Alternate fast and slow stepping of a heterodimeric kinesin molecule

A conventional kinesin molecule travels continuously along a microtubule in discrete 8-nm steps. This processive movement is generally explained by models in which the two identical heads of a kinesin move in a 'hand-over-hand' manner(1-4). Here, we show that a single heterodimeric kinesin molecule (in which one of the two heads is mutated in a nucleotide-binding site) exhibits fast and slow (with the dwell time at least 10 times longer than that of the fast step) 8-nm steps alternately, presumably corresponding to the displacement by the wildtype and mutant heads, respectively. Our results provide the first direct evidence for models in which the roles of the two heads alternate every 8-nm step.