Figure 8.

Translocating microtubules fail to transport F-actin in the absence of extract factors. (A) Time-lapse, dual-label fluorescence views of Alexa 488 phalloidin–labeled skeletal muscle F-actin/alpha actinin bundles (green) and microtubules (red) assembled from 99.5% porcine brain tubulin and 0.5% X-rhodamine–labeled tubulin that are gliding across a coverslip coated with rat brain kinesin. A microtubule translocates across the field of view, as judged by following the translocation of its speckle mark (arrow). En route, it makes contact with multiple F-actin bundles, three of which are indicated (#1–3). In spite of these multiple contacts, the moving microtubule does not transport any F-actin bundles. (B) Plot of the distance of the microtubule speckle mark (arrow in A) and of the three different F-actin bundles indicated in A (#1–3) from the origin (position of the speckle at time 00:00) vs. time. This demonstrates that the contact with the microtubule doesn't change the position of the actin bundles. Also see supplemental video 13 at http://www.jcb.org/cgi/content/full/150/2/361/DC1.