Direct evidence for fluid membranes.

We describe a new kind of cell motility that provides direct, visual evidence for the fluid nature of cell membranes. The movement involves continual, unidirectional rotation of one part of a devescovinid flagellate in relation to a neighboring part, at speeds up to one rotation/1.5 sec (room temperature). Rotation includes the plasma membrane, using the flagellar bases and ectosymbiotic bacteria embedded in pockets of the membrane as visible markers. The plasma membrane between the rotating and stationary surfaces is continuous, without fusions with other membranes, and has the typical trilaminar structure of other cell membranes. The nucleus, helical Golgi complex, and stiff central axostyle also rotate. The head of the flagellate always rotates clockwise (as viewed from the anterior end) in relation to the body, but when the head becomes stuck to debris, the body rotates counterclockwise. Evidence suggests that the microtubular axostyle generates the motive force for rotation.