Figure 1.

Stepping pathways. Candidate trajectories for kinesin stepping predict consecutive 8-nm steps or alternating 7- and 9-nm steps, measured along the microtubule axis from a point on the stalk. The surface lattice for a 13-protofilament microtubule and successive positions (numbered) occupied by the walking heads (red and blue) of a dimeric kinesin molecule are shown. Stalk position is indicated (yellow), along with head motions (black arrows). Tubulin α-β heterodimers (green dumbbells) form longitudinal protofilaments that are offset by 0.94 nm. In one-head-bound models, the position of the free head is not displayed; the stalk reports a position near the bound head. For the tightrope and straddle models, the stalk position is assumed to be located at the midpoint between the bound heads; in the asymmetric straddle model, the stalk is associated with a single head.