Kinesin-coated beads move in both directions on proplatelet microtubules. (A) Differential-interference-contrast micrographs of a proplatelet before and after permeabilization with Triton X-100 (B) in a microtubule stabilizing buffer containing taxol. Scale bar, 5 μm. (C) Sequence of movement by 2 kinesin-coated latex beads on the permeabilized proplatelet (Movie S6). Kinesin-coated bead 1 (arrow at 0 sec) is attached to the microtubules and moves left with time. Bead 2 attached to the same proplatelet at 10 sec (arrowhead) and moves in the opposite direction, right. (D-E) Kinesin-coated beads that move into proplatelet tips do not exit. (D) Differential-interference-contrast micrograph of a proplatelet-producing megakaryocyte before permeabilization. A single proplatelet extends from the cell body (CB). Scale bar, 10 μm. (E) High magnification video sequence of a kinesin-coated latex bead (arrow) moving tip-ward on the microtubule cytoskeleton of a permeabilized proplatelet (Movie S7). The kinesin-coated bead (arrow at 0 sec) attached to the microtubules of the permeabilized proplatelet and moved continuously toward the end of the outgrowth, where it remained attached on reaching the tip. A second bead located at the proplatelet tip remains stationary throughout the recording period. Elapsed time indicated in seconds.