Pulling cargo increases the precision of molecular motor progress

Biomolecular motors use free energy to drive a variety of cellular tasks, including the transport of cargo, such as vesicles and organelles. We find that the widely used "constant-force" approximation for the effect of cargo on motor dynamics leads to a much larger variance of motor step number compared to explicitly modeling diffusive cargo, suggesting the constant-force approximation may be misapplied in some cases. We also find that, with cargo, motor progress is significantly more precise than suggested by a recent result. For cargo with a low relative diffusivity, the dynamics of continuous cargo motion—rather than discrete motor steps—dominate, leading to a new, more permissive bound on the precision of motor progress which is independent of the number of stages per motor cycle.