Figure 3. Proposed mechanisms underlying conformational wave-driven bead movement in the assay.

(a) Initially, when a bead is placed under tension it rests against the microtubule wall at a secondary contact point. (b) In the lateral push scenario, the curling protofilaments push laterally against the secondary contact point, causing the bead to pivot about the base of the tether. Lateral deflections from the protofilaments, h, produce larger axial displacements of the bead, a. If A is the tether point and B is the point of bead-microtubule contact and C is the bead center, then ABC defines a right triangle and the amount of leverage is given by the ratio of sides BC/AB. The predicted leverage for 900 nm diameter beads attached via 36 nm tethers is a·h⁻¹=2.4. (c) In the axial pull scenario, the microtubule first disassembles past the secondary contact point, allowing the bead to rotate under laser trap tension into an end-on configuration relative to the microtubule tip. (d) Then the working stroke occurs when curling protofilaments encounter the tether and pull axially on the bead.

DOI: http://dx.doi.org/10.7554/eLife.28433.006

Figure 3—figure supplement 1. A rare example record in which the initial pulse, from a stable baseline, was followed by bead relaxation toward the trap center and then by a second pulse (double arrow).

Such secondary pulses were seen in only 2% of all recorded events (18 of 760). These rare secondary pulses might be generated by axial pulling. However, the lack of any relaxation before the primary pulses indicates that these were not preceded by bead rotation into an end-on configuration, and thus were not generated by axial pulling.

DOI: http://dx.doi.org/10.7554/eLife.28433.007