  Single kinesin molecular motors can processively move along a microtubule
(MT) a few micrometers on average before dissociating. However, cellular length
scales over which transport occurs are several hundred microns and more. Why
seemingly unreliable motors are used to transport cellular cargo remains poorly
understood. We propose a new theory for how low processivity, the average
length of a single bout of directed motion, can enhance cellular transport when
motors and cargoes must first diffusively self assemble into complexes. We
employ stochastic modeling to determine the effect of processivity on overall
cargo transport flux. We show that, under a wide range of physiologically
relevant conditions, possessing "infinite" processivity does not maximize flux
along MTs. Rather, we find that low processivity i.e., weak binding of motors
to MTs, is optimal. These results shed light on the relationship between
processivity and transport efficiency and offer a new theory for the
physiological benefits of low motor processivity.
