Differential effect of multiple kinesin motors on run length, force and microtubule binding rate.

The in vitro transport of cargo by motor proteins constitutes a model system to understand mechanisms of vesicle trafficking inside cells. Here we apply the classic bead assay with a short, stiff kinesin protein to test the effect of multiple motors on essential transport parameters: distance, force and microtubule binding rate. Measurements of unloaded run length show that the transition from single- to multiple-motor behavior can be characterized by the appearance of extended runs, in accordance with a recently proposed model that quantifies the probability of multiple-motor engagement. In this transition, application of mechanical load using optical tweezers allows us to register maximum force values above single kinesin levels (8 pN). Yet, averages of run length and maximum force undergo little change as the probability of multiple-motor participation increases. In contrast, the measured rate of bead binding to microtubules scales linearly with the average number of motors per bead. These observations suggest that multiple motors bound randomly to the same cargo mainly increase the probability of attachment of these cargoes to the cytoskeletal filament network.