Substeps within the 8-nm step of the ATPase cycle of single kinesin molecules

Kinesin is a molecular motor that moves processively(1-4) by regular 8-nm steps along microtubules(5-11). The processivity of this movement is explained by a hand-overhand model in which the two heads of kinesin work in a coordinated manner. One head remains bound to the microtubule while the other steps from the alpha beta -tubulin dimer behind the attached head to the dimer in front. The overall movement is 8 nm per ATPase cycle(9-13). To investigate elementary processes within the 8-nm step, we have developed a new assay that resolves nanometre displacements of single kinesin molecules with microsecond accuracy. Our data show that the 8-nm step can be resolved into fast and slow substeps, each corresponding to a displacement of similar to4 nm. The substeps are most probably generated by structural changes in one head of kinesin, leading to rectified forward thermal motions of the partner head(14). It is also possible that the kinesin steps along the 4-nm repeat of tubulin monomers.