Collective Force Generation by Molecular Motors Is Determined by Strain-Induced Unbinding

In the living cell, we encounter a large variety of motile
processes
such as organelle transport and cytoskeleton remodeling. These processes
are driven by motor proteins that generate force by transducing chemical
free energy into mechanical work. In many cases, the molecular motors
work in teams to collectively generate larger forces. Recent optical
trapping experiments on small teams of cytoskeletal motors indicated
that the collectively generated force increases with the size of the
motor team but that this increase depends on the motor type and on
whether the motors are studied in vitro or in vivo. Here, we use the theory of stochastic processes
to describe the motion of N motors in a stationary
optical trap and to compute the N-dependence of the
collectively generated forces. We consider six distinct motor types,
two kinesins, two dyneins, and two myosins. We show that the force
increases always linearly with N but with a prefactor
that depends on the performance of the single motor. Surprisingly,
this prefactor increases for weaker motors with a lower stall force.
This counter-intuitive behavior reflects the increased probability
with which stronger motors detach from the filament during strain
generation. Our theoretical results are in quantitative agreement
with experimental data on small teams of kinesin-1 motors.