Type-IV Pilus Deformation Can Explain Retraction Behavior

TL;DR

This paper models the nonlinear retraction behavior of Type IV Pili by coupling elasticity, interfacial dynamics, and PilT kinetics, explaining experimental observations including reversal of motion near stall forces.

Contribution

It introduces a coupled biophysical model of TFP elasticity and PilT kinetics that explains complex retraction behaviors observed experimentally.

Findings

Model aligns with experimental data on TFP retraction.

Reversal of motion occurs near stall forces under certain PilT concentrations.

Provides a biophysical parameter map for TFP retraction behavior.

Abstract

Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of 100pN 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 lowered 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