Figure 4. Summary of dimer configurations in ATP and tentative stepping model.

(a) Dimer configurations identified in this study. GST dimer is shown as grey ellipse. (b) An example of a two-step advance along the MT, tentatively correlated with consumption of two molecules of ATP and showing the stalkhead binding sequence both as viewed from the rings and in the view seen in our data. The mobile part of the linker is illustrated (magenta) switching to the primed conformation during stalkhead detachment, as well as a possible trajectory of each detached stalkhead along the MT protofilament. In this example, 8 nm stepping occurs along two adjacent protofilaments and superposed and offset dimers alternate during stepping, illustrated here as an inchworm progression. Examples of alternative stepping patterns are illustrated in Supplementary Fig. 9. (c) In superposed dimers, the stalkhead-stalk hinge allows flexing. In offset dimers, the compliant GST-linker bridge between rings (black coil spring) additionally allows quasi-independent flexing of the heads and pulls the heads towards one another with increasing stalkhead separation.