Dynein is required for movement of ER membranes toward the minus end of MTs. (A) Demembranated sperm was added after 10 min to a crude interphase X. laevis egg extract containing Alexa fluor 488–labeled tubulin and the hydrophobic dye DiIC₁₈. The extension of membrane tubules was followed over time. Arrowheads indicate the tip of extending ER membrane tubule. The kymograph shows the movement of the same tubule. Bars, 2 µm. (B) As in A, but the extract was incubated for 30 min with and without 6 µM of the CC1 of p150 glued to inhibit dynein function. Network formation was visualized by confocal fluorescence microscopy. Bars, 10 µm.