NUCLEAR CENTERING IN SPIROGYRA - FORCE INTEGRATION BY MICROFILAMENTS ALONG MICROTUBULES

The contribution of microtubules and microfilaments to the cytomechani cs of transverse nuclear centering were investigated in the charophyce an green alga Spirogyra crassa (Zygnematales). Cytoplasmic strands of enhanced rigidity and fasciate appearance radiate from the rim of the lenticular nucleus through the vacuole, frequently split once or twice and are attached to the helical chloroplast bands in the peripheral c ytoplasm. The nucleus is encased in tubulin and a web of F-actin. Bund les of microtubules, emerging from the nuclear rim, are organized into dividing fascicles within the strands and reach to the inner surface of the chloroplast envelope. Organelles are translocated in both direc tions along similarly arranged fascicles of microfilament bundles whic h extend from the nucleus to the peripheral actin cytoskeleton. Applic ation of microtubule- and/or microfilament-depolymerizing drugs affect ed the position of the nucleus only slowly, but in distinct ways. The differential effects suggest that nuclear centering depends on the ten sional integrity of the perinuclear scaffold, with microfilaments conv eying tension along stabilized microtubules and the actin cytoskeleton integrating the translocation forces generated within the scaffold.