Model of Kar3Vik1 mechanism of motion. These schematic drawings present a model for Kar3Vik1 microtubule interaction during the motile cycle consistent with the experimental results presented here. In this mechanism, Vik1 binds first to the microtubule followed by Kar3 in the ADP form. Binding by Kar3 and release of ADP (Step 2) produces strain within the coiled coil, thereby weakening the affinity of Vik1 with the microtubule. ATP binding at Kar3 (Step 3) drives a large rotation of the coiled coil stalk toward the minus end of the microtubule. After ATP hydrolysis and P_i release (Step 4), the Kar3•ADP head detaches from the microtubule. Throughout the cycle, the coiled coil does not unwind significantly and imposes the restraint that Kar3 and Vik1 must bind to adjacent protofilaments where Vik1 cannot adopt a canonical microtubule-binding conformation on the lattice. Adapted from Allingham et al. (2007).