The Arabidopsis kinesin-4, FRA1, requires a high level of processive motility to function correctly.

Processivity is important for kinesins that mediate intracellular transport. Structure-function analyses of N-terminal kinesins (i.e. kinesins comprising their motor domains at the N-terminus) have identified several non-motor regions that affect processivity in vitro However, whether these structural elements affect kinesin processivity and function in vivo is not known. Here, we used an Arabidopsis thaliana kinesin-4, called Fragile Fiber 1 (FRA1, also known as KIN4A), which is thought to mediate vesicle transport, to test whether mutations that alter processivity in vitro lead to similar changes in behavior in vivo and whether processivity is important for the function of FRA1. We generated several FRA1 mutants that differed in their 'run lengths' in vitro and then transformed them into the fra1-5 mutant for complementation and in vivo motility analyses. Our data show that the behavior of processivity mutants in vivo can differ dramatically from in vitro properties, underscoring the need to extend structure-function analyses of kinesins in vivo In addition, we found that a high density of processive motility is necessary for the physiological function of FRA1.