Flexibility of the neck domain enhances kinesin-1 motility under load

Kinesin-1 is a dimeric motor protein that moves stepwise along microtubules. A two-stranded alpha-helical coiled-coil formed by the neck domain links the two heads of the molecule, and forces the motor heads to alternate. By exchanging the particularly soft neck region of the conventional kinesin from the fungus Neurospora crassa with an artificial, highly stable coiled- coil we investigated how this domain affects motor kinetics and motility. Under unloaded standard conditions, both motor constructs developed the same gliding velocity. However, in a force-feedback laser trap the mutant showed increasing motility defects with increasing loads, and did not reach wild-type velocities and run lengths. The stall force dropped significantly from 4.1 to 3.0 pN. These results indicate the compliance of kinesin's neck is important to sustain motility under load, and reveal a so far unknown constrain on the imperfect coiled-coil heptad pattern of Kinesin-1. We conclude that coiled- coil structures, a motif encountered in various types of molecular motors, are not merely a clamp for linking two heavy chains to a functional unit but may have specifically evolved to allow motor progression in a viscous, inhomogeneous environment or when several motors attached to a transported vesicle are required to cooperate efficiently.