Active movement in vitro of bundle of microfilaments isolated from Nitella cell.

Subcortical fibrils composed of bundles of F-actin filaments and endoplasmic filaments are responsible for endoplasmic streaming. It is reported here that these fibrils and filaments move actively in an artificial medium containing Mg-ATP and sucrose at neutral pH, when the medium was added to the cytoplasm squeezed out of the cell. The movement was observed by phase-contrast microscopy or dark-field microscopy and recorded on 16-mm film. Chains of chloroplasts linked by subcortical fibrils showed translational movement in the medium. Even after all chloroplasts and the endoplasm were washed away by perfusion with fresh medium, free fibrils and/or filaments (henceforth, referred to as fibers) not attached to chloroplasts continued travelling in the direction of the fiber orientation. Sometimes the fibers formed rings and rotated. Chloroplast chains and free fibers or rings continued moving for 5-30 min at about half the rate of the endoplasmic streaming in vivo. Calcium ion concentrations < 10(-7) M permitted movement to take place. Electron microscopy revealed that both fibers and rings were bundles of F-actin filaments that showed the same polarity after decoration with heavy meromyosin.

Cytoplasmic streaming in characean cells: role of subcortical fibrils

Three or four parallel fibrils of ca. 0.1 μm in width attached to each file of chloroplasts in intact internodal cells generate the motive force for cytoplasmic streaming. Experimental evidence for this conclusion is drawn from experiments in which fibrillar motion and streaming are interrupted by centrifugation, microbeam irradiation, and electrical stimulation. The role of Pb2+ in preventing cessation of cytoplasmic streaming after electrical stimulation is interpreted in terms of localized changes in viscosity of the cytoplasm.

Localization of actin filaments in internodal cells of characean algae. A scanning and transmission electron microscope study.

New methods of visualizing subcortical actin filament bundles, or fibrils, in Characean internodes confirm that they are associated with chloroplasts at the surface facing the streaming endoplasm, and reveal that they are continuous over long distances. With the scanning electron microscope, an average of four to six fibrils are seen bridging a file of chloroplasts. The same configuration appears in negatively stained preparations of large blocks of chloroplast files connected by actin fibrils. Few branches of the subcortical fibrils are evident. These findings are discussed with respect to the mechanism of cytoplasmic streaming in Characeae.