Coordination of kinesin's two heads studied with mutant heterodimers

A conventional kinesin molecule has two identical catalytic domains (heads) and is thought to use them alternately to move processively, with 8-nm steps. To clarify how each head contributes to the observed steps, we have constructed heterodimeric kinesins that consist of two distinct heads. The heterodimers in which one of the heads is mutated in a microtubule-bincling loop moved processively, even when the parent mutant homodimers bound too weakly to retain microtubules in microtubule-gliding assays. The velocities of the heterodimers were only slightly higher than those of the mutant homodimers, although mixtures of these weak-binding mutant homodimers and the WT dimers moved microtubules at a velocity similar to the WT. Thus, the mutant head affects the motility of the WT head only when they are in the same molecule. The maximum force a single heterodimer produced in optical trapping nanometry was intermediate between the WT and mutant homodimers, indicating that both heads contribute to the maximum force at the same time. These results demonstrate close collaboration of kinesin's two heads in producing force and motility.