The perinuclear microtubule system in the green algaAcetabularia: anchor or motility device?

Migrating secondary nuclei inAcetabularia are tightly associated with actin bundles and possess a comet-like tail composed of microtubules. When secondary nuclei begin to settle in preparation for cyst morphogenesis, the tails expand into radially symmetrical arrays of microtubules. Concomitantly, nuclei become gradually dissociated from the actin bundles and eventually stop moving, even though the actin bundles remain intact and persist through this stage. If, however, the radial perinuclear microtubule arrays are destroyed by inhibitors, the nuclei reassociate with the actin bundles and regain their motile activity. Because this movement is sensitive to Cytochalasin D, we propose that actin is required for nuclear movements, whereas microtubules most likely function as a trailing anchor that begins to act as a braking device above a certain threshold in the number and length of perinuclear microtubules.