Fig. 1.

Walking model defining kinesin's second step. Head 1 binds the Mt with rapid ADP release. ATP binding at head 1 leads to the plus-end-directed motion of the neck linker to position head 2 forward at the next Mt-binding site. ATP binding at head 1 promotes head 2 binding to the Mt, followed by rapid ADP release. ATP hydrolysis at head 1 locks head 2 onto the Mt in a tight-binding state. Phosphate release occurs on head 1, followed by detachment from the Mt. The active site of head 2 is then accessible for ATP binding, and the cycle is repeated.