Key result
Coupling Type-IV pilus elasticity and interfacial behavior with PilT kinetics in a biophysical model explains experimentally observed nonlinear retraction behaviors, including bimodal switching.
Model explains TFP retraction nonlinearity; leaves open in vivo tests of PilT effects on motility.
Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of 100 pN during their retraction before stalling, powering surface translocation(twitching). Single TFP level experiments have shown remarkable nonlinearity in the retraction behavior influenced by the external load as well as levels of PilT molecular motor protein. This includes reversal of motion near stall forces when the concentration of the PilT protein is loweblack significantly. In order to explain this behavior, we analyze the coupling of TFP elasticity and interfacial behavior with PilT kinetics. We model retraction as reaction controlled and elongation as transport controlled process. The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load. Elongation is controlled by monomer transport which suffer entrapment due to excess PilT in the cell periplasm. Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map.
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Ghosh et al. (2014) studied Bacterial twitching motility. Biophysical modeling of TFP elasticity and PilT kinetics was evaluated on Force-velocity characteristics of TFP retraction. Coupling Type-IV pilus elasticity and interfacial behavior with PilT kinetics in a biophysical model explains experimentally observed nonlinear retraction behaviors, including bimodal switching.
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