Microtubules and filaments in ciliate contractility.

The basis for cell body contractility in heterotrich ciliates resides in the interaction of two discrete contractile fiber systems, the myonemes and km fibers. The motive force for rapid cell contraction in these ciliates has been associated with a calcium-induced change in the macromolecular conformation of the contractile filaments of the myonemes. In Stentor coeruleus, changes in the contractile state of the myonemes are reflected in a reversible calcium-dependent transformation of thin, oriented filaments to shorter tubular forms. Dimensional changes in the km fibers in Stentor coeruleus are generated by the relative sliding of the component parallel microtubule arrays. Changes in the morphology of cross-bridges extending between the sliding components suggest that these structures function in the process of cell extension either in developing the sliding forces required for active displacement of adjacent microtubule ribbons or regulating the extent to which sliding occurs.