Figure 6.

Proposed new, to our knowledge, model for kinesin stepping under load. The key feature of the model is that forward steps and backward steps originate from different states. During each cycle of ATP turnover under load, kinesin first attempts to step forward from its K⋅ATP-K⋅ADP and K⋅ADP⋅Pi-K⋅ADP states (blue arrows). In our model, backsteps do not occur from either of these states. At higher loads, forward stepping increasingly fails to complete before Pi release, and kinesin then enters a weak binding K⋅ADP-K⋅ADP state that can slip back along the lattice (orange arrows), re-engage, release ADP, and begin a fresh attempt to step forward. For completeness, a presumptively minor process, ADP release while standing still (gray arrows), is also shown. To see this figure in color, go online.