradient so that the ParA concentration is higher in front of the chromosome than behind it. Since the chromosome is attracted to ParA via ParB, it moves up the ParA gradient and across the cell. It is found that translocation is most robust when ParB binds side-on to ParA filaments. In this case, robust translocation occurs over a wide parameter range and is controlled by a single dimensionless quantity: the product of the rate of ParA disassembly and a characteristic relaxation time of the chromosome. This time scale measures the time it takes for the chromosome to recover its average shape after it is has been pulled. Our results suggest explanations for observed phenomena such as segregation failure, filament-length-dependent translocation velocity, and chromosomal compaction.
