  The type IV pilus retraction motor is found in many important bacterial
pathogens. It is the strongest known linear motor protein and is required for
bacterial infectivity. We characterize the dynamics of type IV pilus retraction
in terms of a stochastic chemical reaction model. We find that a two state
model can describe the experimental force velocity relation and qualitative
dependence of ATP concentration. The results indicate that the dynamics is
limited by an ATP-dependent step at low load and a force-dependent step at high
load, and that at least one step is effectively irreversible in the measured
range of forces. The irreversible nature of the sub-step(s) lead to interesting
predictions for future experiments: We find different parameterizations with
mathematically identical force velocity relations but different fluctuations
(diffusion constant). We also find a longer elementary step compared to an
earlier analysis, which agrees better with known facts about the structure of
the pilus filament and energetic considerations. We conclude that more
experimental data is needed, and that further retraction experiments are likely
to resolve interesting details and give valuable insights into the PilT
machinery. In light of our findings, the fluctuations of the retraction
dynamics emerge as a key property to be studied in future experiments.
