The Orphan Kinesin PAKRP2 Achieves Processive Motility Via Noncanonical Stepping

Abstract

PAKRP2 is an orphan kinesin in Arabidopsis thaliana that is thought to transport vesicles along phragmoplast microtubules for cell plate formation. Here, using single-molecule fluorescence microscopy, we show that PAKRP2 exhibits processive plus-end-directed motility on single microtubules as individual homodimers despite having an exceptionally long (32 residues) neck linker. Furthermore, using high-resolution nanoparticle tracking to visualize motor stepping dynamics, we find that PAKRP2 achieves processivity via a noncanonical stepping mechanism that includes small step sizes and frequent lateral steps to adjacent protofilaments. We propose that the small steps sizes are due to a transient intermediate step that involves a prolonged diffusional search of the tethered head due to its long neck linker. Despite this different stepping behavior, ATP is tightly coupled to each 8-nm step. Collectively, this study reveals PAKRP2 as the first orphan kinesin to demonstrate processive motility and broadens our understanding of the diverse kinesin stepping mechanisms.