KINESIN HYDROLYZES ONE ATP PER 8-NM STEP

Kinesin is a two-headed, ATP-dependent motor protein(1,2) that moves a long microtubules in discrete steps(3) of 8 nm. In vitro, single molec ules produce processive movement(4,5); motors typically take similar t o 100 steps before releasing from a microtubule(5-7). A central questi on relates to mechanochemical coupling in this enzyme: how many molecu les of ATP are consumed per step? For the actomyosin system, experimen tal approaches to this issue have generated considerable controversy(8 ,9). Here we take advantage of the processivity of kinesin to determin e the coupling ratio without recourse to direct measurements of ATPase activity, which are subject to large experimental uncertainties(8,10- 12). Beads carrying single molecules of kinesin moving on microtubules were tracked with high spatial and temporal resolution by interferome try(3,13). Statistical analysis of the intervals between steps at limi ting ATP, and studies of fluctuations in motor speed as a function of ATP concentration(14,15), allow the coupling ratio to be determined. A t near-zero load, kinesin molecules hydrolyse a single ATP molecule pe r 8-nm advance. This finding excludes various one-to-many and many-to- one coupling schemes, analogous to those advanced for myosin, and plac es severe constraints on models for movement.