Dynamisches Verhalten der Actinfibrillen von Nitella auf Grund schneller Filament-Rotation

Abstract

Summary The active movable actin fibrils in cytoplasmic drops squeezed out of Nitella flexilis internodial cells were observed under the best optical conditions in negative-phase contrast. As far as possible, motion picture analysis was done. The following new observations have been made: Ramifications may also travel with the waves along the linear fibrilar loops having adhearing particles. The linear fibrilar bundles without adhearing particles always arise in the vicinity of chloroplast accumulations and extend from here to other parts of the cytoplasmic drop. They generate a particle stream on their surface which always runs in one direction. The angles of this kind of fibrils apparently move as waves in the opposite direciton to the particle stream. This is no true wave motion but a displacement of the whole loop. Lateral ramifications often show slow rotations or pendulum motions around the main fibril. The angles of these fibrilar bundles are variable. Like joints they are obviously the only flexible points along the fibrils. The thin fibrils behave quite differently than the thick ones. They are much more dynamic: The points of branching run along the main fibril in the direction of the particle stream. This can cause very rapid changes in the fibrillar network. Slender fibrils show a rolling motion over the other ones in a direction perpendicular to their long axis. Most slender fibrils lose their stiffnes and begin to flutter very strongly. The ability to generate a particle stream is lost at the same time. This behaviour is often reversible. The fluttering fibrils show an extreme elasticity and capability of self-branching. The interpretation of the protoplasmic streaming as a displacement along very rapidly rotating helical protein-filaments is fully confirmed by the observations on the most slender fibrils. Many phenomena may be imitated to the last detail in model-experiments with rotating helical springs. In order to understand the phenomena on the thicker fibrilar bundles, it is necessary to distinguish between intertwined filaments and filaments which only lie side by side in the bundle in parallel contact. Accordingly the particle-transport along the fibrils and therewith the cause for the “active shearing” or “sliding force” is the submicroscopic wave motion on the single rapidly rotating helical filaments which are jointed together in the bundle. The behaviour of the most slender elements indicates that isolated filaments may show, by mechanical reasons only, a rotation without submicroscopic waves. That means they can move by a drilling motion and branch by self-intertwining. The possible molecular reasons for the rotation are discussed in relation to structural changes in the helical configuration of the filaments.