Properties of the kinesin-3 NcKin3 motor domain and implications for neck function.

Members of the Kinesin-3 family are microtubule motors involved in the transport of membranous cargo. NcKin3 from the fungus Neurospora crassa is dimeric but inactivates one of its motor heads to generate nonprocessive motility. To determine how one of the heads is inactivated, we investigated truncated monomeric constructs. None of the constructs generated processive single-molecule motility, and multimotor velocities depended linearly on the number of residues remaining in the neck. The kinetic analysis suggests futile ATP hydrolysis cycles, because a representative monomer showed a faster ATP turnover than the dimer while supporting slower motility. The K(0.5,MT) was 70-fold lower, the microtubule-bound portion of the kinetic cycle eight-fold longer and the microtubule detachment rate almost 15-fold slower than that of the dimer. Moreover, the monomer's microtubule-dependent ADP release occurred three-fold to four-fold faster than k(cat) (125 versus 34 s(-1)), whereas phosphate release was approximately equally fast (29 s(-1)). A dimeric construct containing a structure-breaking insert between motor head and neck showed a similar behaviour. These data suggest that the heads of the wild-type NcKin3 motor are strictly coupled via the neck domain, and that the dimeric structure is required for proper detachment after one ATPase cycle. This is the first direct comparison of a monomeric Kinesin-3 with its dimeric full-length counterpart, and the kinetic changes observed here may also apply to other Kinesin-3 motors.